Dr. Jie Zou PHY 1151G Department of Physics1 Chapter 7 Work and Kinetic Energy (Continued)

Dr. Jie Zou PHY 1151G Department of Physics2 Kinetic Energy Kinetic energy K: The energy of motion. K = (1/2)mv 2, where v is the speed. SI units for kinetic energy: joule (J). Kinetic energy is always greater than or equal to zero. Example: Consider your kinetic energy when jogging. Assuming a mass of about 62 kg and a speed of 2.5 m/s, your kinetic energy is K = (1/2)mv 2 = (1/2)(62 kg)(2.5 m/s) 2 = 190 J.

Dr. Jie Zou PHY 1151G Department of Physics3 Work-Energy Theorem Work-energy theorem: The total (net) work done on an object is equal to the change in its kinetic energy. W total =  K = (1/2)mv f 2 - (1/2)mv i 2 Here v f and v i are the final and initial speed of the object, respectively.

Dr. Jie Zou PHY 1151G Department of Physics4 Example 7-5 A 4.1-kg box of books is lifted vertically from rest a distance of 1.6 m by an upward applied force of 60.0 N. Find (a) The work done by the applied force. (b) The work done by the force of gravity. (c) Use work-energy theorem and find the final speed of the box. Can you use kinematics equations to solve (c)?

Dr. Jie Zou PHY 1151G Department of Physics5 Example 2 A boy exerts a force of 11.0 N at 29.0  above the horizontal on a 6.40-kg sled. Find the work done by the boy and the final speed of the sled after it moves 2.00 m, assuming the sled starts with an initial speed of m/s and slides horizontally without friction.

Dr. Jie Zou PHY 1151G Department of Physics6 Power Power: A measure of how quickly work is done. Average power = W / t. W is the work done in time t. SI units: J/s = watt (W). 1 horsepower (hp) = 746 W.

Dr. Jie Zou PHY 1151G Department of Physics7 Example 7-8 To pass a slow- moving truck, you want your 1.30  10 3 kg-car to accelerate from 13.4 m/s (30.0 mph) to 17.9 m/s (40.0 mph) in 3.00 s. What is the minimum power required for this pass?

Dr. Jie Zou PHY 1151G Department of Physics8 Homework Chapter 7, Page 194, Problems: #15, 18, 19, 36.