ENERGY,WORK, POWER, AND MACHINES

IPC Spring 2014

Energy, Work, Power & Machines

1. 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= mv2 /2 m is mass (kg) v is velocity (m/s)

Energy, Work, Power, & Machines

2. Energy conversion is a changing of one form of energy to another.

3. The Law of Conservation of Energy states that energy cannot be created nor destroyed, but is only changed from one form to another.

1. After you place a book on a high shelf, we say the book has increased _____.

A. gravitational potential energy.

B. elastic potential energy.C. chemical energy.D. kinetic energy.

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

3. When you ride a playground swing, your potential energy is greatest at the highest point.

A. True

B. False

4. As mass decreases, kinetic energy increases.

A. True

B. False

Energy, Work, Power, & Machines 4. Work is done when a object

moves through a distance because of a force acting upon the object.

5. The following formula is used to calculate work: W=F×d.

W stands for the work done (J) F stands for the force (N) d stands for distance (m) The SI unit for work is the joule (J).

Energy, Work, Power, & Machines

6. Power - the rate at which work is done.

7. Power can be calculated by using the following formula:

P = W/t or 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).

5. A student weighing 900 N runs to the top of an 8 m staircase in 5 sec. How much power was produced?

A. 56.25 W

B. 11.25 W

C. 1440 J

D. 1440 W

6. The metric unit for work is the _________________.

A. Watt

B. Joule

C. Newton

D. Foot-Pound

7. The metric unit for power is a _____________.

A. a. Joule

B. Watt

C. Newton

D. Horsepower

8. How much work does Billy perform if he pushes the 8000 N stalled car a distance of 25 meters?A. 320 J

B. 100,000 J

C. 200,000 J

D. 8025 J

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.

ScrewA 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 It is 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.

Energy, Work, Power, & Machines 8.The mechanical advantage is the number

of times a machine multiplies an effort force. Mechanical advantage can be calculated by the following equations:

AMA = Fr IMA = de

Fe dr

AMA = actual mechanical advantage (real life) IMA = ideal mechanical advantage (pretend) Fr = resistance force de = effort distance Fe = effort force d r = resistance distance 9. The mechanical advantage of a pulley

system is determined by counting the number of strands of rope that support the resistance force.

Energy, Work, Power, & Machines 10.Work input is always greater than

work output due to friction. Win= W out

Fe x de = Fr x d r

Win = work input (J) W out = work output (J) Fe = effort force (N) de = effort distance (m) Fr =force of the resistance (N) d r=how far the resistance moved (m)

Energy, Work, Power, & Machines 11.Efficiency is a measure of how

much of the work put into a machine is changed to work put out by the machine.

Efficiency is calculated using the following formula:

efficiency = W out x 100%

Win

efficiency = AMA x 100% IMA

Efficiency of a Machine12.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 100Remember that work = force x distance

Energy, Work, Power, & Machines 13. Compound machines

consist of two or more simple machines.

9. 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%.

10. Which pulley has the greatest mechanical advantage?A. A. Picture 1

B. B. Picture 2

C. C. Picture 3

D. D. Picture 4