# Kinetic Energy Lecturer: Professor Stephen T. Thornton

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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.

Discussed the concept of work
Last Time Discussed the concept of work

Discuss kinetic energy Work-energy theorem
Today Discuss kinetic energy Work-energy theorem

Conceptual Quiz In a baseball game, the catcher stops a 90-mph pitch. What can you say about the work done by the catcher on the ball? A) catcher has done positive work B) catcher has done negative work C) catcher has done zero work Click to add notes

Conceptual Quiz In a baseball game, the catcher stops a 90-mph pitch. What can you say about the work done by the catcher on the ball? A) catcher has done positive work B) catcher has done negative work C) catcher has done zero work 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  = 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:

Definition of kinetic energy
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 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. Do spring demo here.

the Work-Energy Principle
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.

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: Figure Caption: A moving hammer strikes a nail and comes to rest. The hammer exerts a force F on the nail; the nail exerts a force -F on the hammer (Newton’s third law). The work done on the nail by the hammer is positive (Wn = Fd >0). The work done on the hammer by the nail is negative (Wh = -Fd).

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
We need to make sure we can find the work done by every force. (sloppy diagram) Figure is from Walker textbook.

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. Answer: Gravity does no work on the Moon, as the force is always perpendicular to the displacement (assuming a circular orbit).

Cable Tension, Work. A 265-kg load is lifted 23
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. Giancoli, 4th ed, Problem 7-62

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 What is the final speed of the crate after being pulled these 21.0 m? Giancoli, 4th ed, Problem 7-66

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. Giancoli, 4th ed, Problem 7-80

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? positive work was done B) negative work was done C) zero work was done Answer: 1

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? 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 = DKE = 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) 20 m B) 30 m C) 40 m D) 60 m E) 80 m Answer: 5

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) 20 m B) 30 m C) 40 m D) 60 m E) 80 m F d = Wnet = DKE = 0 – mv2, and thus, |F| d = mv2. Therefore, if the speed doubles, the stopping distance gets four times larger.

Work Done by a Constant Force
Solving work problems: Draw a free-body diagram. Choose a coordinate system. Apply Newton’s laws to determine any unknown forces. Find the work done by a specific force. To find the net work, either find the net force and then find the work it does, or find the work done by each force and add.

Conceptual Quiz By what factor does the kinetic energy of a car change when its speed is tripled? A) no change at all B) factor of 3 C) factor of 6 D) factor of 9 E) factor of 12 Click to add notes

Conceptual Quiz By what factor does the kinetic energy of a car change when its speed is tripled? A) no change at all B) factor of 3 C) factor of 6 D) factor of 9 E) factor of 12 Because the kinetic energy is 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 Click to add notes

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: DKE = Wnet = F d cosq DKE = 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 accelerates to 60 mph. Which takes more energy, the 0  30 mph, or the 30  60 mph? A) 0  30 mph B) 30  60 mph C) both the same Click to add notes

Conceptual Quiz A) 0  30 mph B) 30  60 mph C) both the same
A car starts from rest and accelerates to 30 mph. Later, it gets on a highway and accelerates to 60 mph. Which takes more energy, the 0  30 mph, or the 30  60 mph? A) 0  30 mph B) 30  60 mph C) both the same The change in KE ( mv2 ) involves the velocity squared. So in the first case, we have: m (302 − 02) = m (900) In the second case, we have: m (602 − 302) = 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 A) 2 W0 B) 3 W0 C) 6 W0 D) 8 W0 E) 9 W0 The work W0 accelerates a car from 0 to 50 km/hr. How much work is needed to accelerate the car from 50 km/hr to 150 km/hr? Click to add notes

Conceptual Quiz A) 2 W0 B) 3 W0 C) 6 W0 D) 8 W0 E) 9 W0 The work W0 accelerates a car from 0 to 50 km/hr. How much work is needed to accelerate the car from 50 km/hr to 150 km/hr? Let’s call the two speeds v and 3v, for simplicity. We know that the work is given by W = DKE = KEf – Kei. Case #1: W0 = m (v2 – 02) = m (v2) Case #2: W = m ((3v)2 – v2) = m (9v2 – v2) = 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) slide on a frictionless floor and have the same kinetic energy when they hit a long rough stretch (m > 0), which slows them down to a stop. Which one goes farther? A) m1 B) m2 C) they will go the same distance m1 m2 Click to add notes

Conceptual Quiz Two blocks of mass m1 and m2 (m1 > m2) slide on a frictionless floor and have the same kinetic energy when they hit a long rough stretch (m > 0), which slows them down to a stop. Which one goes farther? A) m1 B) m2 C) they will go the same distance With the same DKE, both blocks must have the same work done to them by friction. The friction force is less for m2 so stopping distance must be greater. m1 m2 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 Click to add notes

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 factor of 2, because KE = mv2.

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