Download Explaining Motion

Explaining Motion
P4
Speed
• In real life, it’s pretty rare
for an object to go at
exactly the same speed
for a long period of time
• Objects usually start off
moving 0 m/s but
accelerate up to speed
• It’s more useful to know
the average speed, which
is the same formula as for
normal speed
Speed, Distance and Velocity
Distance time graph:
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Gradient = Speed
Curves
– Acceleration
– Deceleration
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Displacement can be positive or
negative
– The object can be going in one direction
or the opposite direction
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A distance given in a particular
direction is called displacement
Speed is just a number, but velocity
has direction too
Velocity is sometimes more useful
when measuring motion
Instantaneous velocity is its speed
and direction at a given moment in
time
Can be negative velocities
Acceleration and Velocity
• Acceleration is the change
in velocity or speed
• Deceleration is negative
acceleration
• The units for acceleration
are m/s2
• Gradient = Acceleration
• Speed-time graphs ignore
direction but velocitytime graphs don’t
Forces and Friction
Forces
• Forces occur when two
objects interact
• Interaction pair
– When two forces interact
• An object resting on a
surface experiences a
reaction force
– The upward force on the
object, response
Friction
• Moving objects normally
experience friction
• Friction opposes a movement
• A reaction force
• A frictional force will match
the size of the force trying to
move an object, up to a
maximum point. After this
point the friction will become
less than the other force
allowing the object to move
Types of Friction
• Friction between solid surfaces which are gripping
– This friction allows people to walk
– If there was no friction people would just slip and slide
• Friction between solid surfaces which are sliding past
each other
– This kind of friction can be reduced but putting a lubricant
between the surfaces
• Resistance or ‘drag’ from fluids, liquids or gasses
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Drag is basically the same as the other types of friction
An object pushing through molecules, causes friction
Drag only happens if an object is moving through a fluid
There is no drag in space
Forces
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Arrows show the size and direction of forces
– The length of the arrow shows the size of the force
– The direction of the arrow shows the direction of the force
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If the forces are opposite and equal then the forces are balanced
The reaction on a surface – Balanced forces
– If an object is resting on a surface, it’s weight causes a downward force
– This causes an equal reaction force to push up on the object
– The two forces are opposite and the same size so they are equal interaction pairs, meaning
the object won’t move
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Steady speed – Balanced forces
– If an object is moving at a steady speed then the forces must be balanced
– Just because something is moving doesn’t mean that there’s an overall force acting on it –
unless it’s changing speed or direction, the overall (resultant) force is zero
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A resultant force is the difference between the two forces in an interaction pair
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Overall force acting on an object
Decides the motion of the object
Accelerate, decelerate, stay at a steady speed
If there is a resultant force acting on an object, its speed / direction (or Both) changes
Forces and Momentum
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Acceleration – Unbalanced forces
The bigger the resultant fore, the
greater the acceleration
The grater the mass of an object, or
the greater it’s velocity, the more
momentum the object has
Momentum is a vector quantity
– It has size and direction (like velocity
but not speed)
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A resultant force of zero means that a
stationary object will stay still
If the object was moving, it stays at a
constant velocity and constant
momentum
If the resultant force on an object is
not zero, its momentum changes in
the direction of the force
Change in Momentum and Force
• The change in momentum
depends on the force
– When a resultant force acts on
an object, it causes a change in
momentum in the direction of
the force
– The change of momentum it
causes is proportional to the
size of the force and the time it
acts for
• The bigger the force and the
longer it acts for, the bigger
the change in momentum
• Car safety features increase
time to reduce forces in
collisions
Work
• Work = a measure of the
amount of energy being
transferred from one place to
another
• Doing work = when you push
or pull something to make it
work
• When lifting something
vertically…
– Force = weight of the object
lifted
• Some energy is lost as heat or
noise so the object doesn’t
have the full energy
transferred
Kinetic Energy (K.E.)
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Is energy of movement
Any moving object has K.E.
Depends on mass and speed
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The greater its mass and the aster it’s going,
the bigger its kinetic energy
To increase K.E., the velocity must be
increased
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So force must be applied
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K.E. is just movement energy
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Thus work is being done on the object
If work is done on an object and it doesn’t
accelerate then the kinetic energy is not
increased
The increase in an object’s K.E. is normally a
bit less that the amount of work done on it,
because some energy is wasted as heat
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If there is no friction or air resistance acting
on an object, OR you’re told to ignore it, then
the increase in an object’s K.G. is equal to the
amount of work done on it
Gravitational Potential Energy (G.P.E.)
• The energy stored in an object
when it is raised to a height
against the force of gravity
– When an object is lifted, its
G.P.E. increases
– When an object falls its G.P.E.
decreases
• The increase in G.P.D. is equal
to the amount of work done
by the lifting force in order to
raise the height
• K.E. gained = G.P.D. lost
– If friction and air resistance is
ignored