Week 3 Lecture Physics PDF

Instructor: Hafsa Mir  Work W = Fs (constant force in direction of straight-line displacement) 1 joule = 1 newton * 1 meter or 1 J = 1 N.m  Work: positive, negative or zero Positive: When the force has a component in the s...

Instructor: Hafsa Mir  Work W = Fs (constant force in direction of straight-line displacement) 1 joule = 1 newton * 1 meter or 1 J = 1 N.m  Work: positive, negative or zero Positive: When the force has a component in the same direction as the displacement. Negative: When the force has a component opposite to the displacement. Zero: When the force is perpendicular to the displacement. Example 1 Example Power When a quantity of work is done during a time interval the average work done per unit time or average power is defined to be Example Practice problem A tow truck pulls a car 5.00 km along a horizontal roadway using a cable having a tension of 850 N. (a) How much work does the cable do on the car if it pulls horizontally? If it pulls at 35.0° above the horizontal? (b) How much work does the cable do on the tow truck in both cases of part (a)? (c) How much work does gravity do on the car in part (a)? Solution Problem 2 In Fig. E6.7 assume that there is no friction force on the 20.0-N block that sits on the tabletop. The pulley is light and frictionless. (a) Calculate the tension T in the light string that connects the blocks. (b) For a displacement in which the 12.0-N block descends 1.20 m, calculate the total work done on (i) the 20.0-N block and (ii) the 12.0-N block. (c) For the displacement in part (b), calculate the total work done on the system of the two blocks. How does your answer compare to the work done on the 12.0-N block by gravity? Solution Problem 3 How many joules of energy does a 100-watt light bulb use per hour? How fast would a 70-kg person have to run to have that amount of kinetic energy? Solution