B1.4 Work and Energy
Image [CC BY-SA 3.0] from http://globalcareersandeducation.wikispaces.com/Job+vs.+Career
All other images in presentation from Microsoft ClipArt, MS Office 2010 Edition unless otherwise stated.
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Force and Motion
YouTube:
https://www.youtube.com/watch?v=8iKhLGK7
HGk
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The question of why objects exhibit
uniform or accelerated motion puzzled
philosophers and scientists for centuries.
In the 1600s, Isaac Newton described
the important relationships between
forces and motion.
https://www.youtube.com/wat
ch?v=kKKM8Y-u7ds
Right image by Alkarex Malin [GNU Free Documentation License] http://commons.wikimedia.org/wiki/File:Leaning_tower_of_pisa_2.jpg from Wikimedia Commons.
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Left image by Godfrey Kneller [Public Domain] http://commons.wikimedia.org/wiki/File:GodfreyKneller-IsaacNewton-1689.jpg from Wikimedia Commons.
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A force is a push or pull on an object.
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Forces have a direction .: a vector quantity
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Forces have a strength (magnitude) and are
measured in Newtons (N).
1. An object at rest will remain at rest.
It does not move because all forces acting upon it
are balanced.
1. An object at rest will remain at rest.
It does not move because all forces acting upon it
are balanced.
Force 1
Force 2
1. An object at rest will remain at rest.
It does not move because all forces acting upon it
are balanced.
Force 1
Force 2
Balanced forces are equal in magnitude but opposite in
direction.
2. An object at rest will move if an unbalanced
force is applied to it.
2. An object at rest will move if an unbalanced
force is applied to it.
Force 1
Force 2
2. An object at rest will move if an unbalanced
force is applied to it.
Force 2
Force 1
2. An object at rest will move if an unbalanced
force is applied to it.
Force 1
Force 2
Force 2
Force 1
Unbalanced forces are not equal in magnitude, or are
not opposite in direction.
3. An object in motion tends to remain in motion
moving at a constant speed in a straight line.
3. An object in motion tends to remain in motion
moving at a constant speed in a straight line.
moving ball speeds up
unbalanced force
3. An object in motion tends to remain in motion
moving at a constant speed in a straight line.
moving ball slows down
unbalanced force (friction)
3. An object in motion tends to remain in motion
moving at a constant speed in a straight line.
moving ball speeds up
unbalanced force
moving ball slows down
unbalanced force (friction)
If an unbalanced force is applied to the moving ball it will
either speed up or slow down
When an unbalanced force is applied to an object, energy is
transferred to the object. This causes a change in the
motion of the object.
When an unbalanced force is applied to an object, energy is
transferred to the object. This causes a change in the
motion of the object.
moving ball speeds up
unbalanced force
Energy supplied by
the cue is
transferred to the
ball
When an unbalanced force is applied to an object, energy is
transferred to the object. This causes a change in the
motion of the object.
Energy supplied
by friction
between surfaces
is transferred to
the ball
moving ball slows down
unbalanced force (friction)
Work occurs when a force moves an object through
a distance that is in the direction of the force.
force
direction of movement
Work occurs when a force moves an object through
a distance that is in the direction of the force.
force
direction of movement
Three conditions for work to be done on an
object:
1. There must be movement.
2. There must be a force.
3. The force and the distance the object travels
must be in the same direction.
Is work being done?
A
B
C
D
Calculating work:
Work = force x distance the object travels
W = Fd
The unit for work is the joule (J) or newton-metre (Nm)
The unit for force is the newton (N)
The unit for distance is the metre (m)
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1 J = 1 Nm = 1 kg·m2/s2
1 joule = 1 newton metre
1 J = 1 N·m
Note: 1 N = kg·m
1 J = 1 kg∙m ∙m
s2
s2
1 J = 1 kg∙m2 or 1J = 1 kg∙m2/s2
s2
1 joule is equal to 1 newton-metre
1 joule is equal to 1 kilogram metre squared second squared
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When a force is applied to move an object through
a distance, work is done on the object. This is
called work input or energy input.
The work input can be calculated using the
formula: W = Fd
The object gains energy as a result of this work
done on it. This energy is called energy output or
work output.
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total work input = total work output
(in the absence of any outside forces such as
friction)
A force of 10 N is applied to lift a box of paper 0.5 m on to a
chair. Calculate the work input. (2 sig.dig.)
W=Fxd
The work output (energy gained by box) = ______ J
A tugboat is towing a tanker. Calculate the work done by
the tugboat if it applies an average horizontal force of
6.50 × 103 N on the towrope while towing the tanker
through a horizontal distance of 150m. (3 sig.dig.)
Manipulating Formulas
Calculating force applied if the distance is 5.00 metres, and
500 J of work was done on the object.
F = 500 J
5.00 m
F = 500 Nm
5.00 m
F = 500 Nm
5.00 m
F = 100 N
The force is 100 newtons.
A force of 70 N was applied to an object resulting in 280 J of
work input. Calculate the distance the object was moved.
d = 280 J
70 N
d = 280 Nm
70 N
d = 280 Nm
70 N
d= 4m
The distance is 4.0 metres.
Energy is the ability to do work.
Energy and work are the same thing.
If a body does work on an object, then the body
doing the work loses energy.
The object that has the work done to it gains this
energy. An energy transfer has occurred.
change in energy = work
Δ E =W
Force applied over
a distance
Work is done on
the ball by the
cue.
Energy supplied
by the cue is
transferred to
the ball
The ball has gained
the energy lost by
the cue.