14.1 & 14.2

Work The weight lifter applies a large force to hold the barbell over his head. Because the barbell is motionless, no work is done on the barbell.

Work In science, work is the product of force and distance. 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. Any part of a force that does not act in the direction of motion does no work on an object.

Work Work is done when a force acts on an object in the direction the object moves.

Work Requirements Work Requires Motion No motion = No work Work Depends on Direction If all of the force acts in the same direction as the motion, all of the force does work. If part of the applied force acts in the direction of motion, that part of the force does work. If none of the force is applied in the direction of the motion, the force does no work.

Visual A. All of the force does work on the suitcase. B. The horizontal part of the force does work. C. The force does no work on the suitcase. Force This force does work This force does no work Force Direction of motion Force and motion in the same direction Part of force in direction of motion Lifting force not in direction of motion

Work Equation Force is in newtons (N) Distance is in meters (m) The joule (J) is the SI unit of work. A joule is equal to 1 newton-meter.

Calculating Work A weight lifter raises a 1600-newton barbell to a height of 2.0 meters. Work = Force × Distance Work = 1600 N × 2.0 m Work = 3200 N·m = 3200 J

Work vs Power Power is the rate of doing work. Doing work at a faster rate requires more power. To increase power, you can increase the amount of work done in a given time Do a given amount of work in less time.

Example Because the snow blower can remove more snow in less time, it requires more power than hand shoveling does.

Power Equation Work is measured in joules (J) Time is measured in seconds (s). Power is the watt (W), which is equal to one joule per second.

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?

More Problems 1. Your family is moving to a new apartment. While lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this? Answer: Work = Force × Distance = 200 N × 1.5 m = 300 J Power = Work/Time = 300 J/1.0 s = 300 W

More Problems 2. You lift a book from the floor to a bookshelf 1.0 m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s? Answer: Work = Force × Distance = 15 N × 1.0 m = 15 J Power = Work/Time = 15 J/2.0 s = 7.5 W

More Problems 3. You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used? (Hint: The suitcase moves 0.5 m/s. Consider how much work the force does each second and how work is related to power.) Answer: Work = Force × Distance = 10.0 N × 0.5 m = 5 J Power = Work/Time = 5 J/1.0 s = 5 W

James Watt & Horsepower Another common unit of power is the horsepower. One horsepower (hp) is equal to about 746 watts. James Watt ( ) was looking for a way to compare the power outputs of steam engines he had designed. Horses were a logical choice for comparison as they were the most commonly used source of power in the 1700s.

Horse Power The horse-drawn plow and the gasoline-powered engine are both capable of doing work at a rate of four horsepower.

Machines A machine is a device that changes a force. Machines make work easier to do. They change the size of a force needed The direction of a force Or, the distance over which a force acts.

Increase Force Increasing Force Turning the jack handle allows the man to raise the car.

Each complete rotation of a jack handle applies a small force over a large distance. A small force exerted over a large distance becomes a large force exerted over a short distance. Each rotation lifts the car only a very short distance.

Increase Distance Increasing Distance A rower pulls an oar through a small distance. The end of the oar in the water moves through a large distance. The increased travel of the oar through the water requires the rower to exert a greater force. A machine that decreases the distance through which you exert a force increases the amount of force required.

Direction Change Changing Direction Some machines change the direction of the applied force. Pulling back on the handle of the oar causes its other end to move in the opposite direction.

Rowboat The oars of the boat act as machines that increase the distance over which the force acts. Boat moves in this direction. Input force Input distance Output force Output distance

Machine Efficiency Because of friction, the work done by a machine is always less than the work done on the machine.

Machine Work Work Input to a Machine The force exerted on a machine is the input force. The distance the input force acts through is the input distance. The work input equals the input force multiplied by the input distance.

Machine Work Work Output of a Machine The force exerted by a machine is called the output force. The distance the output force is exerted through is the output distance. The work output of a machine is the output force multiplied by the output distance.

Row Input & Output For an oar, the input force is the force exerted on the handle, and the input distance is the distance the handle moves. The work input is the work done to move the handle. The output work of the oars results from the oars pushing against the water so that the water pushes back against the oars.

You get what you deserve!! The only way to increase the work output is to increase the amount of work you put into the machine. You cannot get more work out of a machine than you put into it!