Download P4 Explaining Motion - Blackpool Aspire Academy

Forces in all directions
•
A force is needed to start anything moving.
An interaction force arises between two objects.
The green arrow shows
the force of the
weightlifter pushing
upwards.
The red arrow
shows the
downwards force
of the weights.
The two forces in a pair are
called interaction forces.
Interaction forces are:
1.Equal in size
2.Opposite in direction
3.Act on different objects
How things start moving
•
When objects explode, the pieces usually travel outwards in all directions, however when an
object is designed so that everything that comes out of it goes in one direction………. Then
you have a rocket!!
Firstly, the engine has to make the wheels turn, this causes a
forward force on the car.
Force exerted
on car
Force exerted
by tyre on
road.
Friction
•
Friction adjusts its size depending on the situation until it reaches a limit.
25N
50N
25N
•
70N
50N
Box
moves
Friction at its maximum
When two surfaces are put together, the bumps on one fit into another and make it hard to
move over each other. (think of sliding two brushes over each other)
Reaction of surfaces
•
When you drop an object the force pulling it downwards is called Gravity
•
However when you put an object on the table, it does not fall. Therefore something must be
stopping gravity. The table must exert an upward force on the ball that balances gravity.
• Adding forces
•
•
If forces acting on an object balance each other, then they add to zero.
The resultant force is the direction that the larger force is facing.
3N
5N
Resultant force = 2N
More adding forces
5N
6N
5N
Resultant
force = 0
3N
Resultant
force = 3N
5N
3N
Resultant
force = 8N
Speed
Average speed = distance travelled
Time taken
Instantaneous speed = the speed at a particular moment
How to measure vehicle’s speed:
•Gatso speed cameras- uses radar. The camera takes two photographs of the car half a second
apart.
•Truvelo speed cameras- detector cables. Pressure sensors detect when the car passes over. It
measures the time taken to travel in-between these cables.
•Police radar guns- bounce microwaves off approaching cars.
Picturing motion
•
•
Distance time graph- shows how far a moving object is from its starting point.
Speed time graph- shows the speed of the moving object.
•
Distance time graphs
Speed- time graph
Forces and motion
Velocity= Speed in a certain direction
If an object is heavy, it will move
slower, whereas the lighter ones will
move quicker.
Momentum= mass x velocity
(Kg m/s)
(kg)
(m/s)
Change of momentum = force x time for which it acts
(kg m/s)
(N)
(s)
Imagine you are pushing an object hard enough for
it to move. If you keep pushing it, it will get faster.
Its momentum is increasing.
Example:
•
•
A football has a mass of around 1 kg. A free kick gives it a speed of 20 m/s. What is its
momentum?
Momentum = mass x velocity
–
1 kg
x
20 m/s
–
–
= 20 kg m/s
Question 1: What is the momentum of a hockey
ball of mass 0.4 kg moving at 5 m/s?
Question 2: What is the momentum of a jogger of
mass 55kg, running at 4 m/s?
Car safety
•
•
•
•
•
Crumple zones- the front part of the car is designed to absorb the energy of the impact. It is
designed to crumple slowly, therefore making the force on the car smaller.
In a collision, the momentum becomes zero as the car stops.
The size of the force exerted on the car depends on the time the collision lasts.
The BIGGER the time, the smaller the force.
Seat Belts and air bags- are designed to make the change of momentum take longer. So the
force that causes the change is less.
Laws of motion
•
•
Law 1: If the resultant force acting on an object is zero, the momentum of the object does
not change.
Law 2: If there is a resultant force acting on an object, the momentum of the object will
change.
Counter- force
Driving force
As you go faster,
air resistance
force gets bigger,
so does the
counter force.
Eventually you
reach a speed
where the counter
force balances with
the driving force.
Work and Energy
•
•
Work done by a force = force x distance moved in direction of force
(J)
(N)
(m)
•
If you are pushing your car:
600 N
Work done = force x distance moved
= 600N x 50m
= 30,000 J
50 m
Gravitational potential energy
•
When you lift an object up, its gravitational potential energy increases.
•
Gravitational potential energy = weight x vertical height difference
•
(J)
(N)
(m)
•
If you push a trolley you are transferring your energy to the trolley which is stored as kinetic energy:
•
•
Kinetic energy = ½ x mass x (velocity)²
(J)
(kg)
(m/s)
•
For example when a roller coaster runs down a slope it loses gravitational potential energy and gains
kinetic energy.