# Physics Study Guide

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 for v1

(2 kg) * v1 = (0.01 kg) * (200 m/s)

(2 kg) * v1 = (2 kg-m/s)

v1 = (2 kg-m/s) / (2 kg)

v1 = 1 m/s

Multiple Choice Question 6.6

A rifle recoils while firing a bullet. The speed of the rifle's recoil is small because the force against the rifle is smaller than against the bullet.

momentum of the rifle is smaller.

rifle has much more mass than the bullet. ***(answer)

momentum is mainly concentrated in the bullet.

Multiple Choice Question 6.8

Two objects have the same size and shape, but one is much heavier than the other. When they are dropped simultaneously from a tower, they reach the ground at the same time, but the heavier one has a greater

momentum. ***(answer)

acceleration.

speed.

all of these

none of these

(a) from Vf² = Vo² + 2ad, if we assume Vo = 0 (ie both started from rest) and d = the same (ie both started from the same height, and also weight = mass x g so that the heavier object has more mass, then Vf heavy object = Vf light object. right? mass isn't part of the equation Vf² = Vo² + 2ad. so (a) isn't the answer.

(b) the only acceleration is the acceleration due to gravity. and it's equal for both objects

(c) momentum = mass x velocity. we already know from (a that v heavy = v light. since mass heavy > mass light, momentum heavy > momentum light. this makes (c) true.

(d) and (e) are not correct since (a) and (b) are false and (c) is true...

Multiple Choice Question 6.13

A car traveling along the highway needs a certain amount of force exerted on it to stop it in a certain distance. More stopping force is required when the car has more mass.

more momentum.

less stopping distance.

all of these ***(answers)

none of these

Multiple Choice Question 6.19

A ball is moving at 3 m/s and has a momentum of 48 kg m/s. What is the ball's mass? 144 kg

12 kg

16 kg ***(answer)

4 kg

none of these

Momentum P = m*V

48 kg m/s = m * 3m/s

48/3 = m

m = 16 kg

Multiple Choice Question 6.18

A 4 kg ball has a...

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