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AP Physics I.C
Work, Energy and Power
6.1 Work Done by a Constant
Force
In Physics, more work is done when .
..
. . . Applied over a greater
distance . . . And when more
force is used
Work is
• Scalar
• Measured in Joules (rhymes with “schools”
or “fools” i.e. high schoolers who enroll in
AP Physics)
Only the component of the force
that acts in the direction of the
displacement is used to calculate
work so . . .
Example : Mr. Clean – A man pulls on his vacuum cleaner at an
angle of 30.0 degrees with the force of 50.0 N for a distance of
3.00 m. How much work does he do on the vacuum cleaner?
Ex. Find the work required to raise a bucket that weighs 250 N
a distance of 0.80 m if the bucket is raised at a constant
velocity. What is the work required to lower the bucket?
Summary
• If the force and displacement are in the
same direction, work is positive (cos 0º = 1)
• If the force and displacement are in opposite
directions, work is negative (cos 180º = –1)
• If work and displacement are perpendicular,
work is zero.
• Be sure and note the force which does the
work (ex. Applied force, friction, Net force)
Another Example – Accelerating a Crate: A crate with mass of 120
kg on the bed of a truck does not slip. The truck accelerates at
1.5 m/s squared for 65 m. How much work is done on the crate by
the forces that act on it?
6.2 The Work-Energy Theorem
And now, to the amazement of
everyone, I shall prove to you once
and for all . . .
So, work changes the kinetic energy
of an object. Conversely, an object
with kinetic energy has the capacity
to do work.
Ex. A net force of 4500 N is applied to a 1400 kg car that is
initially at rest. What is the kinetic energy and speed of the car
after it has traveled 1.0 EE 2 m?
If several forces act on an object,
they must be added to find the net
force. Net work is then calculated
from this force, as seen in the
following example . . .
Ex. A 2.00 EE 3 kg car moves along a level highway under the
action of two forces: a 1.00 EE 3 N force exerted on the
drivewheels by the road and a 950 N resistive force. Find the
speed of the car after it has moved 2.0 EE 1 m if it starts from
rest.
Again, the work-energy theorem
applies to work done by a net
external force, not an individual
force. If the work done by the net
force is positive, K increases. If the
work done by the net force is
negative, K decreases.
6.3 Gravitational Potential
Energy
And, change in gravitational
potential energy.
6.4 Conservative and NonConservative Forces
Conservative Forces (gravity, elastic
and electrical)
• Work done on an object is independent of
the path
• No net work is done on an object moving it
around a closed path (I.e. starting and
finishing at the same point)
Non-Conservative Forces
• The work done is dependent on the path
• For a closed path, the net work is not zero
A conceptual equation – it’s a liner,
not a boxer.
6.5 Conservation of Mechanical
Energy
One of four conservation laws in
physics
A ball rolling down an inclined
plane
A note on gravitational potential
energy, work and an inclined plane.
High Diver (Dropper?) – A diver drops from a board 10.0 m
high. Use conservation of energy to find the velocity of the diver
at a height of 5.00 m above the water.
Ex. A Viking and a sled have a combined weight of 8.00 EE 2
N. If he slides on the sled down a frictionless hill a vertical
distance of 10.0 m, find the speed at the bottom of the hill if the
Viking pushes off with an initial velocity of 5.00 m/s.
p. 182: 32-33, 36, 38, 40-41, 88B2.ab
32. 6.6 m/s
36. a) 52.2 J
38. 42.3 m
40. 3.5 m/s
88B2.a-b
a) 9.9 m/s
b) 48.7 m/s
b) 0.56
6.7 Power
Two cars with identical mass
Three formulas for power (note:
power is calculated from the work
done by the applied force)
Ex. An elevator with a mass of 1.0 EE 3 kg carries a load of 8.0
EE 2 kg. A frictional force of 4.0 EE 3 kg resists the motion
upwards. What is the power (in kW) the motor must deliver if
the elevator moves at a constant speed of 3.0 m/s?
p. 184: 55, 57, 59-61; Rev. p. 52: 16;
p. 121: 13
60. a) 4.00 kW
p. 52: 16 8.0 m/s
b) 33.6 kW