Section 6-2 The Work Energy Theorem and Kinetic Energy

Warm-Up #1 Two forces, F 1 and F 2 are acting on the box, causing the box to move across the floor. The two force vectors are drawn to scale. Which one of the following statements is correct? A) F 2 does more work than F 1 does B) F 1 does more work than F 2 does C) Both forces do the same amount of work D) Neither force does any work

Warm-Up #2 A box is being moved with a velocity v by a force P (in the same direction as v) along a level floor. The normal force is F N, the kinetic frictional force is f k, and the weight is mg. Which one of the following statements is correct? A) F N does positive work, P and f k do zero work, and mg does negative work B) F N does positive work, P and f K do zero work, and mg does negative work C) f K does positive work, F N and mg do zero work, and P does negative work D) P does positive work, F N and mg do zero work, and f K does negative work

6.2 The Work-Energy Theorem In physics, when a net force performs work on an object, the result is a change in the kinetic energy of the object.

Definition of Kinetic Energy

6.2 The Work-Energy Theorem Using Newton’s 2 nd Law (F=ma), the definition of work (W=Fcos Ɵs), and kinematics (v 2 =v as), we can relate the work done by a net external force to the change in KE of the object This is called the Work Energy Theorem

The Work Energy Theorem

Example 1 p. 165 Deep Space 1 was a probe launched in Its engine produced a constant, low-level thrust force of N. The probe has a mass of 474 kg. If it traveled at an initial speed of 275 m/s and no forces acted on it except the engine. This force is directed parallel to the displacement, d (of course!). The probe moved a distance of 2.42 x 10 9 meters. Determine the final speed of the probe, assuming that the mass remains constant.

Example 2 p. 166 A 58 kg skier is coasting down a 25° slope. Near the top of the slope, her speed is 3.6 m/s. She accelerates downward because of the gravitational force even though a kinetic frictional force of magnitude 71 N opposes her motion. Ignore air resistance. Determine the speed at a point that is displaced 57 meters downhill.

Assignment p. 187 Focus on Concepts #8,9 p. 188 #13-27 odds