Chapter 9 - Force and momentum

Question 1

Define momentum and give it’s units

Answer 1

Mass x velocity
Kgms^-1

Question 2

Define conservation of momentum

Answer 2

In a closed system the total momentum before an event = total momentum after the event, provided no external resultant force acts on the system

Question 3

Write an equation linking force to change in momentum using f = ma

Answer 3

f = m(u-v)/t 
f = mv -mu/ t 
force = change in momentum/time

Question 4

Define the impulse of a force

Answer 4

Force x the time for which the force acts
Impulse = F x t =change in momentum
- units: Ns

Question 5

What is impulse equal to?

Answer 5

Change in momentum

Question 6

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

Answer 6

Impulse or change in momentum

Question 7

A force of 10N acts for 20s on an object of mass 50kg which is initially at rest.
Calculate:
a.) Change in momentum
b.) Velocity of the object at 20s

Answer 7

a.) 10 x 20 = 200 Ns
b.) momentum at 20s = 200Ns
velocity = momentum/mass
= 200/50
= 4ms^-1

Question 8

How do you calculate force of impact?

Answer 8

F = change in momentum/ contact time 
F = mv - mu/t

Question 9

Calculate change in momentum of ball hitting wall at an angle and re-bounding back.There is no loss of speed

Answer 9

Change in momentum = final - initial
= -ucos angle - ucos angle
= -2mucos angle

Question 10

What is an elastic collision?

Answer 10

Collision where there is no loss of kinetic energy

Question 11

What is an inelastic collision?

Answer 11

Collision where the colliding objects have less kinetic energy after the collision than before the collison.

Question 12

If two vehicles collide, causing damage, is this an elastic or inelastic collision?

Answer 12

Inelastic

Question 13

For a falling object, what does the gain in kinetic energy equal?

Answer 13

Loss of gravitational potential energy

Question 14

What is the equation for kinetic energy?

Answer 14

Kinetic energy = 1/2 x mass x velocity^2

Question 15

How do you work out gravitational potential energy?

Answer 15

gravitational potential = mass x g x height

Question 16

A railway wagon of 8000 kg moving at 3ms^-1 collides with an initially stationary wagon of mass 5000 kg. The two wagons separate after the collision. The 8000 kg wagon moves at a speed of 1ms^-1 without change of direction after the collision.

calculate:
a. ) the speed + direction of the 5000 kg wagon after the collision
b. ) the loss of kinetic energy due to the collision.

Answer 16

a.) initial momentum = 8000 x 3 = 24000 kgms^-1
final momentum = (8000 x1) + 5000v
5000v = 16000
v = 3.2ms^-1
b.) kinetic energy before = 1/2 x 8000 x 3^2 = 36000J
Ek of 8000kg wagon after = 1/2 x 8000 x 1^2 = 4000J
Ek of 5000kg wagon after = 1/2 x 5000 x 3.2^2 = 25600J
loss of kinetic energy = 36000 - (4000 +25600)
= 6400J

Question 17

What happens to two objects when they fly apart from each other after initially being at rest?

Answer 17

Recoil from each other with equal and opposite amount of momentum

Question 18

How is total kinetic energy affected when 2 objects collide and move stuck together?

Answer 18

Decreases (inelastic collision)

Question 19

Describe motion of two objects after they collide elastically head on?

Answer 19

Move in opposite directions/rebound (objects always seperate in leastic collisions)