# Work and Energy Problems

Questions:

Work and Energy Review Problems (Answers are at the bottom of this document)

1. From the given list, select a possible pair for the value of coefficient of static and kinetic frictions μs and μk (There may be more than one choice)

a. (2.61, 2.00) b. (2.00, 2.61) c. (0.261, 0.150) d. (0.150, 0.261)

2. A runner has a kinetic energy of 750 J. What will be his/her kinetic energy if

(a) his/her mass is doubled.

(b) his/her speed is doubled.

3. Three objects were thrown from top of a building. First one was thrown with 20 m/s horizontally. The second one was thrown vertically upward with an initial speed of 20 m/s. The third object was thrown at an angle 30o to the horizontal. When the objects reach the ground, which object will have the highest speed?

4. A 2.5 x 103 kg car starts from rest at the top of a 20 m rough slope inclined at 30o with the horizontal. If the average frictional force experienced by the car is 3.2 x 103 N, find the speed of the car at the bottom of the slope.

5. The electric motor of a toy car accelerates the car from rest to 0.5 m/s in 0.030 s. If the mass of the toy car is 3.0 kg, find the average power delivered to the car during its acceleration.

#### Solution Preview

Answers.

1. Coefficient of friction can only assume values between 0 and one. i.e.

0 ≤ μs ≤ 1 0 ≤ μk ≤ 1. Further, coefficient of static friction is larger than coefficient of kinetic friction. i.e. μs > μk

So the only possible choice is choice C.

2. Formula for kinetic energy is: K = ½ m v2

Note that K is proportional to m. Hence if we double the mass, kinetic energy also should double. So the new kinetic energy must be 2 x 750 = 1500 J

Note that K is proportional to v2 ...

#### Solution Summary

This document contains five problem on the topic of 'Work and Energy'. First three problems test the conceptual understanding of this topic while the other two tests the problem solving skills. I have provided detailed step-by-step solution to all the problems. I also have included colorful clear labeled diagrams to further clarify the problems at hand. I created these problem set for my college level students for review and I believe these will benefit you as well.