Transcript Physics_AP_A_Evans_Day_39_Period_4

AP Physics I.C
Work, Energy and Power
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 or Pre-AP Physics)
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 the force and displacement are
perpendicular, work is zero (cos 90º = 0)
• Be sure and note the force which does the
work (ex. applied force, friction, net force)
Ex. A 15 kg crate is moved along a horizontal floor by a
warehouse worker who pulls on it with a rope that makes an
angle of 30.0º with the horizontal . The tension in the rope is
69 N and the crate slides a distance of 10 m. How much work
is done on the crate by the worker? If the coefficient of
kinetic friction between the crate and floor is 0.40, how much
work is done by the normal force? How much work is done
by the frictional force?
Ex. A box slides down a ramp that makes an angle of 37.0º
with the horizontal. The mass of the block is 35 kg, the
coefficient of kinetic friction between the ramp and box is
0.30 and the length of the ramp is 8.0 m. a) How much work
is done by gravity? b) How much work is done by the
normal force? c) How much work is done by friction? d)
What is the total work done on the box?
Force vs. position graph
Ex. A force applied to an object increases at a constant rate
from 0.0 N to 15.0 N while the object is pushed a distance of
5.0 m. The force then decreases at a constant rate to 0.0 N
while the box is pushed an additional 10.0 m. Graph the
motion on a force vs. distance graph and find the net work
done on the object.
The Work-Energy Theorem
And now, to the amazement of
everyone, I shall prove to you once
and for all . . .
Ex. How much work is required to accelerate a 1.00 EE 3 kg
car from an initial speed of 20.0 m/s to 30.0 m/s?
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?
Ex. A pool cue striking a stationary billiard ball (mass 0.25 kg)
gives the ball a speed of 2.0 m/s. If the average force on the cue
ball was 200 N, over what distance did this force act?
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
Conservation of Mechanical
Energy
One of four conservation laws in
physics
A ball falls from a tall wall of a
stall in the hall of the mall and is
observed by a man named Paul.
Ex. A pole vaulter clears a cross bar at a height of 4.2 m. If the
vaulter is momentarily at rest at the height of the bar, what is
the speed as she lands in the pit directly below the bar?
Ex. A Big Mac is thrown horizontally with a speed of 15.0 m/s
from a 20.0 m high cliff. What is the speed of the Big Mac when
it is 8.0 m above the ground?
Ex. A baboon on a vine is released from rest (by an elephant
that holds the vine with its trunk) when the vine makes an
angle of 35º with the vertical. The length of the vine is 3.0 m.
What is the speed of the baboon as it swings through the lowest
point of its circular arc?
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 speed of 5.00 m/s.
Ex. An overbaked chocolate cake, with a mass of 3.5 kg rests
on a frictionless table and compresses a spring with a spring
constant of 25 N/m a distance of 0.20 m. The cake is released
by a hungry humanoid. What maximum speed does the cake
attain?
Ex. a) A box of mass 2.0 kg is released from rest and slides
down a frictionless inclined plane of height 0.5 m. The box
compresses a spring at the bottom of the ramp with spring
constant of 200 N/m. How far does the box compress the
spring? b) If the box and compressed spring are placed in a
position such that the box is free to move at the edge of a
frictionless table, how fast is the box moving when the box is
released from rest? c) Suppose the table is 0.80 m high. How
far from a point directly below the edge of the table will the
block strike the floor?
Ex. A catfish with a mass of 2.0 kg compresses a vertical
spring a distance 0.25 m. If the catfish is released from rest,
and the spring constant is 300 N/m, what is the maximum
vertical height attained by the catfish?
Mechanical energy, potential
energy, kinetic energy graphs
(energy vs. time and energy vs.
distance)
Power
The weight lifters
Ex. A cable on a crane raises a crate with a mass of 350 kg to a
height of 10.2 m in a time of 14.0 s at a constant velocity. What
is the power supplied by the tension in the cable?
Ex. A skier of mass 70.0 kg is pulled up a slope by a motor
driven cable. How much power must the motor deliver to pull
him 60.0 m up a 30.0º slope at a constant speed of 2.0 m/s?