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Explaining Motion
Revision
Forces
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Forces arise from an interaction
between 2 objects.
In an interaction pair the forces are
equal in size but opposite in direction.
These forces act on different objects.
They explain how rockets work etc.
Forces (cont.)
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Draw diagrams showing forces with
arrows.
Some forces are a response to other
forces eg friction, reaction.
Forces (cont.)
•The resultant force on an object is the
sum of all the individual forces acting on it
•All forces are in pairs
Speed
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Average speed = distance/time
Distinguish between average speed
and instantaneous speed.
Remember that movement in one
direction is positive and the other
direction is negative.
Distance-Time Graphs
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Draw and interpret graphs that show
objects that are
– Stationary
– Moving at constant speed
– Increasing or decreasing speed
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Recall that a steeper gradient means a
faster speed.
Make calculations from a distancespeed graph.
Velocity-Time Graphs
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Recall that velocity is speed with an
indication of direction.
Interpret velocity-time graphs for
objects that are
– Stationary.
– Moving in a straight line at steady speed.
– Moving in a straight line with changing
speed.
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Be able to read a tachograph.
Momentum
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Momentum = mass x velocity
When a force acts on an object it
causes a change in momentum in the
direction of the force.
The size of the change in momentum
is related to the size of the force and
the time it was applied.
Change in momentum= force x time
Changing momentum
safely
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In a collision, if the time taken is
increased that size of the force will be
smaller – causing less damage.
This is how air bags, seat belts,
crumple zones etc work
Forces and Motion
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Motion can be described in terms of a
driving force and a counter force.
The driving force is the engine or pedals.
The counter force is air resistance and the
brakes.
If the driving force is
– Greater than the counter force the object speeds
up
– Equal to the counter force the object moves at
constant speed (or remains stationary).
– Less than the counter force the object slows
Energy Changes
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Moving objects have kinetic energy
KE= ½ mv2.
Objects that are lifted have gravitational
potential energy.
GPE=weight x change in height.
When on object does work it loses energy.
When work is done on an object it gains
energy.
Change in energy = work done.
Energy Changes
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When a force causes an object to
move, work is done.
Work done = force x distance.
Lifting an object gives it GPE, moving
an object gives it KE.
The bigger or faster an object the
greater the KE.
The greater or higher an object the
greater the GPE
Energy Changes
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A falling object loses GPE but gains
KE.
The rise in KE is equal to the fall in
GPE (assuming no air resistance or
friction).
Air resistance and friction will stop the
gain in KE being equal to the work
done.