Work and Machines-Chapter 8

Work is done on an object if two things happen:

(1) the object moves as a force is applied and (2) the direction of the object's motion is the same as the direction of the force.

mechanical advantage

a number that tells how many times a machine multiplies force MA = output force / input force

Differences Between Force and Work

Applying a force doesn't always result in work being done. Suppose that you help push a stalled car. You push and push, but the car doesn't budge. The pushing may have made you tired. But you haven't done any work on the car, because the car hasn't moved.

What are the two things that a machine can change about how work is done?

Force and Distance

work

the transfer of energy to an object by using a force that causes the object to move in the direction of the force W = F x d

joule

the unit used to express energy; equivalent to the amount of work done by a force of 1 N acting through a distance of 1 m in the direction of the force (symbol, J)

watt

the unit used to express power; equivalent to a joule per second (symbol, W)

work output

the work done by a machine; the product of the output force and the distance through which the force is exerted

work input

the work done on a machine; the product of the input force and the distance through which the force is exerted

screw

a simple machine that consists of an inclined plane wrapped around a cylinder

The Force-Distance Trade-Off

When a machine changes the size of the force, the distance through which the force is exerted must also change. Force or distance can increase, but both cannot increase. When one increases, the other must decrease.

How Machines Help

You might think that machines help you because they increase the amount of work done. But that's not true. Work output can never be greater than work input. Machines allow force to be applied over a greater distance, which means that less force will be needed for the same amount of work.

machine

a device that helps do work by either overcoming a force or changing the direction of the applied force (a device that makes work easier by changing the size or direction of a force)

compound machine

a machine made of more than one simple machine (e.g., a can opener) Note: The mechanical efficiency of most compound machines is low because compound machines have more moving parts than simple machines do, thus there is more friction to overcome.

power

the rate at which work is done or energy is transformed (or how fast work is done!) P = W / t

mechanical efficiency

a quantity, usually expressed as a percentage, that measures the ratio of work output to work input in a machine (The work output of a machine can never be greater than the work input. In fact, the work output of a machine is always less than the work input. Why?) Because: Some of the work done by the machine is used to overcome the friction created by the use of the machine. But keep in mind that no work is lost. The work output plus the work done to overcome friction is equal to the work input. ME = (work output / work input) x 100

wheel and axle

a simple machine consisting of two circular objects of different sizes; the wheel is the larger of the two circular objects MA = radius of wheel / radius of axle

lever

a simple machine that consists of a bar that pivots at a fixed point called a fulcrum 1st class: balance; seesaw (load-fulcrum-input force) 2nd class: wheel barrel; bottle opener (fulcrum-load-input force) 3rd class: hammer (fulcrum-input-load) Note: load = output force

pulley

a simple machine that consists of a wheel over which a rope, chain, or wire passes Fixed: input force and output force move in opposite direction Movable: input forces and output force move in same direction Block and Tackle: Fixed and Movable pulleys used together

inclined plane

a simple machine that is a straight, slanted surface, which facilitates the raising of loads; a ramp Question: What effect does an inclined plane have on work? Answer? (Ask your father if you don't know!) MA = length of plane / height

wedge

a simple machine that is made up of two inclined planes and that moves; often used for cutting (e.g., knife) MA = length of wedge / greatest thickness