Upload
vokien
View
279
Download
30
Embed Size (px)
Citation preview
3 of 27 © Boardworks Ltd 2011
Why is it a good idea to avoid a large object moving quickly?
Stopping moving objects
4 of 27 © Boardworks Ltd 2011
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?
5 of 27 © Boardworks Ltd 2011
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).
6 of 27 © Boardworks Ltd 2011
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.
7 of 27 © Boardworks Ltd 2011
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
11 of 27 © Boardworks Ltd 2011
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?
13 of 27 © Boardworks Ltd 2011
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
18 of 27 © Boardworks Ltd 2011
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
19 of 27 © Boardworks Ltd 2011
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
20 of 27 © Boardworks Ltd 2011
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
force =change in momentum
time
= (0.1 × 0.5) – ( 0.1 × 0)
0.1
= 0.05
0.05
22 of 27 © Boardworks Ltd 2011
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
force =change in momentum
time
Compare this equation with the one below. What do you
notice?
mass × change in velocity
change in momentum
time= mass × acceleration
23 of 27 © Boardworks Ltd 2011
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?
24 of 27 © Boardworks Ltd 2011
Reducing forces in collisions
force =change in momentum
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.