Work, Power, and Simple Machines

Objectives To be able to define Work and Power To be able to calculate work and power To be able to explain how simple machines make work easier

Work Work is done when a force causes a change in the motion of an object. The force is a push or a pull on an object. Work = Force x Distance (W=Fd)

Is this weight lifter doing any work?

Joules Work is measured in Joules Joules is Newtons x meters (F x d) 1J = 1kg x m^2 s^2

Practice Problem (all together) Q) A father lifts his daughter in the air about 2 meters with a force of about 190 N. How much work does he do? W = F x d W = (190 N) x (2 m) W = 380 Joules

Power Power is the amount of TIME to get work done. Power = work/time or P = W/t Power is measured in Watts (W) A Watt is the amount of power required to do 1J of work in 1s.

Practice Problem (all together) Q) It takes 100kJ of work to lift an elevator 18m. If this is done in 20s, what is the average power of the elevator? P = Work/time P = (100kJ)/ (20s) P = 5 kW (kilowatts)

Simple Machines

There are 6 types of simple machines These 6 are divided into 2 families: The Lever Family and the Inclined Plane Family.

The Lever Family Includes: Simple levers, Pulleys, Wheel and Axle All levers have an ARM, that turns around a point called the FULCRUM. Levers are divided into 3 classes depending on the location of the arm and fulcrum.

1st Class Levers Most common type. The fulcrum is in the middle of an arm. Think of a See-Saw, the fulcrum is in the middle and force pushing down on one side pushes up the other side with ease. A hammer is also a good example when trying to remove a nail.

2nd class levers The fulcrum is at one end of the arm. The force is applied to the other end of the arm. A good example is a wheelbarrow.

3rd Class Levers The fulcrum is still at one end, but the Force applied is to the middle of the arm, these levers multiply distance rather than force. The human body has many 3rd class levers, the forearm is a good example as the biceps muscle contracts a short distance to move the hand a long distance.

Pulleys The point in the middle of the pulley is like a fulcrum of a lever. The rope acts like the arm is a 1st class lever.

Wheel and Axle This is a lever, or pulley connected to a shaft. Ex) The steering wheel of a car When the wheel is turned, the axle (shaft) also turns which applies a large force to the steering column which turns the front wheels of the car.

The Inclined Plane Family This family helps do work by multiplying and redirecting force. An inclined plane turns a small input force into a large output force by spreading out the work over a large distance. It’s easier to push something up a ramp than lift it straight up!

A Wedge A knife blade, ax blade, and nails are examples of wedges. These function like 2 inclined planes back to back. It turns a single downward force into 2 forces directed out to the sides.

A Screw This is an inclined plane wrapped around a cylinder. Like a spiral inclined plane.