Ch 9

What is Work ?

What is the equation for work?Work is done only when force cause a change in the motion of an object.Is this a good example of Work? Trying to lift a car and the car moves.Work is measured in what?

Work

Joules are a derived unit of what two units multiplied together? Because these units are equal, you can choose whichever unit is easiest for solving a particular problem.Solve this problemA crane uses an average force of 5200 N to lift a girder 25 m. How much work does the crane do on the girder?

Practice Questions

1. A father is playing with his daughter by lifting her repeatedly in the air. How much work does he do with each lift, assuming he lifts her 2.0 m and exerts an average force of 190 N?

2. A apple weighing 1 N fall through a distance of 1m. How much work is done on the apple by the force of gravity?

Power

What is Power?The equation for Power?Power is measured in what units?A ___is the amount of power required to do 1J of work in 1 s.

Practice Problems

1. It takes 100 kJ of work to lift an elevator 18m. If this is done in 20s, what is the average power of the elevator during the process?2. Suppose you are moving a 300 N box of books. Calculate your power output in the following situations:a. you exert a force of 60.0 N to push the box 12.0 m in 20.0 s.b. you lift the box 1 m onto a truck in 3 s.

Machines and Mechanical Advantage

Which is easier lifting a car yourself or using a jack ?Machines can change the direction of an input force, or they can increase an output force by changing the distance over which the force is applied.What is the equation for Mechanical Advantage?

Simple Machines

There are Six Simple Machines:1. Lever2. Pulley3. Wheel and axle4. Screw5. Wedge6. inclined plane