Here's the actual problem:
Suppose that you have a cart with very little friction, and you pull this cart with a
constant force.
A) After the cart is moving, is the force that is applied to the cart by the string constant,
increasing, or decreasing? Explain.
B) How does the acceleration graph vary in time? Does a constant applied force produce
a constant acceleration? Explain.
C) How does the velocity graph vary in time? What kind of change in velocity
corresponds to a constant applied force? Explain.
Pulling force
F
Friction force
f
Solution:
a) After the cart is moving, the force that is applied to the cart
by the string is constant.
From the balance of the cart, F-f=ma, no forces changed in
this situation.
b) Again, from the balance of the cart, a=(F-f)/m, so the
acceleration does not change. A constant applied force does
not necessarily produce a constant acceleration. Because the
acceleration does not only depend on the applied force, but
also depends on the friction and the mass.
c) From the equation V2=V1+at, we can conclude that velocity
lineally increases in time. In this case, constant applied force
produced a constant acceleration, and then velocity increase
corresponds to a constant applied force.