where dos the 0.5 come from? Example on Conservation of Mechanical EnergyThe man at the window wishes to throw the 30 kg sack on theground. To do this he allows it to swing from rest at B to pointC, when he releases the cord at 0 = 30°. Determine the speedat which it strikes the ground.Conservation ofMechanical EnergyВ8 mKg + UB = Kp + Up%3D8 m0 + (16)(30)(9.8) = (0.5)(30)v,² + 0Vo? = (16)(30)(9.8) / (0.5)(30)Vo = 17.7 m/s16 mNote that the tension in thecord does no work on the\Dsack because it is alwaysperpendicular to thedisplacement!0 2007 by R.C. Htbeier. To be published by Pearson Prentice Hal, Pearson Education, Inc, Upper Sadde River New Jersey All rights reserved

Using law of conservation of mechanical energy is KEB+UB=KED+UDmgh=12mvD2

The man at the window wishes to throw the 30 kg sack on the ground. To do this he allows it to swing from rest at B to point C, when he releases the cord at 0 = 30°. Determine the speed at which it strikes the ground. Conservation of A Mechanical Energy B 8m Kg + Ug = Kp + U, 8 m 0 + (16)(30)(9.8) = (0.5)(30)v,² + 0 Vo? = (16)(30)(9.8) / (0.5)(30) 16 m Vo = 17.7 m/s Note that the tension in the cord does no work on the sack because it is always perpendicular to the displacement! R 0 2007 by RC Htbeier To be published by Peason Prentice Hal Peason Education, Inc, Upper Sadde River. New Jeey All ights reseved

The man at the window wishes to throw the 30 kg sack on the ground. To do this he allows it to swing from rest at B to point C, when he releases the cord at 0 = 30°. Determine the speed at which it strikes the ground. Conservation of A Mechanical Energy B 8m Kg + Ug = Kp + U, 8 m 0 + (16)(30)(9.8) = (0.5)(30)v,² + 0 Vo? = (16)(30)(9.8) / (0.5)(30) 16 m Vo = 17.7 m/s Note that the tension in the cord does no work on the sack because it is always perpendicular to the displacement! R 0 2007 by RC Htbeier To be published by Peason Prentice Hal Peason Education, Inc, Upper Sadde River. New Jeey All ights reseved

Principles of Physics: A Calculus-Based Text

5th Edition

ISBN:9781133104261

Author:Raymond A. Serway, John W. Jewett

Publisher:Raymond A. Serway, John W. Jewett

Chapter6: Energy Of A System

Section: Chapter Questions

Problem 5P: Spiderman, whose mass is 80.0 kg, is dangling on the free end of a 12.0-m-long rope, the other end...

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A student has the idea that the total work done on an object is equal to its final kinetic energy. Is this idea true always, sometimes, or never? Ii it is sometimes true, under what circumstances? If it is always or never true, explain why.
An a simple pendulum swings back and forth, the forces acting on the suspended object are (a) the gravitational force, (b) the tension in the supporting cord, and (c) air resistance, (i) Which of these forces, if any, does no work on the pendulum at any time? (ii) Which of these forces does negative work on the pendulum at all Limes during its motion?
Integrated Concepts (a) Calculate the force the woman in Figure 7.46 exerts to do a push-up at constant speed, taking all data to be known to three digits. (b) How much work does she do if her center of mass rises 0.240 m? (c) What is her useful power output if she does 25 push-ups in 1 min? (Should work done lowering her body be included? See the discussion of useful work in Work, Energy, and Power in Humans. Figure 7.46 Forces involved in doing push-ups. The woman's weight acts as a force exerted downward on her center of gravity (CG).
One person drops a ball from the top of a building while another person at the bottom observes its motion. Will these two people agree (a) on the value of the gravitational potential energy of the ball-Earth system? (b) On the change in potential energy? (c) On the kinetic energy of the ball at some point in its motion?
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Review. You can think of the workkinetic energy theorem as a second theory of motion, parallel to Newtons laws in describing how outside influences affect the motion of an object. In this problem, solve parts (a), (b), and (c) separately from parts (d) and (e) so you can compare the predictions of the two theories. A 15.0-g bullet is accelerated from rest to a speed of 780 m/s in a rifle barrel of length 72.0 cm. (a) Find the kinetic energy of the bullet as it leaves the barrel. (b) Use the workkinetic energy theorem to find the net work that is done on the bullet. (c) Use your result to part (b) to find the magnitude of the average net force that acted on the bullet while it was in the barrel. (d) Now model the bullet as a particle under constant acceleration. Find the constant acceleration of a bullet that starts from rest and gains a speed of 780 m/s over a distance of 72.0 cm. (e) Modeling the bullet as a particle under a net force, find the net force that acted on it during its acceleration. (f) What conclusion can you draw from comparing your results of parts (c) and (e)?
Consider the following scenario. A car for which friction is not negligible accelerates from rest down a hill, running out of gasoline after a short distance. The driver lets the car coast farther down the hill, then up and over a small crest. He then coasts down that hill into a gas station, where he brakes to a stop and fills the tank with gasoline. Identify the forms of energy the car has, and how they are changed and transferred in this series of events. (See Figure 7.34.) Figure 7.34 A car experiencing non-negligible friction coasts down a hill, over a small crest then dill again, and comes to a stop at a gas station.
The Flybar high-tech pogo stick is advertised as being capable of launching jumpers up to 6 ft. The ad says that the minimum weight of a jumper is 120 lb and the maximum weight is 250 lb. It also says that the pogo stick uses a patented system of elastometric rubber springs that provides up to 1200 lbs of thrust, something common helical spring sticks simply cannot achieve (rubber has 10 times the energy storing capability of steel). a. Use Figure P8.32 to estimate the maximum compression of the pogo sticks spring. Include the uncertainty in your estimate. b. What is the effective spring constant of the elastometric rubber springs? Comment on the claim that rubber has 10 times the energy-storing capability of steel. c. Check the ads claim that the maximum height a jumper can achieve is 6 ft.
What does work on a shuffleboard puck as it slides to rest? Why is the board dusted, and how does this affect work?
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(a) Can the kinetic energy of a system be negative? (b) Can the gravitational potential energy of a system be negative? Explain.