- #1

postfan

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## Homework Statement

A truck is initially moving at velocity v. The driver presses the brake in order to slow the truck to a stop. The

brake applies a constant force F to the truck. The truck rolls a distance x before coming to a stop, and the time

it takes to stop is t.

8. Which of the following expressions is equal the initial kinetic energy of the truck (i.e. the kinetic energy before the driver starts braking)?

(A) F x

(B) F vt

(C) F xt

(D) F t

(E) Both (a) and (b) are correct

9. Which of the following expressions is equal the initial momentum of the truck (i.e. the momentum before the driver starts braking)?

(A) F x

(B) F t/2

(C) F xt

(D) 2F t

(E) 2F x/v

## Homework Equations

W = delta K

p= m*v

F*t = delta p

## The Attempt at a Solution

For 8 I used the work energy theorem to get the kinetic energy to equal Fx. And since x=vt, I picked (E) but apparently (A) is right. Why doesn't x=vt apply?

For 9 I used the fact that force* time = delta p, to get that p=F*t, and that's where I get stuck. Any thoughts on how to continue?