Download Explaining Motion

Explaining motion
P4
Big picture
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How forces arise
Friction and normal reaction
Adding forces
Describing and summarizing motion
Explaining the motion of objects
Work
Energy
How forces arise
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Forces arise from an interaction
between two objects
Always come in pairs
The two forces in an interaction pair
are always equal and opposite and
act on different objects
How things start moving
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To make a vehicle/person start
moving it needs to push against the
ground
When it pushes on the ground the
ground pushes back and it will start
to move
Friction
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Friction is an unusual force
It adjusts its size in response to the
situation – up to a limit
This limit depends on the objects and
the surfaces involved
The force of friction arises due to lots
of tiny welds that have to be broken
as an object slides against another
Reaction of surfaces
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If an object is placed on a surface it
squashes or distorts the surface
The surface exerts a reaction force
on the object
Adding forces
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If there is a force acting on an object
and it is not moving there must be
another force balancing the first one
If they balance we say the “resultant
force” is zero
Speed
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Average speed = distance / time
Instantaneous speed – when average
speed is measured over very short
time intervals
Speed cameras detect speeding cars
Motion graphs
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Distance – time graph:
gradient/slope shows speed
Speed – time graph: gradient shows
acceleration
Velocity – time graph: also shows
direction of motion
Force and change of momentum
Momentum = mass x velocity
 Change of momentum caused by a
force:
Change of momentum = force x time
(time is for how long the force acts)
 Conservation of momentum – in an
interaction the total change in
momentum is zero
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Car Safety
In a collision the force on passengers can be
great. Cars are designed to reduce these
forces:
 Crumple zones – increase the collision
time
 Seat belts – stretch to make the change of
momentum longer
 Air bags – cushion impact to reduce your
momentum slowly
Factors involved
Collision time – the size of force on
the car depends on the time the
collision lasts
 Momentum – the bigger the time,
the smaller the force
In summary, the longer it takes to
reduce the passenger’s speed to
zero, the smaller the force they
experience.
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Laws of motion
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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
will change (c.o.m.=r.f x time) and is
in the same direction
Motion
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Stationary objects have a resultant
force that is zero
Objects moving at a constant speed
also have a resultant force that is
zero
Speeding up or slowing down- overall
resultant force exists
Work done
When a force causes movement of an
object, work is done
 Use the equation:
work done by a force = force × distance
moved by
the force
(joule, J)
(newton, N) (metre, m)
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Change of energy
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The energy of a moving object is
called kinetic energy
As an object falls, its gravitational
potential energy decreases
Work and change of energy (cont)
Understand that when work is done
on an object, the energy of the
object increases and
 When work is done by an object, the
energy of the object decreases
according to the relationship:
change in energy = work done
(joule, J)
(joule, J)
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From potential to kinetic energy
When an object is lifted to a higher
position above the ground, work is done
by the lifting force against the
gravitational force acting on the object (its
weight);
 this increases the object’s gravitational
potential energy (GPE);
 use the equation:
change in GPE = weight × vertical height
difference
(joule, J)
(newton, N) (metre, m)
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Changes in kinetic energy
When work is done to make an
object move faster the kinetic energy
increase.
 Change in energy = work done
 So,
change in energy = force x distance
However, some work is wasted due to
the force of friction.
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Conservation of energy
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When an object falls it –
• Loses gravitational potential energy
• Gains kinetic energy
• If friction is small enough to ignore then
Amount of GPE lost = amount of KE gained
We use this formula to calculate KE:
Gain in KE = ½ mass x velocity squared