LUMS SCHOOL OF SCIENCE AND ENGINEERING

PHY-101 Problem Set # 3 October 1, 2014 Due: Thursday October 23, 2014, 5pm.

Topics: Newtons laws, Combination of forces, Friction

Q1. A bag of cement of weight hangs from three wires as shown in Figure. Two of the

wires make angles and with the horizontal. If the system is in equilibrium, show that the tension in the left-hand wire is

Q2. Three blocks are in contact with each other on a frictionless, horizontal surface, as in Figure. A horizontal force is applied to . Take kg, kg, kg and N. Draw a separate free-body diagram for each block and find (a) the acceleration of the blocks, (b) the resultant force on each block, and (c) the magnitudes of the contact forces between the blocks. (d) You are working on a construction project. A coworker is nailing up plasterboard on one side of a light partition, and you are on the opposite side, providing backing by leaning against the wall with your back pushing on it. Every blow makes your back sting. The supervisor helps you to put a heavy block of wood between the wall and your back. Using the situation analyzed in parts (a), (b), and (c) as a model, explain how this works to make your job more comfortable.

Q3. Two blocks of mass kg and kg are connected by a massless string that passes over a frictionless pulley as shown in figure. The inclines are frictionless. Find (a) the magnitude of the acceleration of each block and (b) the tension in the string.

Q4. Three forces acting on an object are given by N,

N, and N. The object experiences an acceleration of magnitude

. (a) What is the direction of the acceleration? (b) What is the mass of the object? (c) If the object is initially at rest, what is its speed after ? (d) What are the velocity components of the object after ? Q5. A kg object is moving in a plane, with its and coordinates given by and , where and are in meters and is in seconds. Find the magnitude of the net force acting on this object at s. Q6. The distance between two telephone poles is m. When a kg bird lands on the telephone wire midway between the poles, the wire sags m. Draw a freebody diagram of the bird. How much tension does the bird produce in the wire? Ignore the weight of the wire. Q7. A 1.00 kg object is observed to have an acceleration of in a direction east of north as shown in figure. The force exerted on the object has a magnitude of

N and is directed north. Determine the magnitude and direction of the force acting on the object.