Work, Power, Energy Study Guide

Work, Power, Energy Study Guide Topics:  What is work? How can you calculate it?  How do you know if the work done on an object is positive or negative?  Understand the relationship between power, work, and time.  What causes an object’s total amount of energy to change?  What is mechanical energy?  Know how to solve problems using the formulas for potential and kinetic energy.  Know how to solve problems (for an unknown) using the Work-Energy Theorem.  Understand the concept of energy conservation.  Know how to solve problems (for an unknown) using conservation of energy. This includes but is not limited to o Roller coaster problems, o Pendulum problems,  Understand what elastic potential energy and Hooke’s Law are.

Practice Problems: 1. A 2000kg car accelerates along the on-ramp to the NJ turnpike and speeds up from 15m/s to 30m/s, while traveling a distance of 200m. Find the force applied by the driver as he presses on the accelerator. (Ans: 3375N)

2. A 45kg skater moving with a velocity of 9m/s coasts to a stop after traveling a distance of 50m. What is the frictional force acting on the skater? (Ans: 36.45N)

3. A 1.5kg rock is dropped from the top of a 220m high building. Find the speed of the rock as it reaches the ground. (Ans: 65.7m/s)

4. A pendulum is pulled to one side so that its height is 0.25m. Find the speed of the pendulum at the bottom of its swing. (Ans: 2.2m/s)

5. For the 1000kg rollercoaster below, find the unknown quantities at the various points. (Ans: A = 45.9m, B = 0, C = 250000J, D = 20.4m, E = 22.4m/s, F = 450,000J, G = 0, H = 30m/s)

6. A child pulls a red ‘radio flyer’ wagon for 125m along the sidewalk with a horizontal force of 40N directed along the handle. If the child takes two minutes to move this distance, find the work done and the power of the child. (Ans: 5000J, 41.7W) 7. A crane lifts a 1300 kg car a distance of 5m in 40 s. a. How much work does the crane do? (Ans: 63700 J) b. How much power does the crane exert? (Ans: 1592.5 W)

8. Fill in all of the unknowns for the roller coaster below. The mass of the coaster car is 1200 kg. (Ans: point B v =24.2 m/s, point C h = 19.9m)

9. Frank throws his 0.4 kg camera downward at a speed of 3 m/s from the top of a 20 m high cliff. What is the speed of the camera as it strikes the ground? (Ans: 20 m/s) b. The same camera is dropped but this time it encounters 2 N of air resistance. What is its speed as it strikes the ground now? (Ans: 14.2 m/s)

10. If a spring has a spring constant of 250 N/m, how much force is required to compress the spring 0.4 m from its relaxed position? (Ans: 100N)

11. Shown below is an 8.0 kg stone resting on a spring. The spring is compressed by 10 cm by the stone. (a) What is the spring constant? (Ans: 784 N/m) (b) Suppose that you now push the stone down an additional 30 cm. What is the elastic potential energy of the spring? (Ans: 62.7 J)