### Problems

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

A boy pushes his little sister on a sled. The sled accelerates from 0 to 3.2 m/s . If the combined mass of his sister and the sled is 40.0 kg and 18 W of power were generated, how long did the boy push the sled?

- 205 s
- 128 s
- 23 s
- 11 s

#### 9.2 Mechanical Energy and Conservation of Energy

The potential energy stored in a compressed spring is $$U=\frac{1}{2}k{x}^{2}$$, where *k* is the force constant and *x* is the distance the spring is compressed from the equilibrium position. Four experimental setups described below can be used to determine the force constant of a spring. Which one(s) require measurement of the fewest number of variables to determine *k*? Assume the acceleration due to gravity is known.

- An object is propelled vertically by a compressed spring.
- An object is propelled horizontally on a frictionless surface by a compressed spring.
- An object is statically suspended from a spring.
- An object suspended from a spring is set into oscillatory motion.

- I only
- III only
- I and II only
- III and IV only

#### 9.3 Simple Machines

A man is using a wedge to split a block of wood by hitting the wedge with a hammer. This drives the wedge into the wood creating a crack in the wood. When he hits the wedge with a force of 400 N it travels 4 cm into the wood. This caused the wedge to exert a force of 1,400 N sideways increasing the width of the crack by 1 cm . What is the efficiency of the wedge?

- 0.875 percent
- 0.14
- 0.751
- 87.5 percent

An electrician grips the handles of a wire cutter, like the one shown, 10 cm from the pivot and places a wire between the jaws 2 cm from the pivot. If the cutter blades are 2 cm wide and 0.3 cm thick, what is the overall IMA of this complex machine?

- 1.34
- 1.53
- 33.3
- 33.5