1 Momentum, N3L and suvat

Question 1

What effect does a cushion on the inside of a helmet have on the impact force felt by a cyclist if they collide with a windscreen?

Answer 1

• cushion increases the collision time
• F = (change in momentum)/ time
• Larger collision time for the same change in momentum
• decreases the impact force
• Less force, less injury

Question 2

What is different about N3L pair forces?

Answer 2

• they act in different directions
• they act on different objects

Question 3

What is the equation for impulse?

Answer 3

Question 4

Is momentum a vector quantity?

Answer 4

Yes- it has both magnitude and direction

Question 5

Use the conservation of momentum to explain why a cannon would have a recoil velocity when a cannon ball is fired.

Answer 5

p_before = p_after

p_before = 0

Therfore p_after = 0

[M_cannon x v _cannon] + [m_{cannon ball} x V_{cannon ball}]= 0

[M_cannon x v _cannon] = - [m_{cannon ball} x V _{cannon ball}]

negative sign shows that the cannon must move in opposite direction to cannonball so that their momentum’s cancel out

Question 6

If the action and reaction force in the image below are equal an opposite.

Can these forces cancel? Why does the small boat move backwards if these forces are equal and opposite?

Answer 6

The reaction force acts on the boat

The action force acts on the person

They cannot cancel!

Therefore the reaction force on the boat makes the boat move to the right.

Question 7

Define momentum

Answer 7

The property an object has due to its mass and velocity. It is the product of its mass and velocity

mass in motion!!!

Question 8

Name the N3L pair force for the force named below

Answer 8

The water pushes up and forward on the swimmer

Question 9

A 0.8 kg ball moves to the right at 6m/s and hits a wall. It rebounds at 4m/s.

How can you calculate its change in momentum and what is the direction of the force which causes this change in momentum?

Answer 9

final velocity is to the left therefore -4m/s

change in momentum = p_final - p_initial

change in momentum = [0.8 x (-4)] - [0.8 x (+6)]

change in momentum = 0.8 x [-4 - 6]

change in momentum = 0.8 x -10 = -8kgm/s

Change in momentum is negative therefore this indicates that the direction of force which caused this change is to the left as well.

Question 10

What effect does making a hammer of hard steel have on the impact force on a nail?

Answer 10

• hard steel hammer decreases the collision time
• F = (change in momentum)/ time
• smaller collision time for the same change in momentum
• increases the impact force
• more force, easier to drive in nail

Question 11

Use the conservation of momentum to create a equation for the collision below

Answer 11

p_before = p_after

[0.01 kg x 300m/s] + [0] = [(0.01 kg + 1.99kg) x v]

• Note the velocity of the block before is 0m/s*
• After the bullet is in the block and the total mass of the system is 2.00 kg*

Question 12

Explain why the velocity of the cart decreases after the mass is dropped onto it.

Answer 12

Momentum is conserved

p before = p after

m_before x v_before =m_after x v_after

m V = M v

If the mass of the system increases after the collsion the velocity must decrease to keep momentum the same.

Question 13

Define force in terms of momentum

Answer 13

Force is the rate of change of momentum

Question 14

What does the area of a force-time graph represent?

Answer 14

F t = change in momentum

This is called impulse

The area of a force-time graph is the impulse

Question 15

An object undergoes a change in motion indicated as Case A and Case B.

In which case is the change in momentum the greatest?

Answer 15

Case A- Since mass is the same we can look at the change in velocity.

The change in velocity is found by using

(v-u)

(-3 m/s) - (5m/s)

= -8m/s

Question 16

If a person on a skateboard pushes on a wall. Name the N3L pair force of….

Persons’ force on wall [left]

Answer 16

Walls’ force on person [right]

Question 17

What is the relationship between momentum, mass and velocity?

Answer 17

p=mv

Momentum = mass x velocity

Question 18

When a person runs they exert a back and downwards force on the road surface. Name the N3L pair of this force….

Runner pushes back and down on the road

Answer 18

Road pushed up forwards and up on the runner

Question 19

What is the unit for momentum?

Answer 19

kgm/s

Question 20

What is the total momentum before the explosion?

What is the total momentum after the collsion?

Answer 20

zero before and after!!

Question 21

What effect does a crumple zone have on the impact force felt by a driver of a car?

Answer 21

• crumple zone increases the collision time
• F = (change in momentum)/ time
• Larger collision time for the same change in momentum
• decreases the impact force
• Less force, less injury

Question 22

Is mass a vector quantity?

Answer 22

NO-it is a scalar quantity- it only has magnitude (You can’t cancel mass)

Question 23

A catapult can be attached to a cart. If a ball is launched from the heavy cart, the cart moves in the opposite direction.

Using N3L the force on a ball by the cart is equal and opposite to the force on the cart by the ball

Use this idea of N3L to explain why the velocity of heavy cart is significally smaller than the velocity of the ball

Answer 23

F = ma

Since the mass of the ball is significantly smaller than the cart, the velocity of the cart must be significantly smaller than the velocity of ball so that the product of the mass and velocity are equal in magnitude.

Question 24

What effect does bending your legs when landing a parachute jump have on the force felt by the persons legs?

Answer 24

• bending legs increases the collision time
• F = (change in momentum)/ time
• Larger collision time for the same change in momentum
• decreases the impact force on the legs
• Less force, less injury

Question 25

Name the N3L pair force

Answer 25

air exerts an upward force on the wings

Question 26

What is the unit for mass?

Answer 26

kg

Question 27

A person and an elephant are having a tug-o-war. They both pull with 500N.

If the person pulls on the rope with 500N to the left, name the N3L pair force and determine the tension in the rope?

Answer 27

The rope pulls on the person with 500N to the right

The tension in the rope is still only 500N! (It is no different if the rope is tied to a wall)

Question 28

Is velocity a vector quantity?

Answer 28

Yes- it has both magnitude and direction

Question 29

What is the unit for velocity?

Answer 29

m/s

Question 30

Define impulse

Answer 30

Impulse is the change in momentum of an object

Question 31

In the suvat equation:

v²=u²+2as

what do the symbols stand for?

Answer 31

Question 32

Calculate the total momentum before the collsion

Answer 32

Only one of the curling stones is moving…therefore

Momentum before = 18 kg x (+3 m/s)

momentum before = 54 kgm/s [right]

Question 33

What equation can we use which links initial and final speed, acceleration and distance travelled.

Answer 33

Question 34

State Newton’s Third Law

Answer 34

For every action (force) there is an equal and opposite reaction (force)

Question 35

A cheetah accelerates from rest at 6 m/s² over 12 metres to catch a rabbit. What is the cheetah’s final velocity.

Answer 35

initial velocity = 0 m/s

v²=u²+2as

v²=0²+(2x6x12)

v²=144

v= 12m/s

Question 36

What is the same about N3L pair forces?

Answer 36

• same magnitude
• same type of force
• they act along the same line