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Kinetic Energy Lecturer: Professor Stephen T. Thornton Reading Quiz: Two marbles, one twice as heavy as the other, are dropped to the ground from the roof of a building. Just before hitting the ground, the heavier marble has A) the same kinetic energy as the lighter one. B) half as much kinetic energy as the lighter one. C) twice as much kinetic energy as the lighter one. D) four times as much kinetic energy as the lighter one. Answer: C The velocities will be the same in this case, so the only difference in the kinetic energy is due to the mass. Because the mass is twice as much, the kinetic energy is twice as much. K 1 mv2 2 Last Time Discussed the concept of work Today Discuss kinetic energy Work-energy theorem Conceptual Quiz In a baseball game, the catcher stops a 90-mph A) catcher has done positive work pitch. What can you say B) catcher has done negative work about the work done by C) catcher has done zero work the catcher on the ball? Conceptual Quiz In a baseball game, the catcher stops a 90-mph A) catcher has done positive work pitch. What can you say B) catcher has done negative work about the work done by C) catcher has done zero work the catcher on the ball? The force exerted by the catcher is opposite in direction to the displacement of the ball, so the work is negative. Or using the definition of work (W = F d cos q ), because q = 180º, then W < 0. Note that because the work done on the ball is negative, its speed decreases. Follow-up: What about the work done by the ball on the catcher? Previous kinematic equation: v v 2ad 2 f 2 i or rearranging, 2ad v v 2 f 2 i Fnet 2 2 2 d v f vi , m Fnet d 1 1 multiply by m / 2 mv mv 2 2 1 2 1 2 Wnet Wtotal mv f mvi 2 2 2 f 2 i Definition of kinetic energy 1 2 K mv 2 Unit of kinetic energy is the joule, J. Q. Why do we define this? A. It turns out to make our calculations easier! Work-Energy Theorem Wtotal K K f Ki The total work done on an object is equal to the change in its kinetic energy: This is a general result, even for a force not constant in magnitude and direction. Do spring demo. 1 1 mv mv 2 2 2 f 2 i Kinetic Energy and the Work-Energy Principle Energy was traditionally defined as the ability to do work. We now know that not all forces are able to do work; however, we are dealing in these chapters with mechanical energy, which does follow this definition. Copyright © 2009 Pearson Education, Inc. Because work and kinetic energy can be equated, they must have the same units: kinetic energy is measured in joules. Energy can be considered as the ability to do work: To find work, we have to be sure about what force is exerting the effort. Here we might ask about the work done by friction, gravity, air resistance, or the engine. Not a good figure. Forces are not accurate. We need to make sure we can find the work done by every force. (sloppy diagram) Wengine Fd (don't like) Wfriction Ffriction d Ffriction d Wgravity Fgravity d mgd sin q Wair resis Fair resis d Fair resis d Doing Work Against Gravity Energy is reclaimed in this case. Doing Work Against Friction Energy is not reclaimed in this case. Does the Earth do work on the moon? The Moon revolves around the Earth in a nearly circular orbit, with approximately constant tangential speed, kept there by the gravitational force exerted by the Earth. Gravity does no work, because the displacement and force are perpendicular. Cable Tension, Work. A 265-kg load is lifted 23.0 m vertically with an acceleration a = 0.150g by a single cable. Determine (a) the tension in the cable; (b) the net work done on the load; (c) the work done by the cable on the load; (d) the work done by gravity on the load; (e) the final speed of the load assuming it started from rest. Crate Friction. A 46.0-kg crate, starting from rest, is pulled across a floor with a constant horizontal force of 225 N. For the first 11.0 m the floor is frictionless, and for the next 10.0 m the coefficient of friction is 0.20. What is the final speed of the crate after being pulled these 21.0 m? Paintball. In the game of paintball, players use guns powered by pressurized gas to propel 33-g gel capsules filled with paint at the opposing team. Game rules dictate that a paintball cannot leave the barrel of a gun with a speed greater than 85 m/s. Model the shot by assuming the pressurized gas applies a constant force F to a 33-g capsule over the length of the 32-cm barrel. Determine F (a) using the work-energy principle, and (b) using the kinematic equations and Newton’s second law. Conceptual Quiz A child on a skateboard is moving at a speed of 2 m/s. After a force acts on the child, her speed is 3 m/s. What can you say about the work done by the external force on the child? A) positive work was done B) negative work was done C) zero work was done Conceptual Quiz A child on a skateboard is moving at a speed of 2 m/s. After a force acts on the child, her speed is 3 m/s. What can you say about the work done by the external force on the child? A) positive work was done B) negative work was done C) zero work was done The kinetic energy of the child increased because her speed increased. This increase in KE was the result of positive work being done. Or, from the definition of work, because W = KE = KEf – KEi and we know that KEf > KEi in this case, then the work W must be positive. Follow-up: What does it mean for negative work to be done on the child? Conceptual Quiz If a car traveling 60 km/hr can brake to a stop within 20 m, what is its stopping distance if it is traveling 120 km/hr? Assume that the braking force is the same in both cases. A) B) C) D) E) 20 m 30 m 40 m 60 m 80 m Conceptual Quiz If a car traveling 60 km/hr can brake to a stop within 20 m, what is its stopping distance if it is traveling 120 km/hr? Assume that the braking force is the same in both cases. 1 2 F d = Wnet = KE 1= 0 – mv2, and thus, |F| d = 2 mv2. Therefore, if the speed doubles, the stopping distance gets four times larger. A) B) C) D) E) 20 m 30 m 40 m 60 m 80 m Work Done by a Constant Force Solving work problems: 1. Draw a free-body diagram. 2. Choose a coordinate system. 3. Apply Newton’s laws to determine any unknown forces. 4. Find the work done by a specific force. 5. To find the net work, either a) find the net force and then find the work it does, or b) find the work done by each force and add. Copyright © 2009 Pearson Education, Inc. Conceptual Quiz By what factor does A) no change at all the kinetic energy of B) factor of 3 a car change when its speed is tripled? C) factor of 6 D) factor of 9 E) factor of 12 Conceptual Quiz By what factor does the A) no change at all kinetic energy of a car B) factor of 3 change when its speed C) factor of 6 is tripled? D) factor of 9 E) factor of 12 Because the kinetic energy is 1 2 mv2, if the speed increases by a factor of 3, then the KE will increase by a factor of 9. Follow-up: How would you achieve a KE increase of a factor of 2? Conceptual Quiz Two stones, one twice the mass of the other, are dropped from a cliff. Just before hitting the ground, what is the kinetic energy of the heavy stone compared to the light one? A) quarter as much B) half as much C) the same D) twice as much E) four times as much Conceptual Quiz Two stones, one twice the mass of the other, are dropped from a cliff. Just before hitting the ground, what is the kinetic energy of the heavy stone compared to the light one? A) quarter as much B) half as much C) the same D) twice as much E) four times as much Consider the work done by gravity to make the stone fall distance d: KE = Wnet = F d cosq KE = mg d Thus, the stone with the greater mass has the greater KE, which is twice as big for the heavy stone. Follow-up: How do the initial values of gravitational PE compare? Conceptual Quiz A car starts from rest and accelerates to 30 mph. Later, it gets on a highway and A) 0 30 mph accelerates to 60 mph. Which takes more B) 30 60 mph energy, the 0 30 mph, or the 30 60 C) both the same mph? Conceptual Quiz A car starts from rest and accelerates to 30 mph. Later, it gets on a highway and A) 0 30 mph accelerates to 60 mph. Which takes more B) 30 60 mph energy, the 0 30 mph, or the 30 60 C) both the same mph? 1 The change in KE ( 2 mv2 ) involves the velocity squared. So in the first case, we have: In the second case, we have: 1 2 1 2 1 m (302 − 02) = 2 m (900) m (602 − 302) = 21 m (2700) Thus, the bigger energy change occurs in the second case. Follow-up: How much energy is required to stop the 60-mph car? Conceptual Quiz The work W0 accelerates a car from A) 2 W0 0 to 50 km/hr. How much work is B) 3 W0 needed to accelerate the car from C) 6 W0 50 km/hr to 150 km/hr? D) 8 W0 E) 9 W0 Conceptual Quiz The work W0 accelerates a car from A) 2 W0 0 to 50 km/hr. How much work is B) 3 W0 needed to accelerate the car from C) 6 W0 50 km/hr to 150 km/hr? D) 8 W0 E) 9 W0 Let’s call the two speeds v and 3v, for simplicity. We know that the work is given by W = KE = KEf – Kei. 1 2 Case #1: W0 = Case #2: W = 1 2 m (v2 – 02) = 1 2 m ((3v)2 – v2) = m (v2) 1 m 2 (9v2 – v2) = 1 2 m (8v2) = 8 W0 Follow-up: How much work is required to stop the 150-km/hr car? Conceptual Quiz Two blocks of mass m1 and m2 (m1 > m2) A) m1 slide on a frictionless floor and have the B) m2 same kinetic energy when they hit a long C) they will go the rough stretch (m > 0), which slows them same distance down to a stop. Which one goes farther? m1 m2 Conceptual Quiz Two blocks of mass m1 and m2 (m1 > m2) A) m1 slide on a frictionless floor and have the B) m2 same kinetic energy when they hit a long C) they will go the rough stretch (m > 0), which slows them same distance down to a stop. Which one goes farther? With the same KE, both blocks m1 must have the same work done to them by friction. The friction force is less for m2 so stopping m2 distance must be greater. Follow-up: Which block has the greater magnitude of acceleration? Conceptual Quiz A golfer making a putt gives the ball an initial velocity of v0, but he has badly misjudged the putt, and the ball only travels one-quarter of the distance to the hole. If the resistance force due to the grass is constant, what speed should he have given the ball (from its original position) in order to make it into the hole? A) 2 v0 B) 3 v0 C) 4 v0 D) 8 v0 E) 16 v0 Conceptual Quiz A golfer making a putt gives the ball an initial velocity of v0, but he has badly misjudged the putt, and the ball only travels one-quarter of the distance to the hole. If the resistance force due to the grass is constant, what speed should he have given the ball (from its original position) in order to make it into the hole? A) 2 v0 B) 3 v0 C) 4 v0 D) 8 v0 E) 16 v0 In traveling four times the distance, the resistive force will do four times the work. Thus, the ball’s initial KE must be four times greater in order to just reach the hole—this requires an increase in the initial speed by a 1 factor of 2, because KE = 2 mv2.