Work and Machines

What is work? Work is done only when force is applied to an object and the object moves in the same direction as the applied force. Work is calculated by multiplying the force by the distance over which the force is applied. Work = force x distance W = Fd

What is work? Work is zero when an object is not moving. For example, if you were to try and lift a car and it does not move, no work is being done.

What is work? Because work is calculated as force times distance, it is expressed in newtons times meters (N x m). This combination of of SI units is also called joules. One joule is equal to one kilogram times meter squared per second squared. 1 N x m = 1 J = 1 kg x m2/s2

You do about 1 J of work when you lift an apple, which weighs about 1 N, from your arm’s length down at your side to the top of your head, a distance of about 1 meter.

Practice A crane uses an average force of 5,200 N to lift a steel beam 25 m. How much work does the crane do on the beam?

Power The amount of time that a given amount of work takes is an important factor when you consider work and machines. The quantity that measures work in relation to time is power. Power is the rate at which work is done, or how much work is done in a given amount of time.

Power Power equation - power = work time The SI unit to express power is the watt (W). One watt is the amount of power needed to do one joule of work.

DO NOT confuse the symbol for work, W , which is italic, with the symbol for watt, W. You can tell which one is meant from the context and by the use of italics.

Practice While rowing across the lake during a race, John does 3,960 J of work on the oars in 60.0 s. What is his power output in watts?

Machines and Mechanical Advantage Lifting a car by hand and lifting the car with a jack requires the same amount of work. The jack makes the work easier by allowing you to apply less force at any given moment. Machines help do work by changing the size of an input force, the direction of the force, or both.

Mechanical Advantage Mechanical advantage is the ratio between output force and input force. It is also equal to the ratio between the input distance and output distance. Mechanical Advantage = output force = input dist. input force output dist.

Ex. Calculate the mechanical advantage of a ramp that is 5 Ex. Calculate the mechanical advantage of a ramp that is 5.0m long and 1.5m high. Input distance = 5.0 m Output distance = 1.5m Mechanical advantage = 3.3

Simple machines There are six basic types of simple machines: simple lever, the pulley, the wheel and axle, the simple inclined plane, the wedge, and the screw. Simple machines are divided into two families: the lever family and the inclined plane family.

Simple machines The Lever family – All levers have a rigid arm that turns around a point called the fulcrum. 1st class levers – Examples include a hammer, and a pair of pliers.

2nd class levers - Examples include wheelbarrows, nutcrackers, and hinged doors.

3rd class levers - A person’s forearm is an example of a third class lever.

Pulleys Pulleys are modified levers. Examples include flagpoles and sails on a sailboat. The point in the middle of the pulley is like the fulcrum of a lever. Using moving pulleys or more than one pulley at a time can increase the mechanical advantage.

Wheel and Axle A wheel and axle is made of a lever or a pulley (the wheel) connected to a shaft (the axle). When a small input force is applied to the steering wheel, the force is multiplied to become a large output force applied to the steering column.

Inclined Plane Family Pushing an object up an inclined plane requires less input force. The same amount of work must be done whether you lift something straight up or push it up a ramp. When you push an object up a ramp, you apply force in the direction parallel to the ramp over the length of the ramp.

Inclined Plane Pushing an object up a long, gradual ramp takes less force than pushing the object up a short, steep ramp. The mechanical advantage of an inclined plane is equal to the length of the inclined plane divided by the height to which the load is lifted.

Wedge A wedge is a modified incline plane. A wedge is formed of 2 inclined planes placed back to back. A wedge turns a single downward force into two forces directed out to the sides.

Screws A screw is an inclined plane wrapped around a cylinder. Tightening a screw with gently sloping threads requires a small force to act over a long distance.

Compound Machines A compound machine is a machine that combines two or more simple machines. A pair of scissors for example, uses two first-class levers joined at a common fulcrum; each lever arm has a wedge that cuts into the paper.

Rube Goldberg Machines Rube Goldberg Machines are complicated devices that include more than one simple machine.