WORK

IN EACH OF THE FOLLOWING SITUATIONS, OBSERVE:

•HOW THE FORCE IS APPLIED,

•WHERE IT IS APPLIED, AND

•HOW MUCH IS APPLIED

THEN OBSERVE:•HOW FAR THE APPLIED FORCE MOVES AND•HOW FAR THE OBJECT MOVED.

THE PURPOSE OF A MACHINE IS TO DO WORK, TO TRADE OFF FORCE AND DISTANCE IN A CONVENIENT WAY.

WORK=FORCE X DISTANCE

The horse is exerting a force to pull the cart. The amount of work done by the horse is equal to the force it exerts times the distance it pulls the cart. The work goes toward overcoming friction between the cart's wheels and the road and thus maintaining the cart's kinetic energy, the product of its mass times the square of its velocity.

MACHINES DO WORK

• Machines have been known since antiquity and have come to form the basis of many components in modern machinery.

• All are designed to amplify the effect of forces or to do work that is, to exert forces over certain distances in order to move weights or to overcome resistances

• Among these devices are the– lever, – the wheel and axle,– the inclined plane,– the screw, – and the rope-and-pulley system.

LEVERS DO WORK

In a first-class lever the fulcrum is between the effort and resistance, where the lever exerts a force. In a second-class lever the resistance is between the fulcrum and effort. For such levers, if the effort arm is longer than the resistance arm, less effort over a certain distance exerts a greater force over a shorter distance. In a third-class lever the effort is between the fulcrum and resistance. More effort is required to moveless weight, but the speed and distance moved are increased.

PULLEYS DO WORK

Pulling a rope that passes over a single pulley (A) lifts a weight “w” a distance “l.” The applied force must equal the weight. If a second pulley is added (B), the same force can lift twice the weight the same distance “l.” The force must be exerted over twice the distance the weight is lifted, however. If a pulley system is rigged with four strands of rope (C), the same force can lift four times the weight, but the force must be exerted over four times the distance.

INCLINED PLANES DO WORK

An inclined plane (A) reduces the effort needed to raise a mass: instead of lifting it directly, a smaller force is exerted over the length of the incline. The ratio of the length to the height is the mechanical advantage. Two inclined planes set back-to-back form a wedge (B), which provides a double mechanical advantage. A moderate downward force results in increased sideways forces. A screw (C) is essentially an inclined plane wrapped around a cylinder and has a high mechanical advantage.