I. Work and Power

A. Work—occurs when a force causes an object to move in the same direction that the force is applied

1. Work involves motion, not just effort.

2. Work is done only when the force you exert on an object is in the same direction as the object’s motion; lifting a clothes basket is work, but carrying it while walking is not work.

3. When a force is exerted at an angle, only the part of the force that is in the same direction as the motion does work.

B. Work can be calculated using the formula work = force * distance.

1. Force is measured in newtons, distance is measured in meters, and the unit for work is the joule.

2. Distance in the work equation is the distance an object moves only while the force is being applied.

C. Power—how quickly work is done

1. Power can be calculated using the formula power = work done / time needed.

2. The unit of power is the watt.

3. Doing work on an object increases the object’s kinetic energy.

4. The amount of work done is the amount of energy transferred and can be expressed in the power formula in place of work done: power = energy transferred / time needed.

5. Power is always the rate at which energy is transferred.

 

II. Using Machines

A. Machine—device that makes doing work easier

B. Machines change the way a person does work, not the amount of work that needs to be done.

1. Input force—the effort, or work, force you exert on a machine

2. Output force—the resistance force, or the work a machine does to move an object over some distance

3. When using a machine, the output work can never be greater then the input work.

4. Mechanical advantage—number of times the input force is multiplied by a machine; calculated as mechanical advantage = output force / input force

a. Some machines make work easier by allowing you to exert a smaller force over a longer distance, resulting in a mechanical advantage of more than one.

b. Other machines allow you to exert your force over a shorter distance resulting in a mechanical advantage of less than one.

c. Still other machines allow you to change the direction of input force resulting in a mechanical advantage equal to one.

C. Efficiency—ability of a machine to convert input work to output work; calculated as efficiency = output work / input work ✕ 100%

1. Friction reduces efficiency by converting some work into heat.

2. The efficiency of a real machine is always less than 100 percent because of friction.

3. Oil, or another lubricant, can increase efficiency by reducing the number of contact points between surfaces.