Transcript What is Work? - Net Start Class

Energy, Work,
Power, & Machines
Energy is the ability to do work.
Potential energy – stored energy or
energy due to position
PE=m g h
m is mass (kg)
g is gravity (9.8 m/s2)
h is the height of the object (m)
Kinetic energy – energy of motion
KE=1/2 mv2
m is mass (kg)
v is velocity (m/s)
1. After you place a book on a high shelf,
we say the book has increased
a. elastic potential energy.
b. chemical energy.
c. kinetic energy.
d. gravitational potential energy.
Answer: D
2. An empty truck traveling at 10 km/h has
kinetic energy. How much kinetic energy
does it have when it is loaded so its
mass is twice, and its speed is increased
to twice?
a. the same KE
b. twice the KE
c. four times the KE
d. more than four times the KE
Answer: D
What is Work?
Work is force exerted on an object that causes
the object to move some distance
Force without moving a distance yields NO
WORK!!
Work = Force x Distance
W=Fd
SI unit for work is the Joule
1 Joule = 1Newton x 1 Meter
How much work performed:
How much work is performed if you apply 85 N of
force on a box causing it to move 3 m?
W = Fd
W = 85 N x 3 m = 255 Nm = 255 J
How much work performed:
How much work is performed if you apply 37 N of
force and move a wagon 4.3 m?
W = Fd
W = 37N x 4.3m = 159.1 Nm = 159.1 J
How much work performed:
How much work is performed if you apply 118 N
of force on a car that is stuck in the mud and
doesn’t move?:
W = Fd
W = 118N x 0m = 0 J
You might be tired from pushing but no
work was done!!
What is Power?
Power - the rate at which work is done.
P= W
t
or
P=
Fd
t
P stands for power (W)
W stands for work (J)
t stands for time (s)
The SI unit for power is the watt (W).
What is a Machine?
A device that makes work easier or more effective
A machine makes work easier by changing the amount
of force, the distance covered or by changing the
direction of the force
Simple Machines
Inclined Plane
A plane is a flat surface. When that plane is
inclined, or slanted, it can help you move
objects across distances and that's work! A
common inclined plane is a ramp. Lifting a
heavy box onto a loading dock is much
easier if you slide the box up a ramp--a
simple machine.
Wedge
you can use the edge of an inclined plane to push
things apart. Then, the inclined plane is a
wedge. So, a wedge is actually a kind of
inclined plane. An axe blade is a wedge. Think
of the edge of the blade. It's the edge of a
smooth slanted surface.
Screw
A screw an inclined plane wrapped around a
cylinder
A screw can convert a rotational force (torque) to
a linear force and vice versa.
Lever
Any tool that pries something loose is a lever. A
lever is a rigid bar that "pivots" (or turns)
against a "fulcrum" (or a fixed point).
Wheel and Axle
two circular objects attached together about a
common axis
Wheel is the large cylinder
Axle is the small cylinder
Pulley
In a pulley, a cord wraps around a wheel. As the
wheel rotates, the cord moves in either
direction. Now, attach a hook to the cord, and
you can use the wheel's rotation to raise and
lower objects.
Efficiency is a measure of how much of the work
put into a machine is changed to work put out
by the machine.
efficiency = W out x 100%
Win
Efficiency of a Machine
The amount of work obtained from a machine is always less than
the amount of work put into it. This is because work is lost to
friction.
Efficiency = output work / input work x 100
Remember that
work = force x distance
Efficiency Problem
When 100 J are put into a device that puts
out 40 J, the efficiency of the device is
a. 40%.
b. 50%.
c. 60%.
d. 140%.
Answer: A
(40 J/100J) x 100 =
40%
An energy supply is needed for the operation
of a(n)
a. automobile.
b. living cell.
c. machine.
d. all of these
Answer: D
The main sources of energy on Earth are
a. solar and nuclear.
b. gasoline and fuel cells.
c. wind and tidal.
d. potential energy and kinetic energy.
Answer: A