momentum and collisions - st edmund's girls'...

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Momentum and Collisions


Why is it a good idea to avoid a large object moving quickly?

Stopping moving objects


All moving objects have momentum. This is a measure of

how difficult it is to stop a moving object.

The bigger an object is and the faster it moves, the more

momentum it will have and the more difficult it will be to stop.

If these two cars have the

same mass but one is

quicker than the other, which

has the most momentum?

The faster car.

If both cars travel at the same velocity, but one is full with

luggage and the other is empty, which will have the most

momentum? The heavier car.

What is momentum?


How is momentum calculated?

The momentum of an object can be calculated using this

equation:

momentum = mass × velocity

Velocity is measured in metres per second (m/s).

Momentum is measured in kilogram metres per

second (kg m/s).

Mass is measured in kilograms (kg).


Scalar or vector?

Velocity is a vector quantity – this means it has a

magnitude (size) and direction. Scalar quantities, such

as speed, only have a magnitude.

As velocity is needed to calculate momentum, momentum

must also be a vector quantity and it therefore has a direction.

If two objects of the same

mass are moving in opposite

directions but at the same

speed (i.e. their velocities are

different), the momentum of

each object will be different.

A ‘+’ and a ‘–’ are often used

to indicate the opposite directions of momentum.


An aircraft carrier has a

mass of 1,000,000kg

and a velocity of 15m/s.

What is its momentum?

Calculating momentum question

= 1,000,000 × 15

momentum = mass × velocity

= 15,000,000kg m/s


Momentum calculations


Momentum and collisions


If two objects collide or interact, the

forces acting on each one will be the

same size but in opposite directions.

The same is true for the change in

momentum of each object.

This means that the momentum

lost by one of the objects will

be gained by the other object.

Therefore, whenever two

objects collide or interact,

momentum is conserved.

What is conservation of momentum?


Using conservation of momentum


Conservation of momentum question

Two trolleys collide and stick together. From the data below,

calculate the velocity of the trolleys after the collision.

trolley A trolley B

mass = 3kg mass = 5kg

velocity = 8m/s velocity = –4m/s

momentum = 24kgm/s momentum = –20kgm/s

total momentum before collision = 4kgm/s (24 + –20)

mass after collision = 8kg (3 + 5)

momentum after collision = 4kgm/s

velocity after collision = momentum / mass = 0.5 m/s


Investigating momentum


Momentum in explosions


Momentum: true or false?


When a force is applied to an object, the object’s velocity

changes. This means that its momentum will also change.

Force and change in momentum

The change in momentum depends on the size of the

force and the time for which it is applied. The relationship

between these values is shown by this equation:

Time is measured in seconds (s).

Momentum is measured in kilogram metres per

second (kg m/s).

Force is measured in newtons (N).

force = change in momentum

time


A rugby ball of mass 0.5kg is

kicked from stationary to a velocity

of 8m/s. The kicker’s foot is in

contact with ball for 0.1 seconds.

What force does the kicker use?

Change in momentum question 1

= 40N

force =change in momentum

time

= (0.5 × 8) – ( 0.5 × 0)

0.1

= 4

0.1


A tennis ball is rolled at a toy car of mass 0.1kg. The car

is moved with a velocity of 0.5m/s. If the ball and car are

in contact for 0.05 seconds, with what force is the tennis

ball rolled?

Change in momentum question 2

= 1N


time

= (0.1 × 0.5) – ( 0.1 × 0)

0.1

= 0.05

0.05


Change in momentum calculations


Newton’s second law

Newton’s second law of motion states that:

force = mass × acceleration

Acceleration is calculated by dividing change in velocity

by time. This can be substituted into Newton’s second law:

force =time


time

Compare this equation with the one below. What do you

notice?

mass × change in velocity

change in momentum

time= mass × acceleration


Car crashes and momentum

What happens if two cars travelling very quickly collide?

Both cars come to a stop

in a short space of time.

This means that the cars

and their occupants

experience a large change

of momentum very quickly.

Why could this cause a

very serious injury?

A very large change of momentum in a short space of time

means that the car occupants will experience a large force.

Using this principle, how could you improve the safety of cars?


Reducing forces in collisions


time

The best way to reduce the

forces in a collision is usually

to increase the time taken for

the person to decelerate.

A longer deceleration means that change in momentum

occurs over a longer time. There is therefore a smaller force

acting on the person.

Air bags increase the

time it takes for a driver’s

head to decelerate,

bringing them to a more

gradual stop.


Glossary


Multiple-choice quiz

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