Energy, Work, Power, & Machines

Energy. Energy is the ability to do work. Potential energy Potential energy – stored energy or energy due to position PE=m g h m m is mass (kg) g g is gravity (9.8 m/s 2 ) h h is the height of the object (m)

Kinetic energy Kinetic energy – energy of motion KE=1/2 mv 2 m m is mass (kg) v 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 force distance Work is force exerted on an object that causes the object to move some distance Force distance NO WORK!! 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 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 = Nm = 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 Power - the rate at which work is done. P= Wor P = F d 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 machine 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 screw A screw an inclined plane wrapped around a cylinder torque torque A screw can convert a rotational force (torque) to a linear force and vice versa.torque

Lever 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 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% W in

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

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