The Physics of Everyday Life HW
The Physics of Everyday Life HW
HW-1.1
Chapter 1, Problem 1
If your car has a mass of 827 kg, how much force is required to accelerate it forward at 4.18 m/s2?
F =
Chapter 1, Exercise 2
As you jump across a small stream, does a horizontal force keep you moving forward? If so, what is the force?
a) Yes, the horizontal force of the leap pushes you across the stream.
b) No, there is no horizontal force. You continue to move forward because of inertia.
c) Yes, there is a horizontal force. Inertia is that force.
d) Yes, there is a horizontal force. Gravity is that force.
Chapter 1, Exercise 4
Tapping your toothbrush on the sink dries it off because
a) the vibration of the tapping lowers the friction between the water and the toothbrush.
b) the force of the tapping knocks it off.
c) the water on the toothbrush has inertia.
d) the heat generated by the tapping loosens the water.
Chapter 1, Exercise 5
The back of your car seat has a head rest to protect your neck during a collision. The type of collision which causes your head to press against the headrest is
a) a head-on collision.
b) none of these.
c) a side-impact collision.
d) a rear-end collision.
Chapter 1, Exercise 8
Why is your velocity continuously changing as you ride on a carousel?
a) This is not true, because your speed is not changing
b) This is only true if you are on a carousel animal which is constantly moving up and down.
c) Because your speed is continously increasing and decreasing as your direction changes.
d) Because your direction of motion is continuously changing.
Chapter 1, Exercise 9
When you apply the brakes on your bicycle, which way do you accelerate?
a) Backward
b) Forward
c) You don't accelerate; that would be speeding up.
d) You don't accelerate, you decelerate.
Chapter 1, Exercise 10
One type of home coffee grinder has a small blade that rotates very rapidly and cuts the coffee beans into powder. Nothing prevents the coffee beans from moving. The reason the beans don't get out of the way when the blade begins to push on them is
a) the beans are stuck together.
b) gravity holds them in place.
c) they have inertia.
d) the blades aren't strong enough to make them move.
HW-1.2
Chapter 1, Problem 2
If your car accelerates from rest at a steady rate of 3 m/s2, how soon will it reach 75.6 km/h (47.0 mph or 21.0 m/s)?
t =
Chapter 1, Problem 3
On Mars, the acceleration due to gravity is 3.71 m/s2. What would a rock’s speed be 4 s after you dropped it on Mars?
v =
Chapter 1, Problem 4
On Mars, the acceleration due to gravity is 3.71 m/s2. How far would a rock fall in 6 s if you dropped it on Mars?
s =
Chapter 1, Problem 5
How would your Mars weight compare to your earth weight if the acceleration due to gravity on Mars is 3.71 m/s2?
WMars/Wearth =
Chapter 1, Problem 6
A basketball player can leap upward 0.320 m. What is the magnitude of his initial velocity at the start of the leap? Use g=9.81 m/s2.
v0 =
Chapter 1, Problem 7
A basketball player can leap upward 0.590 m. How long does the basketball player remain in the air? Use g=9.81 m/s2.
t =
Chapter 1, Problem 8
A sprinter can reach a speed of 12 m/s in 1.17 s. If the sprinter’s acceleration is constant during that time, what is the sprinter’s acceleration?
a =
Chapter 1, Problem 10
How much does a 77.0 kg person weigh on earth? Use g=9.81 m/s2.
w =
Chapter 1, Problem 11
If you jump upward with a speed of 2.80 m/s, how long will it take before you stop rising? Use g=9.81 m/s2.
t =
Chapter 1, Exercise 13
If you pull slowly on the top sheet of a pad of paper, the whole pad will move. But if you yank suddenly on the sheet, it will tear away from the pad. The reason for these different behaviors is
a) the sudden pull isn't strong enough to make the pad move.
b) the slow pull is a stronger force.
c) gravity holds the pad in place during the sudden pull.
d) the pad has inertia.
Chapter 1, Exercise 15
Neglect air resistance in this question. If you drop a ball from a height of 4.9 m, it will hit the ground 1 s later. If you fire a bullet exactly horizontally from a height of 4.9 m, it will also hit the ground 1 s later. Explain.
a) This is not true. The dropped ball will hit the ground first.
b) The earth's curvature beneath the bullet compensates for its slower time of fall.
c) This is not true. The bullet will hit the ground first.
d) Gravity acts only in the vertical direction.
Chapter 1, Exercise 16
An acorn falls from a branch located 9.8 m above the ground. After 1 s of falling, the acorn's velocity will be 9.8 m/s downward. Why hasn't the acorn hit the ground?
a) The acorn's average speed for the 1 s is less than 9.8 m/s.
b) Air resistance held it back.
c) The gravity is weaker farther from the earth, so it wasn't going fast enough when it started falling.
d) This is incorrect. The acorn has hit the ground after 1 s of fall.
Chapter 1, Exercise 21
Your suitcase weighs 50 N. As you ride up an escalator toward the second floor, carrying that suitcase, you are traveling at a constant velocity. The upward force you must exert on the suitcase to keep it moving with you is
a) depends on the speed of the escalator.
b) 50 N.
c) greater than 50 N.
d) less than 50 N.
Chapter 1, Exercise 22
A metro train is traveling at constant velocity. What are the net forces on (a) the first car, (b) the middle car, and (c) the last car?
a) (a) backward; (b) zero; (c) forward
b) (a) zero; (b) zero; (c) zero
c) (a) forward; (b) zero; (c) backward
d) (a) forward; (b) forward; (c) forward
HW-1.3
Chapter 1, Problem 15
The builders of the pyramids used a long ramp to lift 21000-kg (21.0-ton) blocks. If a block rose 0.840 m in height while traveling 22.0 m along the ramp’s surface, how much uphill force was needed to push it up the ramp at constant velocity? Use g=9.81 m/s2.
F =
Chapter 1, Problem 16
The builders of the pyramids used a long ramp to lift 21000-kg (21.0-ton) blocks. How much work was done in raising one of the blocks to a height of 58.0 m? Use g=9.81m/s2.
W =
Chapter 1, Problem 17
The builders of the pyramids used a long ramp to lift 21000.0-kg (21.0-ton) blocks. What is the gravitational potential energy of one of the blocks if it’s now 75.0 m above the ground?
U =
Chapter 1, Problem 19
The tire of your bicycle needs air so you attach a bicycle pump to it and begin to push down on the pump’s handle. If you exert a downward force of 25.0 N on the handle and the handle moves downward 0.600 m, how much work do you do?
W =
Chapter 1, Exercise 24
When you kick a soccer ball, which pushes on the other harder: your foot or the soccer ball?
A) Your foot pushes harder.
B) The soccer ball pushes harder.
C) The answer depends on additional factors not mentioned here.
D) Both push equally.
Chapter 1, Exercise 25
The earth exerts a downward force of 850 N on a veteran astronaut as he works outside the space shuttle. What force (if any) does the astronaut exert on the earth?
A) 850 N upward
B) 850 N downward.
C) 0 N
D) less than 850 N upward
Chapter 1, Exercise 28
Comic book superheroes often catch a falling person only a hairsbreadth from the ground. Why would this rescue actually be just as fatal for the victim as hitting the ground itself?
A) A superhero's arms are as hard as the ground.
B) This is incorrect. The falling person has a much better chance of surviving if the superhero catches her.
C) It is the fall that kills a person, not how they stop.
D) The acceleration required to stop in such a short time is still huge, so the force required would be huge.
Chapter 1, Exercise 31
Which does more work in lifting a grain of rice over its head: an ant or a person?
A) The answer depends on additional factors not mentioned here.
B) The person does more work.
C) The ant does more work.
D) Both do equal work.
Chapter 1, Exercise 35
The steel ball in a pinball game rolls around a flat, tilted surface. If you flick the ball straight uphill, it gradually slows to a stop and then begins to roll downhill. Which direction is the ball accelerating (a) as it rolls uphill? (b) as it rolls downhill?
A) (a) downhill; (b) uphill
B) (a) downhill; (b) downhill
C) (a) uphill; (b) downhill
D) (a) zero; (b) downhill
Chapter 1, Exercise 37a
You roll a marble down a playground slide that starts level, then curves downward, and finally the curve flattens out so that it's level again at the end. Where along its travel does the marble experience its greatest acceleration?
A) At the top of the slide
B) At the bottom of the slide
C) The acceleration is the same at all parts of the slide.
D) Along the downward curve
Chapter 1, Exercise 37b
You roll a marble down a playground slide that starts level, then curves downward, and finally the curve flattens out so that it's level again at the end. Where along its travel does the marble experience its greatest speed?
A) Along the downward curve
B) At the bottom of the slide
C) At the top of the slide
D) The speed is the same at all parts of the slide.
HW 2.1
Chapter 2, Problem 1
When you ride a bicycle, your foot pushes down on a pedal that’s 18.0 cm (0.180 m) from the axis of rotation. Your force produces a torque on the crank attached to the pedal. Suppose that you weigh 654 N. If you put all your weight on the pedal while it’s directly in front of the crank’s axis of rotation, what torque do you exert on the crank?
τ =
Chapter 2, Problem 2
An antique carousel that’s powered by a large electric motor undergoes constant angular acceleration from rest to full rotational speed in 5.00 seconds. When the ride ends, a brake causes it to decelerate steadily from full rotational speed to rest in 11.0 seconds. Find the ratio of the torque that starts the carousel to the torque that stops it.
τstart / τstop =
Chapter 2, Problem 3
When you start your computer, the hard disk begins to spin. It takes 6.00 seconds of constant angular acceleration to reach full speed, at which time the computer can begin to access it. If you wanted the disk drive to reach full speed in only 3.00 seconds, how much more torque would the disk drive’s motor have to exert on it during the starting process (Find the ratio of the two torques)?
τfast / τslow =
Chapter 2, Exercise 3
A mechanic balances the wheels of your car to make sure that their centers of mass are located exactly at their geometrical centers. Neglecting friction and air resistance, how would an improperly balanced wheel behave if it were rotating all by itself?
a) It would not wobble and there would be no bad effects.
b) It would wobble because gravity would produce a net torque on it.
c) It would wobble because the mass is uneven.
d) It would not wobble, but tire wear would be uneven.
Chapter 2, Exercise 5
Why is it hard to start the wheel of a roulette table spinning, and what keeps it spinning once it's started?
A) Rotational inertia, as measured by rotational mass.
B) Inertia, as measured by mass.
C) Friction makes it hard to start, but the friction goes away once it's up to speed.
D) Air resistance makes it hard to start, but the resistance goes away once the wheel is up to speed due to its streamlining.
Chapter 2, Exercise 8
It's much easier to carry a weight in your hand when your arm is at your side than it is when your arm is pointing straight out in front of you. Use the concept of torque to explain this effect. When you arm is at your side
A) the force is parallel to the lever arm, not perpendicular.
B) the lever arm is zero.
C) the weight in your hand is counterbalanced by your arm pulling up.
D) the force is zero.
Chapter 2, Exercise 10
How does the string of a yo-yo get the yo-yo spinning?
A) The force of the string pulling up produces an unbalanced torque on the yo-yo.
B) The point where the string touches the axle is the axis of rotation. The weight of the yo-yo produces an unbalanced torque about this point.
C) The string force is less than the weight of the yo-yo, so the yo-yo accelerates downward.
D) Friction between the string and the axle produces an unbalanced torque.
Chapter 2, Exercise 19
How does a crowbar make it easier to lift the edge of a heavy box a few centimeters off the ground?
A) The crowbar has a large lever arm compared to the lever arm of the crowbar on the box on the lid, so the force exerted on the handle is magnified.
B) The long handle of the crowbar is easier to grip.
C) The lever arm of your hand on the crowbar is equal to the lever arm of the crowbar on the box.
D) The force of your hand on the crowbar is equal to the force of the crowbar on the box.
HW-2.2
Chapter 2, Problem 7
Some special vehicles have spinning disks (flywheels) to store energy while they roll downhill. They use that stored energy to lift themselves uphill later on. Their flywheels have relatively small rotational masses but spin at enormous angular speeds. How would a flywheel’s kinetic energy change if its rotational mass were 3 times larger but its angular speed were 3 times smaller?
KE2/KE1 =
Chapter 2, Exercise 21
Skiers often stop by turning their skies sideways and skidding them across the snow. How does this trick remove energy from a skier, and what happens to that energy?
a) The friction of skidding converts the skier's energy into kinetic energy of the snow flying into the air.
b) The friction of skidding converts the skier's energy into thermal energy.
c) The friction of skidding converts the skier's energy into gravitational potential energy of the snow flying into the air.
d) The friction of skidding causes the skier's energy to be lost.
Chapter 2, Exercise 23
Explain why a rolling pin flattens a piecrust without encountering much sliding friction as it moves.
A) The flattening is caused by static friction between the rolling pin and the crust.
B) The flattening is caused by the weight of the rolling pin, not sliding friction.
C) Actually, there is a lot of sliding friction. That's what flattens the crust.
D) The rolling pin is pushing down on the crust, not sliding along it.
Chapter 2, Exercise 26
As you begin pedaling your bicycle and it accelerates forward, the forward force that the bicycle needs to accelerate is exerted by
a) the tires on the ground.
b) your feet on the pedals.
c) the pedals on your feet.
d) the ground on the tires.
Chapter 2, Exercise 28
If you are pulling a sled along a level field at constant velocity, how does the force you are exerting on the sled compare to the force of sliding friction on its runners?
A) The force you exert is equal to the sliding friction.
B) The force you exert is less than the sliding friction.
C) The force you exert is greater than the sliding friction.
D) There is no set relationship between the two forces.
Chapter 2, Exercise 30
When you’re driving on a level road and there’s ice on the pavement, you hardly notice that ice while you’re heading straight at a constant speed. Why is it that you only notice how slippery the road is when you try to turn left or right, or to speed up or slow down?
A) The force of the car's inertia keeps it going.
B) Friction between the tires and the road is reduced when the road is icy.
C) The heavier the car is, the greater the friction between the tires and the road.
D) Friction is only required when you try to change the car's velocity.
HW-2.3
Chapter 2, Problem 8
What is the momentum of a fly if it’s traveling 1.20 m/s and has a mass of 0.000280 kg?
p =
Chapter 2, Problem 9
Your car is broken, so you’re pushing it. If your car has a mass of 897 kg, how much momentum does it have when it’s moving forward at 3.36 m/s (12.1 km/h)?
p=
Chapter 2, Problem 14
A tricycle releases 60.0 J of gravitational potential energy while rolling 0.330 m directly downhill along a ramp. What is the downhill force (if constant) acting on the tricycle?
F =
Chapter 2, Problem 13
You’re at the roller-skating rink with a friend who weighs twice as much as you do. The two of you stand motionless in the middle of the rink so that your combined momentum is zero. You then push on one another and begin to roll apart. If your momentum is then 419 kg·m/s to the left, what is your friend’s momentum?
pfriend = (to the right)
Chapter 2, Problem 16
An elevator releases 11700 J of gravitational potential energy while descending 4.00 m between floors. How much does the elevator weigh?
Weight =
Chapter 2, Exercise 33
In countless movie and television scenes, the hero punches a brawny villain who doesn’t even flinch at the impact. Why is the immovable villain a Hollywood fantasy?
A) Angular momentum must be conserved.
B) The impulse of the hero's fist is greater than the impulse of the villain's jaw.
C) Energy must be conserved.
D) Momentum must be conserved.
Chapter 2, Exercise 32
Falling into a leaf pile is much more comfortable than falling onto the bare ground. In both cases you come to a complete stop, so why does the leaf pile feel so much better?
A) The soft leaves cushion your fall.
B) The stopping time for hitting the ground is bigger.
C) Hitting the ground involves a bigger impulse than hitting the leaves.D) It takes more time to come to rest falling in a leaf pile, so the stopping force is less.
Chapter 2, Exercise 34
Why can't an acrobat stop himself from spinning while he is in midair?
A) His energy is conserved.
B) His angular momentum is conserved.
C) His momentum is conserved.
D) Actually, he can. All he has to do is straighten out his body.
HW-3.1
Chapter 3, Problem 1
Your new designer chair has an S-shaped tubular metal frame that behaves just like a spring. When your friend, who weighs 581 N, sits on the chair, it bends downward 5.00 cm. What is the spring constant for this chair?
k =
Chapter 3, Problem 3
You're squeezing a springy rubber ball in your hand. If you push inward on it with a force of 1.00 N, it dents inward 4.00 mm. How far must you dent it before it pushes outward with a force of 8.00 N?
x =
Chapter 3, Exercise 3
Curly hair behaves like a weak spring that can stretch under its own weight. Why is a hanging curl straighter at the top than at the bottom?
A) Different segments of a given strand of hair have different amounts of curliness.
B) At the bottom the spring constant is higher.
C) At the top the distortion is greater.
D) The top part has more weight hanging from it, so it is more stretched out.
Chapter 3, Exercise 4
When you lie on a spring mattress, it pushes most strongly on the parts of you that stick into it the farthest. Why doesn’t it push up evenly on your entire body?
A) Parts of the mattress that are distorted more have a higher spring constant.
B) The mattress obeys Hooke's Law.
C) Actually, it does push up evenly.
D) The upward push is equal to your weight.
Chapter 3, Exercise 6
While you’re weighing yourself on a bathroom scale, you reach out and push downward on a nearby table. Is the weight reported by the scale high, low, or correct?
A) Correct
B) High
C) More information needed
D) Low
HW-3.2
Chapter 3, Exercise 12
The best running tracks have firm but elastic rubber surfaces. How does a lively surface assist a runner?
A) The track surface increases the runner's coefficient of restitution.
B) A very lively surface will have a coefficient of restitution greater than 1.
C) The runner's feet don't lose their energy.
D) The energy of the collisions between the runner's feet and the track surface is largely not lost, but returned to the runner.
Chapter 3, Exercise 13
Why is it so exhausting to run on soft sand?
A) The sand has a low coefficient of restitution.
B) The sand converts most of the energy of its collision with your feet into thermal energy instead of returning it to you.
C) It takes more energy to run on sand.
D) The sand reduces the coefficient of restitution of your feet.
Chapter 3, Exercise 15
There have been baseball seasons in which so many home runs were hit that people began to suspect that something was wrong with the baseballs. What change in the baseballs would account for them traveling farther than normal?
A) A higher coefficient of restitution.
B) A more rubbery surface.
C) Using springier bats.
D) A higher spring constant.
Chapter 3, Exercise 20
If you drop a steel marble on a wooden floor, why does the floor receive most of the collision energy and contribute most of the rebound energy?
A) The steel marble has a higher coefficient of restitution than the wooden floor.
B) The floor is more massive than the marble.
C) The floor is softer and more massive than the marble.
D) The floor is softer than the marble.
Chapter 3, Exercise 21
A RIF (reduced injury factor) baseball has the same coefficient of restitution as a normal baseball except that it deforms more severely during a collision. Why does this increased deformability lessen the forces exerted by the ball during a bounce and reduce the chances of its causing injury?
A) The RIF ball is softer but just as bouncy.
B) Greater distortion means a collision lasts longer, and impulse equals force times time.
C) The energy is less in an RIF ball.
D) The RIF ball has a smaller spring constant.
Chapter 3, Exercise 25
Why must the surface of a hammer be very hard and stiff for it to drive a nail into wood?
A) The spring constant is higher.
B) If it was soft it couldn't drive a nail.
C) A stiffer surface means less distortion, which takes less time, which means the force on the nail is greater.
D) The coefficient of restitution is higher.
HW-3.3
Chapter 3, Problem 5
Engineers are trying to create artificial "gravity" in a ringshaped space station by spinning it like a centrifuge. The ring is 171 m in radius. How quickly must the space station turn in order to give the astronauts inside it apparent weights equal to their real weights at the earth’s surface? Use g=9.81 m/s2.
v =
Chapter 3, Problem 6
A satellite is orbiting the earth just above its surface. The centripetal force making the satellite follow a circular trajectory is just its weight, so its centripetal acceleration is about 9.81 m/s2 (the acceleration due to gravity near the earth's surface). If the earth's radius is about 6360 km, how fast must the satellite be moving? How long will it take for the satellite to complete one trip around the earth?
v =
t =
Chapter 3, Exercise 26
Some amusement park rides move you back and forth in a horizontal direction. Why is this motion so much more disturbing to your body than cruising at a high speed in a jet airplane?
A) In the ride you're exposed to the moving air, whereas in the jet you're in a sealed box.
B) The effect is entirely psychological.
C) Side-to-side motion is more disturbing than forward/back motion.
D) In the ride your feeling of acceleration is constantly changing, whereas in the jet you have no feeling of acceleration.
Chapter 3, Exercise 30
In some roller coasters, the cars travel through a smooth tube that bends left and right in a series of complicated turns. Why does the car always roll up the right-hand wall of the tube during a sharp left-hand turn?
A) The apparent weight is down and to the right.
B) The apparent weight is up and to the right.
C) The apparent weight is down and to the left.
D) The apparent weight is up and to the left.
Chapter 3, Exercise 35
As you swing back and forth on a playground swing, your apparent weight changes. At what point do you feel the heaviest?
A) At the lowest point of your swing.
B) At the top of your swing going either forward or backward.
C) At the top of your swing going backward.
D) In the middle of your swing going forward, but not yet at the lowest point.
Chapter 3, Exercise 38
People falling from a high diving board feel weightless. Has gravity stopped exerting a force on them? If not, why don’t they feel it?
A) Gravity still exerts a force on them, but they don't feel it because they're falling.
B) Gravity still exerts a force on them, but they don't feel it because their apparent weight has become zero.
C) Gravity still exerts a force on them, but they don't feel it because of air resistance.
D) Gravity no longer exerts a force on them while they're falling.
Chapter 3, Exercise 41
You board an elevator with a large briefcase in your hand. Why does that briefcase suddenly feel particularly heavy when the elevator begins to move upward?
A) The briefcase is accelerating downward, so the resulting apparent weight is upward.
B) The briefcase is accelerating upward, so the resulting apparent weight is upward.
C) The briefcase is accelerating downward, so the resulting apparent weight is downward.
D) The briefcase is accelerating upward, so the resulting apparent weight is downward.
HW-4.1
Chapter 4, Exercise 1
Sprinters start their races from a crouched position with their bodies well forward of their feet. This position allows them to accelerate quickly without tipping over backward. Explain this effect in terms of torque and center of mass.
A) When the runner takes off there will be a large frictional force of their feet against the ground, which will produce a large torque rotating them backward. The fact that their center of mass is well forward of their feet means that there will be a counterbalancing torque rotating them forward, which will keep them from toppling.
B) When the runner takes off there will be a large frictional force of their feet against the ground, which will produce a large torque rotating them forward. The fact that their center of mass is well forward of their feet means that there will be a counterbalancing torque rotating them backward, which will keep them from toppling.
C) When the runner takes off there will be a large frictional force of the ground against their feet, which will produce a large torque rotating them forward. The fact that their center of mass is well forward of their feet means that there will be a counterbalancing torque rotating them backward, which will keep them from toppling.
D) When the runner takes off there will be a large frictional force of the ground against their feet, which will produce a large torque rotating them backward. The fact that their center of mass is well forward of their feet means that there will be a counterbalancing torque rotating them forward, which will keep them from toppling.
Chapter 4, Exercise 3
When you turn while riding a bike, you must lean in the direction of a turn or risk falling over. If you lean left as you turn left, why don’t you fall over to the left?
A) The torque produced by leaning left counterbalances the torque produced by the tires on the ground, so there is no net torque about the center of mass.
B) The torque produced by leaning left is what causes the bikes to rotate as it turns to the left.<
2021-03-09