PHYSICS-1 WORK AND ENERGY EXPERIMENT-7 Work and Energy OBJECTIVE The purpose of this experiment is to apply the concept of the work and energy to calculate the work done by the force of friction between a car and a wooden board as they slide relative to each other at constant velocity. We study it for the case of the car moving up and down the incline. MATERIALS Wooden Board Weight hanger Protractor Car Balance Slotted weights Pulley INTRODUCTION In general, friction is the force that slows down the motion of an object. The force of friction is directed along the surface of contact between the object and surface and directed opposite to the direction of motion of object. Car moving up: For the car of mass M moving up the incline at constant speed, we can show, from 2nd law of motion, that the force of friction f is given by: f = ρ g – M g sinθ (1) where ρ is the mass hanging from the other side of the pulley which makes the car move up. Car moving down: Now, for the car of mass M moving down the incline at constant speed, we can show, from 2 nd law of motion, that the force of friction f is given by: f = M g sinθ – η g (2) where η is the mass hanging from the other side of the pulley which makes the car move down. And finally the work done by friction becomes: Wf = f d (3) Where d is the distance the car travels along the board. 1 PHYSICS-1 WORK AND ENERGY EXPERIMENTAL PROCEDURE 1) Determine the mass M of the car and record it. 2) Set up the board and the car as shown in figure 1 with an incline angle of 300 with the table. Position the car near the bottom of the incline and record its initial (vertical) height h1. Add enough weights of mass ρ on the weight hanger to make the car move up the incline at a very slow speed after given a slight tap. Record the initial height H1 of this weight. Measure also the distance d that the car travels along the board. 3) When the car reaches near the top of the incline, record now its height h2 and that of the weight hanger H2. Start removing weights from the weight hanger until the car starts moving down the incline at a slow speed after given a small tap. Record this mass η. 4) Repeat the above procedures for an incline angle of 600. CALCULATIONS 1) Compute the frictional force f and the work W f done by friction for the incline angle of 300, while car is going up, and down. Record in the table on page 3. 2) Compute the frictional force f and the work Wf done by friction for the incline angle of 600, while car is going up and down. Record in the table on page 3. 3) For each angle, compute the percent difference in the two values of the work W f. Record in the table on page 3. 2 PHYSICS-1 WORK AND ENERGY EXPERIMENT 7 – Work and Energy REPORT FORM Part 1 Name: __________________________________ Car moving up the incline Mass M of car ____________ Angle of incline Mass ρ (g) Distance travelled by car d (cm) Initial height of car h1 (cm) Final height of car h2 (cm) Initial height of suspended mass H1 (cm) Final height of suspended mass H2 (cm) f (from eqn 1) Wf (from eqn 3) (N) (J) f (from eqn 2) Wf (from eqn 3) (N) (J) 300 600 Part 2 Angle of incline Car moving down the incline Mass η (g) Distance travelled by car d (cm) Initial height of car h1 (cm) Final height of car h2 (cm) 300 600 Percent difference in Wf for θ = 300 _________ Percent difference in Wf for θ = 600 _________ 3 Initial height of suspended mass H1 (cm) Final height of suspended mass H2 (cm) PHYSICS-1 WORK AND ENERGY EXPERIMENT -7: Work and Energy Post- laboratory Questions Name: ______________________________________ 1) Compute the percent energy lost to friction for the car moving up the incline for both angles. 2) For θ = 600, compute the work done by each force acting on the car. Should they add up to zero? Explain. 3) For θ = 300, compute the normal force acting on each wheel of the car? 4) Is friction in this experiment only due to the car? Explain. 5) Show that, for the car moving up the incline, that the coefficient µ of friction is given by: 4