Physics Study Guide

Multiple Choice Question 6.31
A 1000-kg car moving at 10 m/s brakes to a stop in 5 s. The average braking force is
3000 N
5000 N
2000 N ***(answer)
1000 N
4000 N

Force = mass x acceleration.
Acceleration = velocity/time = -10/5 = -2 m/s/s. (- sign means a deceleration from velocity of 10 to 0)
Force = 1000 x -2 = -2000 Newtons (i.e. 2000N in opposite direction to motion)

Multiple Choice Question 6.11
When you jump from an elevated position you usually bend your knees upon reaching the ground.
By doing this, you make the time of the impact about 10 times as great as for a stiff-legged landing. In this way the average force your body experiences is less than 1/10 as great. more than 1/10 as great. about 1/10 as great. ***(answer) about 10 times as great.

Answer --> C, about 1/10th as great by bending your legs

Time extended decreases acceleration value.

You can solve and prove it using Newton's 2nd and Uniform Acceleration Laws.
Just plug in some easy values for descending.

Vf = 0.0 m/s
Vi = 10 m/s t = 1 s

Acceleration is given by

a = [Vf - Vi] / t a = [ (0.0 m/s) - (10 m/s) ] / (1 s) a = [ -10 m/s ] / (1 s) a = -10 m/s^2

Solve the same thing, but with time 10x, so t = 10 s

a = [ (0.0 m/s) - (10 m/s) ] / (10 s) a = [ -10 m/s ] / (10 s) a = -1 m/s^2

Solve for first force and second force, using a mass of 100 kg

F = m * a

F = (100 kg) * (-10 m/s^2)
F = -1,000 N

F = (100 kg) * (-1 m/s^2)
F = -100 N

So divide long-force by short-force

(-100 N) / (-1,000 N) = 0.1 (which is 1/10) --> C

Multiple Choice Question 6.33
A rifle of mass 2 kg is suspended by strings. The rifle fires a bullet of mass 0.01 kg at a speed of 200 m/s. The recoil velocity of the rifle is about
0.001 m/s.
0.1 m/s.
1 m/s ***(answer)
0.01 m/s. none of these

You've given

m1 = 2 kg v1 = ?

m2 = 0.01 kg v2 = 200 m/s

Set it up as a conservation of momentum problem

m1v1 = m2v2

Insert values and solve