Exam Review

Exam 4Momentum

Second 6-weeks, Test 1

Question #1Why can rocket can fly into space, when there is

no air to push against?

A propeller airplane pushes against the air to make it go. In the 1940’s, a major newspaper published an article, saying space flight couldn’t work without air to push against, but even Newton would have known better. Why? The rocket pushes the exhaust

backward, and the equal and opposite reaction pushes the rocket forward. Newton’s Third Law

Questions #2 & 3

Define Newton’s Third law of motion:

Think!!For every action force there is an equal reaction

force in the opposite direction

•These are “Force Pairs” – Action and Reaction (#3)

•They are operating in two opposite directions and are operating on two different objects (Newton’s 3rd Law)

Question #4

If every action force has an equal but opposite reaction force, how can two interacting objects be affected differently?

The action and reaction forces are operating on different objects. That means they can have different effects, because the masses of the two objects don’t have to be the same.

These forces are always equal in magnitude, but operating in the opposite direction from each other

Question #5

The momentum of an object depends on what two factors?

Let’s look at the equation P = m v

“m” is mass“v” is velocity (speed)

that’s it

Question #6

• What is the momentum of a 0.8-kilogram dodge-ball moving at 10 m/s?

P = M= V =

?0.8 kg10 m/s

vmP smkgP /108.0

m/skg 8 P

Question #7A rock falls from a cliff, and reaches a velocity of 1.5

m/s when it strikes the hood of a car parked below. The mass of the rock is 1.2 kg. What was the momentum of the rock when it hit the car?

P = M= V =

?1.2 kg1.5 m/s

vmP smkgP /5.12.1

m/skg 8.1 P

Question #8A boy and the skateboard on which he is

standing have a combined mass of 75 kilograms. He throws a 2-kg ball forward at 15 m/s. At what speed will the boy roll backward?

)( 21 PP )( 2211 vmvm

(75 kg)*v1 = - (2 kg) * (15 m/2)

v1 = - (2 kg) * (15 m/2) / 75 kg

v1 = -4.4 m/s

Momentum 1 = Momentum 2(in the other direction)

Questions #9 & #10If two kids shove each other and one is more

massive than the other, describe how their accelerations compare?

Describe those same kids’ momentum:

Force = m X aIf the force must be the same, then the acceleration must be differentIf mass goes up, acceleration goes down, so the acceleration of the more massive kid is less

P1 = -P2

They have the same momentum, but in opposite directions

Question #11

• What is the momentum of a 5000-kg truck traveling at 30 m/s?

P = M= V =

?5000 kg30 m/s

vmP smkgP /30000,5

m/skg 000,150 P

Question #12The momentum of a baseball thrown by the

pitcher is 6 kg·m/sec. It is stopped by the catcher in 0.01 seconds.

a) What equation should be used to solve this equation?

b) How much force did the catcher exert to catch the ball?

Equation #4, t

PF Force = momentum / time

t

PF

s

skgmF

01.0

/6 NF 600

Question #13A ball of mud is thrown with a momentum of 0.6

kg·m/sec and is stopped when it hits a wall in 0.01 seconds. What is the force exerted by the wall when it strikes?

Equation #4, t

PF Force = momentum / time

s

skgmF

01.0

/6.0 NF 60

Question #14A boy on a bicycle has a velocity of 3 m/s and a

combined mass of 65-kg. When he puts on the brakes, friction exerts a force that stops him is 5 seconds. How much is the average force produced by the brakes?

t

PF

s

skgmF

5

/195 NF 39

vmP smkgP /365 m/skg 195 P

Part 1: Find Momentum

Part 2: Find Force

Question #14

If a 4-kg cannonball was fired at 40 m/s from a 400-kg cannon, the correct equation to determine the velocity of the cannon would be:

)( 2211 vmvm Equation # 5

See if you can solve for the velocity of the cannon on your own

Question #16

A 2-kg model rocket is traveling from its launch pad at a speed of 50 m/s. At what speed would 0.2 kg of gases be expelled from the rocket? )( 2211 vmvm

2)2.0()/50)(2( vkgsmkg

2)2.0(/100 vkgsmkg

smkg

sgmkv /500

2.0

/1002

Question #17A 5-kg fire extinguisher is goes off by itself. If it

shoots 2-kg of gas at a speed of 30 meters per second, determine the extinguisher’s velocity?: )( 2211 vmvm

)/30)(2())(5( 1 smkgvkg

)/60())(5( 1 smkgvkg

smkg

sgmkv /12

5

/601