### Concept Items

## 9.1 Work, Power, and the Work–Energy Theorem

A book with a mass of 0.30 kg falls 2 m from a shelf to the floor. This event is described by the work–energy theorem: $$W=\text{}fd\text{=}\frac{1}{2}m{v}_{2}^{2}-\frac{1}{2}m{v}_{1}^{2}$$ Explain why this is enough information to calculate the speed with which the book hits the floor.

- The mass of the book,
*m*, and distance,*d*, are stated.*F*is the weight of the book*mg . v*_{1}is the initial velocity and*v*_{2}is the final velocity. The final velocity is the only unknown quantity. - The mass of the book,
*m*, and distance,*d*, are stated.*F*is the weight of the book*mg . v*_{1}is the final velocity and*v*_{2}is the initial velocity. The final velocity is the only unknown quantity. - The mass of the book,
*m*, and distance,*d*, are stated.*F*is the weight of the book*mg . v*_{1}is the initial velocity and*v*_{2}is the final velocity. The final velocity and the initial velocities are the only unknown quantities. - The mass of the book,
*m*, and distance,*d*, are stated.*F*is the weight of the book*mg . v*_{1}is the final velocity and*v*_{2}is the initial velocity. The final velocity and the initial velocities are the only unknown quantities.

## 9.2 Mechanical Energy and Conservation of Energy

The starting line of a cross country foot race is at the bottom of a hill. Which form(s) of mechanical energy of the runners will change when the starting gun is fired?

- Kinetic energy only
- Potential energy only
- Both kinetic and potential energy
- Neither kinetic nor potential energy

## 9.3 Simple Machines

Which type of simple machine is a knife?

- A ramp
- A wedge
- A pulley
- A screw