Physics 111 Practice Problem Statements 06 Work and Kinetic Energy SJ 8th Ed.: Ch. 7.1 – 7.5 Contents: 7-7E*, 7-15E, 7-19P, 7-22P, 7-23P, 7-25E*, 7-31E*, 7-37P, 7-39P Energy Overview Systems Work Constant force 1D Constant force in 3D Dot Product (Scalar Product) Units Variable force 1D General Vector 3D definition with variable force The Work-Kinetic Energy Theorem Gravitational Force and Examples Variable (Spring) Force and Examples. Power - Read 8/8/2011

Physics 111 Problem Set 6 Chapter 7: Kinetic Energy and Work Contents: 7-7E*, 7-15E, 7-19P, 7-22P, 7-23P, 7-25E*, 7-31E*, 7-37P, 7-39P 8/8/2011

Lecture 6: Chapter 7 Kinetic Energy and Work Energy Overview Work A simple constant force 1D 3D, constant force - Dot Product Units Variable force 1D General Vector 3D definition with variable force Work and Kinetic Energy Kinetic Energy Simple derivation of Work-KE theorem General Form of Work-KE Theorem Gravitational Force and Examples Variable (spring) force and examples. Power 8/8/2011

Problem 7 –7*E: To pull a 50 kg crate across a horizontal frictionless floor, a worker applies a force of 210 N, directed 20° above the horizontal. As the crate moves 3.0 m, what work is done on the crate by (a) the worker's force, (b) the gravitational force on the crate, and (c) the normal force on the crate from the floor? (d) What is the total work done on the crate? 8/8/2011

Problem 7 – 15E: In the figure, a cord runs around two massless, frictionless pulleys; a canister with mass m = 20 kg hangs from one pulley; and you exert a force Fon the free end of the cord. (a) What must be the magnitude of Fif you are to lift the canister at a constant speed? (b) To lift the canister by 2.0 cm, how far must you pull the free end of the cord? During that lift, what is the work done on the canister by (c) your force (via the cord) and (d) the gravitational force on the canister? (Hint: When a cord loops around a pulley as shown, it pulls on the pulley with a net force that is twice the tension in the cord.)                                      8/8/2011

Problem 7 – 19P: A cord is used to vertically lower an initially stationary block of mass M at a constant downward acceleration of g/4. When the block has fallen a distance d, find (a) the workk done by the cord's force on the block, (b) the work done by the gravitational force on the block, (c) the kinetic energy of the block, and (d) the speed of the block. 8/8/2011

Problem 7 – 22P: A 250 g block is dropped onto a relaxed vertical spring that has a spring constant of k = 2.5 N/cm (see figure). The block becomes attached to the spring and compresses the spring 12 cm before momentarily stopping. While the spring is being compressed, what work is done on the block by (a) the gravitational force on it and (b) the spring force? (c) What is the speed of the block just before it hits the spring? (Assume that friction is negligible.) (d) If the speed at impact is doubled, what is the maximum compression of the spring?                              8/8/2011

Problem 7 – 23P: The only force acting on a 2 Problem 7 – 23P: The only force acting on a 2.0 kg body as it moves along the positive x axis has an x component Fx = -6x N, where x is in meters. The velocity of the body at x = 3.0 m is 8.0 m/s. (a) What is the velocity of the body at x = 4.0 m? (b) At what positive value of x will the body have a velocity of 5.0 m/s? 8/8/2011

Problem 7 – 25E. : A 10 kg brick moves along an x axis Problem 7 – 25E*: A 10 kg brick moves along an x axis. Its acceleration as a function of its position is shown in the figure. How much net work is performed on the brick by the force causing the acceleration as the brick moves from x = 0 to x = 8.0 m? 8/8/2011

Problem 7 – 31E. : A 100 kg block is pulled at a constant speed of 5 Problem 7 – 31E*: A 100 kg block is pulled at a constant speed of 5.0 m/s across a horizontal floor by an applied force of 122 N directed 37° above the horizontal. What is the rate at which the force does work on the block? 8/8/2011

Problem 7 – 37P: The force (but not the power) required to tow a boat at constant velocity is proportional to the speed. If a speed of 4.0 km/h requires 7.5 kW, how much power does a speed of 12 km/h require? 8/8/2011

Problem 7 – 39P: A horse pulls a cart with a force of 40 lb at an angle of 30° above the horizontal and moves along at a speed of 6.0 mi/h. (a) How much work does the force do in 10 min? (b) What is the average power (in horsepower) of the force? 8/8/2011