Chapter 5 Work and Machines

WORK The transfer of energy that occurs when a force makes an object move In order for work to be done a force must make an object move

2 Conditions of work 1. The object must move 2. Movement must be in the same direction as the force applied

Work and Energy When work is done there is always a transfer of energy Energy is the ability to do work When you do work on an object, you increase its energy

Calculating work Work (J) = Force (N) x distance (m) W = Fd Example: How much work is done when a force of 33N pulls a wagon 13 meters? W = (33N) x (13m) = 429 J

Work Examples A couch is pushed with a force of 75 N and moves a distance of 5 m across the floor. How much work is done in moving the couch?

Work Examples W = f x d W = 75 N x 5 m W = 375 J

Work Examples The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 50m while the brakes are being applied, what is the force the brakes exert on the car?

Work Examples W = f x d 240,000 J = f x 50m f = 4800 N

Power The rate at which work is done SI unit for power is the Watt (W) P (watts) = Work (J)/Time (s) Usually expressed in kilowatts

Power Examples In lifting a baby from a crib, 50J of work are done. How much power is needed if the baby is lifted in 2.0 s?

Power Examples P = w/t P = 50 J/ 2.0 s P = 25 W

Power Examples The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work?

Power Examples P = w/t 500 W = 10,000 J/ t t = 20 s

A device that makes doing work easier Machine A device that makes doing work easier

2-Forces Involved in Work Done by Machines 1. Input Force: The force that is applied to the machine. (effort force) Given the symbol Fin Output Force: The force applied by the machine. Given the symbol Fout

Mechanical Advantage The ratio of the output force to the input force. Can be calculated using the following equation.

Calculating Mechanical Advantage Calculate the mechanical advantage of a hammer if the input force is 125 N and the output force is 2,000 N.

Calculating Mechanical Advantage MA = Fout/Fin MA = 2,000 N / 125 N = 16

Conserving Energy A machine cannot create energy, so… Wout can never be greater than Win

Which means that… Wout will always be less that Win … because friction changes some of the energy to heat.

A machine without friction. Win = Wout Ideal Machine A machine without friction. Win = Wout

Efficiency A measure of how much of the input work put into a machine is change into useful output.

Efficiency Cont… The efficiency of an ideal machine is 100 percent. The efficiency of a real machine is always less than 100 percent.

How can machines be made more efficient??? by adding a lubricant, such as oil or grease, to surfaces that rub together.

Simple Machines A Machine that does work with only one movement of the machine 6 types of simple machines: lever, pulley, wheel and axle, inclined plane, screw, and wedge

Lever A bar that is free to pivot or turn around a fixed point The fixed point is called a fulcrum There are 3 classes of levers

Ex. – Screwdriver opening a paint can Ex. – Wheelbarrow Ex. – Baseball Bat

Pulley A grooved wheel with a rope, chain, or cable running along the groove The axle of the pulley acts as the fulcrum Fixed and movable Example - Elevators

Wheel and Axle consists of a shaft or axle attached to a larger wheel. The wheel and axle rotate together Examples: Pencil sharpeners, door knobs, screw drivers, faucet handles

Inclined Plane A sloping surface, such as a ramp. Reduces the amount of force required to do work

Screw An inclined plane wrapped in a spiral around a cylindrical post Example – Lid of a peanut butter jar

Wedge An inclined plane with one or two sloping sides It changes the direction of the input forces Example - Knife

Compound Machine Two or more simple machines that operate together Examples – car, can opener

Compound Machines