Momentum

Question 1

Equation to work out momentum

Answer 1

Momentum = mass x velocity

Mass = kg
Velocity = m/s
Momentum = kg m/s

Question 2

Triangle to work out momentum

Answer 2

Momentum at top, left is mass and right is velocity

Question 3

To work out the change in momentum

Answer 3

Change in momentum = final momentum - initial momentum

Use + and - to indicate the directions

Question 4

What is an explosion

Answer 4

A interaction between two or more stationary objects where a force pushes them apart.

For example, a gun firing a bullet, two ice skaters pushing each other apart

Two important bits of physics come into this:

1. The force on each object is equal and opposite and lasts for the same amount of time the objects are pushing on each other.
2. Total momentum of the objects is always conserved. This means that if they were stationary at the start (momentum = 0 kg m/s) the object will have equal momentum in opposite directions after the explosion. One of the directions will be negative.

Question 5

The law of conservation of momentum

Answer 5

The total momentum of all the moving objects before the collision = the total momentum of all the moving objects after the collision.

For example, the momentum of Bowl A is 6 and after it collides with Bowl B it is 4.2. To work out the momentum of A after the collision:

Momentum of A before + Momentum of B before = momentum of A after + momentum of B after

6 + 0 = ? + 4.2
= 1.8

Question 6

State the equation linking force, change in momentum and time taken

Answer 6

Force = change in momentum / time taken

Question 7

When we design safety equipment, the design always causes any momentum to be changed over a longer period of time. Give exam
Les

Answer 7

Crash mats for gymnastics, high jump, pole vault etc

Padded clothing - cricket pads, hockey goalkeeper kit, gum shields

Helmets - most have a hard outer layer and padding inside. For example,cycling helmets use polystyrene which deforms once on impact.

Running shoes - the sole is designed to make the impact of your heel on the ground last over a longer time, meaning that the force between your foot and the road is reduced for each step. This should reduce the risk of injury as less force is transmitted through the joints in your legs and back,

Question 8

Safety features in a car

Answer 8

If a car was involved in a collision, there are 3 main safety features designed into the car to reduce the force on the person and therefore reduce injury to them. These are seatbelts, airbags and crumple zones.

Question 9

Seatbelts

Answer 9

If you give the seatbelt a sharp tug, it will ‘lock’. This is what happens if the car decelerates suddenly - like in a collision.

Seatbelt are made of fabric attached to a torsion bar so they will ‘stretch’ slightly if a large force is applied to them.

If the car suddenly decelerates, the person will carry on moving forwards. The seatbelt locks and the belt then supply’s the decelerating force to the person.

The rate of change of the person’s momentum will be lower, and so will the force applied also be smaller.

Question 10

Airbags

Answer 10

If a car is involved in a front collision, sensors in the front bumpers will trigger the release of the air bag that is fitted into the steering wheel and or dashboard.

The person therefore hits the airbag rather than the steering wheel or dashboard. The force needed to change their momentum will be reduced because their momentum will change over a longer period of time, so the force on them is less. This leads to less injury to them.

Question 11

Crumple zones

Answer 11

Cars are designed with regions of their bodywork at the front and the back designed to ‘crumple up’ in the event of a collision.

If the car is involved in a collision, the crumple zone will collapse over a longer period of time than if the car’s body is rigid.

This extra time means that the rate of change of momentum of the car is reduced. This reduces the force acting on the car and so reduces the risk of injury. It also keeps the ‘passenger cell’ in one piece and stops other parts of the car hitting or squashing the passengers during a collision. The kinetic energy of the vehicles in the crash gets ‘absorbed’ into the plastic deformation of the metals in the crumple zones.

Picture of crumple zones and passenger cell in OneNote