CHAPTER 14:WORK, POWER, AND MACHINES

SECTION 14–1: WORK AND POWER

OBJECTIVES

Describe the conditions that must exist for a force to do work on an object.

Calculate the work done on an object.

Describe and calculate power.

Compare the units of watts and horsepower as they relate to power.

WHAT IS WORK?

An object begins moving only when an unbalanced force acts on it.

In science, work is the product of force and distance. Work is done when a force acts on an object in the direction the object moves.

For a force to do work on an object, some of the force must act in the same direction as the object moves. If there is no movement, no work is done.

WORK DEPENDS ON DIRECTION

The amount of work done on an object, if any, depends on the direction of the force and the direction of the movement.

A force does not have to act entirely in the direction of movement to do work. Only the part of the force that acts in the direction of motion does work.

Any part of a force that does not act in the direction of motion does no work on an object.

CALCULATING WORK

The work done on an object is calculated by multiplying the constant force acting in the direction of motion by the distance that the object moves.

W = FdWork = Force x distance

When using SI units in the work formula, the force is in newtons, and distance is in meters. One newton-meter is called a joule.

The joule (J) is the SI unit of work. When a force of 1 newton moves an object 1 meter in the direction of the force, 1 joule of work is done.

WHAT IS POWER

Power is the rate of doing work.

During work at a faster rate requires more power. To increase power, you can increase the amount of work done in a given time, or you can do a given amount of work in less time.

When comparing someone shoveling snow and a snow blower, they both can do the same amount of work. But typically the snow blower can do it faster, therefore it is more powerful.

CALCULATING POWER

You can calculate power by dividing the amount of work done by the time needed to do the work.

P = W/t Power = Work/Time

When using SI units in the power formula, work is done in joules and time is in seconds.

The SI unit of power is the watt (W), which is equal to one joule per second. Thus, a 40-watt light bulb requires 40 joules each second that it is lit.

CALCULATING POWER

You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box?

Force (F) = 72 N

Distance (d) = 1.0 m

Time (t) = 2.0 s

Power = Work/Time = Force x Distance/TimePower = (72 N)(1.0 m)/(2.0 s) = 36 W

JAMES WATT AND HORSEPOWER

Besides the watt, another common unit of power is the horsepower.

One horsepower (hp) is equal to about 746 watts.

The horsepower was first defined by Scottish scientist James Watt. He was looking for a way to compare the power outputs of steam engines he had designed, and the most logical comparison was the work a horse could do.

VOCABULARY

Work

Joule

Power

Watt

Horsepower