AMATH 250 - Assignment 2
Hello, dear friend, you can consult us at any time if you have any questions, add WeChat: daixieit
AMATH 250 - Assignment 2
1. Consider the following population model
dN N
= 0.1N(1 − ) − 5 , N(0) = 100
dt
where k, A, B are positive constants.
(b) Solve the initial value problem.
(c) Determine the time that the population extincts (N(T) = 0). Round your answer to one decimal place.
2. Consider a simple pendulum of length L. The angle the pendulum creates (measured from the vertical) at a time t can be described by the following second-order DE,
+ ω2 sinθ = 0 , where ω = 4
The DE is independent of the mass connected to the pendulum and here g is the acceleration due to gravity (a known, constant value).
(a) Use the method of reduction of order to solve this DE. You will not be able to solve for θ
directly but will be able to represent t as an appropriate integral.
(b) Using the linearization sin(θ) ≈ θ, solve the original DE subject to the initial conditions
θ(0) = θ0 , θ\ (0) = 0 and determine the period of the pendulum.
3. Solve the following DEs/IVPs.
(a) (ex − 3x2 y2 ) y\ = 2xy3 − y ex
(b) + y = y2 (x − 1) , y(0) = 1
(c) − 1 + (y\ )2 + y y\\ = 0, x > 0, y > 0
(d) x(x2 y\\ + y\ ) + e = 0, y(1) = y\ (1) =
4. For the following IVPs determine if the IVP has a unique solution; if it does, then determine the interval on which the unique solution exists for each IC given.
(a) et x\\ + ln(t2 − 1)x\ + ln((1 − t)2 )x = ^4 − t2
i. x(0) = 1, x\ (0) = 1
ii. x ( ) = 1, x\ ( ) = 1
iii. x(−2) = 1, x\ ( −2) = 1
(b) p\\ = et (p + 1)
i. p(t0 ) = 0,p\ (t0 ) = 1 where t0 ∈ R is given.
5. Consider the model for a rocket launched from the surface of the earth subject only to the force
of
gravity
dv GM
dr r2 ,
v(R) = v0
(1)
Here v is the velocity of the object, r is the distance of the object from the center of the earth, G is the universal gravitation constant, M is the mass of the earth, R is the radius of the earth and v0 is the initial velocity. In this problem, we will consider a different way of determining dimensionless variables.
(a) Write down the dimensions of all the constants and variables in the problem. (Recall that the units of G are ).
(b) Show that introducing the scaled variables τ = , x = , w = in (1) leads to the DE
w dw = − GMTc(2)
dx x2 Lc(3) ,
Hint: Use the chain rule.
w(Lc(−)1 R) =
(c) Determine appropriate choices for Tc and Lc and hence for x and w that leads to the dimen-
sionless model
dw 1
dx x2 ,
w(1) = w0
Make sure your choices ensure that x and w are dimensionless.
(d) Give the expression for the dimensionless constant w0 . What does this represent physically?
2023-02-11