1.3.1 - 1.3.4 - Momentum

Question 1

What is the eq. for momentum

Answer 1

Momentum = mass x velocity
P = MV

Question 2

What is the link between momentum and objects moving?

Answer 2

This means that an object at rest (i.e v = 0) has no momentum
Momentum keeps an object moving in the same direction, making it difficult to change the direction of an object with a large momentum

Question 3

How does momentum link to direction?

Answer 3

Since velocity is a vector this means that the momentum of an object also depends on its direction of travel
This means that momentum can be either positive or negative
If an object travelling to the right has positive momentum, an object travelling in the opposite direction (to the left) will have negative momentum

Question 4

What causes the momentum of an object to change?

Answer 4

The object accelerates (speeds up) or decelerates (slows down)
Changes direction
Its mass changes

Question 5

What is conservation of momentum?

Answer 5

In a closed system, the total momentum before an event is equal to the total momentum after the event

The total momentum before a collision = The total momentum after a collision

Question 6

What type of quantity is momentum?

Answer 6

Since momentum is a vector quantity, a system of objects moving in opposite directions (e.g. towards each other) at the same speed will have an overall momentum of 0 since they will cancel out
Momentum is always conserved over time

Question 7

What does forces acting on a moving object cause?

Answer 7

When a force acts on an object that is moving, or able to move, the object will accelerate (or decelerate)
This causes a change in momentum

Question 8

What is the eq. for changing in momentum?

Answer 8

Force = Change in momentum / Change in time

Final momentum – Initial momentum

Question 9

What is Newton’s third law of motion?

Answer 9

Whenever two bodies interact, the forces they exert on each other are equal and opposite

Question 10

What does newtons third law explain about principles of forces?

Answer 10

All forces arise in pairs - if object A exerts a force on object B, then object B exerts an equal and opposite force on object A
Force pairs are of the same type - for example, if object A exerts a gravitational force on object B, then object B exerts an equal and opposite gravitational force on object A

Question 11

How does Newton’s third law allow someone to walk?

Answer 11

Newton’s third law explains the forces that enable someone to walk
One force is from the foot that pushes the ground backwards
The other is an equal and opposite force from the ground that pushes the foot forwards

Question 12

How is Newton’s law applied in collisions?

Answer 12

When one object exerts a force on another object, the second object will exert an equal force on the first object in the opposite direction
When two objects collide, both objects will react, generally causing one object to speed up (gain momentum) and the other object to slow down (lose momentum)

From Newton’s second law, acceleration depends upon both force and mass, this means:
For objects of equal mass, they will have equal accelerations
For objects of unequal mass, they will have unequal accelerations

Question 13

How can momentum cause large forces?

Answer 13

Since force is equal to the rate of change in momentum, the force of an impact in a vehicle collision can be decreased by increasing the contact time over which the collision occurs
The contact time is the time in which the person is in contact with what they have collided with

Question 14

How do Vehicles reduce the impact of forces?

Answer 14

Vehicle safety features are designed to absorb energy upon an impact by changing shape
The main vehicle safety features are crumple zones, seat belts and airbags
For a given force upon impact, these absorb the energy from the impact and increase the time over which the force takes place
This, in turn, increases the time taken for the change in momentum of the passenger and the vehicle to come to rest
The increased time reduces the force and risk of injury on a passenger
The usefulness of safety equipment depends on two main factors: mass and velocity
If the impact is from a large mass, for example, a truck travelling very fast and colliding with a wall, the momentum will be very large
The change in momentum (ie. from a high speed to rest) will also be very large
This means that a very long contact time is needed to reduce the force of impact

Question 15

How do seat belts reduce the impact of forces?

Answer 15

These are designed to stop a passenger from colliding with the interior of a vehicle by keeping them fixed to their seat in an abrupt stop
They are designed to stretch slightly to increase the time for the passenger’s momentum to reach zero and reduce the force on them in a collision

Question 16

How do airbags reduce the impact of forces?

Answer 16

These are deployed at the front on the dashboard and steering wheel when a collision occurs
They act as a soft cushion to prevent injury on the passenger when they are thrown forward upon impact

Question 17

How do Crumple zones reduce the impact of forces?

Answer 17

These are designed into the exterior of vehicles
They are at the front and back and are designed to crush or crumple in a controlled way in a collision
This is why vehicles after a collision look more heavily damaged than expected, even for relatively small collisions
The crumple zones increase the time over which the vehicle comes to rest, lowering the impact force on the passengers

Question 18

How do crash mats reduce the impact of forces?

Answer 18

When a person lands on a crash mat with a large force, for example after jumping, the soft landing means their body is in contact with the mat for a longer period of time than if it was otherwise not there
This increases the contact time over which their momentum is reduced creating a smaller impact force and a lower chance of injury