MATH 2310 Homework 3 Spring 2022
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MATH 2310
Homework 3
Spring 2022
1. [3 pts] Determine the arc length along the curve
r(t) = ⟨cos(t) + tsin(t), sin(t) − tcos(t),t2⟩
from t = 0 to t = π/2.
2. [3 pts] Interestingly, the notion of arc length can be defined in any dimension. A curve in four- dimensional space R4 is parametrized as
r(t) = ⟨x(t),y(t),z(t),w(t)⟩, a ≤ t ≤ b.
Find the arc length of r(t) = ⟨t,ln(t), 1/t,ln(t)⟩ and 1 ≤ t ≤ 4.
3. Suppose that the trajectory of a particle in R3 is described by the vector-valued function r(t), and let v(t) = r′ (t) and a(t) = r′′ (t) be its velocity and acceleration vectors, respectively. For each of the following statements, either give a proof or exhibit a counter-example.
(a) [2 pts] Let C be the space-curve in R3 which is parametrized by r(t), −∞ < t < ∞. If the
velocity vector v(t) is constant, then the curve C lies entirely in a single plane.
(b) [2 pts] Define the speed of the particle at time t to be the length of its velocity vector s(t) =
|v(t)| at time t. If the speed is a constant function, then the curve lies entirely in a plane.
(c) [2 pts] If the acceleration vector a(t) is constant, then the curve C lies entirely in a single plane.
(d) [2 pts] If the velocity vector v(t) is orthogonal to the acceleration vector a(t) for all time t, then the curve C lies entirely in a single plane.
(e) [2 pts] Prove that r(t) × a(t) = 0 implies r(t) × v(t) = c, where c is a constant vector.
(f) [2 pts] If r(t) × v(t) = c for all time t, prove that the motion takes place in a plane (i.e. that
the space curve parametrized by r(t) lies entirely in a plane). Consider both c = 0 and c 0.
4. Prove that the space curve
r(t) =
lies entirely in a single plane. Find an equation for the plane.
2022-02-10