Work, Power, and Machines

Glencoe Chapter 5

A. Work is the transfer of energy that occurs when a force makes an object move.

1. For work to occur, an object must move.

2. The motion of the object must be in the same direction as the force applied to the object.

Work No work

Direction of force applied

Motion of object

Direction of force applied

Motion of object

B. Work is done on an object when a force is being applied to the object and the object moves in the same direction as the applied force.

C. Work and energy are related.

1. Energy is the ability to do work.

2. When work is done on an object, energy is transferred from the object doing the work to the object on which the work is done.

D. Calculating Work- work equals force (in Newtons) times distance (in meters)

1. W = Fa X d

2. SI units for the work equation

a. Force is measured in Newtons (remember F= mass X acceleration)

b. Distance is measured in meters

c. Work is measured in Joules

1 Joule = 1 Newton-meterJoule is also the SI unit for Energy

W

Fa X d

E. Power - the amount of work done in a certain amount of time. The rate at which work is done.

1. Power equals work divided by time; Power equation:

2. The SI unit for power is watts. 1 watt = 1 joule per second

3. A watt is a small unit, therefore power is usually expressed inkilowatts. 1 kilowatt = 1000 watts

W

P X t

E

P X t

4. Since work and energy are related, power can also be calculated as: power equals energy divided by time.

E.g.: Light bulbs, stereo speakers

Using machines

A. A machine is a device that makes doing work easier. It does NOT decrease the amount of work

B. Machines increase the applied force or change the direction of the applied force to make work easier.

1. Work from a small force over a long distance = Work from a large force over a short distance. eg. car jack

2. Increasing the distance reduces the amount of force needed to do the work.

3. Some machines change the direction of the applied force to do the work. e.g. crowbar, pulley, ax

C. Machines help move things that resist being moved

1. The force applied to a machine is called the input force or effort force.(symbol Fin)

2. The force applied by the machine to overcome the resistance of the object is called the output force. (symbol Fout)

3. Amount of energy the machine transfers to the object cannot be greater than the amount of energy transferred to the machine.

a. Some energy transferred is converted to thermal energy due to friction.

b. In an ideal machine with no friction; Work input = Work output

D. Actual Mechanical Advantage (AMA) is the number of times a machine multiplies the effort (input) force. To calculate AMA, the resistance or output force is divided by the effort or input force.

Equation:

AMA = Fout

Fin

_____

Since mechanical advantage is a ratio, there are no units for MA

E. Efficiency - measure of how much of the work put into a machine is changed into useful output work by the machine.

2. Efficiency of a machine is always less than 100%. Some of the energy is always converted to heat because of friction.

1. Calculating efficiency- efficiency equals (output work divided by input work) times 100%. Equation:

3. Lubricants can make machines more efficient by reducing friction.

Simple Machines

A. A machine that does work with only one movement is a simple machine.

B. Lever- a bar that is free to pivot about a fixed point called a fulcrum.

1. Input arm is the part of the lever on which the force is applied.

2. Output arm is the part of the lever that exerts the resistance force.

3. There are three classes of levers based on the positions of the input force, output force, and the fulcrum. (Remember what is in the center)

a. First class lever - the fulcrum is located between the effort and resistance forces. e.g. crowbar

b. Second class lever - the output force is located between the effort force and the fulcrum. e.g. wheel barrow

c. Third class lever - the input force is located between the output force and the fulcrum. e.g. tweezers,

4. Calculating Ideal Mechanical Advantage (IMA) of a lever: IMA equals the length of the input arm divided by the length of the output arm.

Equation for IMA:

Note: Remember, no units for MA because it is a ratio.

C. Pulley- a grooved wheel with a rope, chain, or cable running through the groove. A pulley is a modified first class lever.

1. A fixed pulley is attached to something that does not move. The direction is changed, but the force is not multiplied.IMA = 1

2. Block and tackle - a system of fixed and moveable pulleys; changes direction of the force and multiplies the force. IMA= number of ropes minus the input rope

D. Wheel and axle - machine with two wheels of different sizes rotating together. e.g. doorknob, steering wheel, pencil sharpener

1. IMA = radius of the wheel divided by the radius of the axle.

2. Gears are modified forms of a wheel and axle.

E. Inclined plane - sloping surface that reduces the amount of force required to do work. e.g. ramp

1. IMA - the length of the slope (effort distance) divided by the height of the slope (resistance distance). Equation:

2. Less force is required if a ramp is longer and less steep.

F. Screw - inclined plane wrapped in a spiral wrapped around a cylinder.

G. Wedge - inclined plane with two sloping sides. e.g. knife, ax

H. Compound machine - uses a combination of two or more simple machines. e.g. scissors, bicycle, car engine