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 KEb. twice the KEc. four times the KEd. 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 Joule1 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 is performed if you apply 37 N of force and move a wagon 4.3 m?

W = Fd

How much work performed:

W = 37N x 4.3m = 159.1 Nm = 159.1 J

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

How much work performed:

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 or P = F d t 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 PlaneA 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.

Wedgeyou 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.

ScrewA screw an inclined plane wrapped around a

cylinder

A screw can convert a rotational force (torque) to a linear force and vice versa.

LeverAny 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 Axletwo circular objects attached together

about a common axis

Wheel is the large cylinderAxle is the small cylinder

PulleyIn 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 MachineThe 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

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%

Efficiency Problem

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