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
Even objects resting on a surface
experience this interaction
E.g. A book resting on a table pushes
down on the table with a force equal to its
weight. The table exerts an equal and
opposite force upwards – ‘reaction force’
Interaction pairs
1.
2.
3.
4.
Indicate where forces are
occurring in this diagram,
using arrows
If one of the forces is the
persons feet, walking on
the ground, what is the
interaction force?
For both forces state
what object is exerting
the force and what the
force is acting on
What can you say about
the forces in an
interaction pair, in terms
of size and direction?
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
What are the forward and backward
forces that make a jet engine move?
Interaction pair
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
Friction examples
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Moving objects – between object and
surface
1. between solid surfaces which are
gripping e.g. Walking, driving
2. between surfaces that slide past
each other e.g. Moving parts in a
machine
3. drag from liquids or air e.g. Air
resistance of a parachute
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
What are the forces acting on this
moving object?
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A number of forces acting on a
body may be replaced by a
single force
The force is called the resultant
force.
If the resultant force acting on a
stationary body is zero the body
will remain stationary.
If the resultant force acting on a
stationary body is not zero the
body will accelerate in the
direction of the resultant force.
If the resultant force acting on a
moving body is zero the body will
continue to move at the same
speed and in the same direction.
If the resultant force acting on a
moving body is not zero the body
will accelerate in the direction of
the resultant force.
Forward
force from
engine
Air resistance
400N
1000N
400N
Friction
All the forces acting on an object can be replaced by a single “resultant fo
E.G. Backward force on car = 400 + 400 = 800N
Forward force on car = 1000N
Overall force acting on car = 1000 – 800 = 200N
The resultant force is 200N
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Air resistance
Forward force
50N
100N
Friction
10N
What forces are
acting on this
woman as she
moves?
•What is the
resultant force?
•40N
Key points
Object at start
At rest
Moving
Moving
Moving
Resultant force Effect on the
object
Zero
Stays at rest
Zero
Velocity stays
the same
More than zero, Accelerates
in the same
direction as the
moving object
More than zero, Decelerates
in the opposite
direction to the
moving object
Newton’s three laws of motion
Balanced forces mean no change in velocity
(no resultant force, overall zero force)
(objects are stationary or moving at a constant
speed)
2.
A resultant force means acceleration
(object will accelerate in direction of resultant
force)
3.
Reaction forces
(if object A exerts a force on object B then object
B exerts the exact opposite force on object A)
1.
Rocket example
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 downoverall 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