Work, Power and Energy

Basic Terms - Work Work (in physics) is defined as a force acting over a distance. W = F x d Scalar Quantity: Units: Making the grade lab: Work done on an inclined plane.

Concept Question 1 Is it possible to do work on an object that remains at rest? 1) yes 2) no

Concept Question 1 If there is no displacement, there is no work done. If there is no displacement, there is no work done. Work requires that there be a force acting over a distance. Is it possible to do work on an object that remains at rest? 1) yes 2) no

Concept Question 2 A box is being pulled across a rough floor at a constant speed. What can you say about the work done by friction? 1) Friction does no work at all 2)Friction does negative work 3)Friction does positive work 4)Work is not defined for friction as friction acts on the floor and the box

Concept Question 2 A box is being pulled across a rough floor at a constant speed. What can you say about the work done by friction? 1) Friction does no work at all 2)Friction does negative work 3)Friction does positive work 4)Work is not defined for friction as friction acts on the floor and the box Work is defined for all forces. The friction acts opposite to the direction of displacement and is therefore negative here. f N mg displacement Pull

Concept Question 3 Can friction ever do positive work? 1) yes 2)no

Concept Question 3 Can friction ever do positive work? 1) yes 2)no Consider the book on your car seat. If you accelerate slowly the book does not slide on the seat but stays stationary with respect to the car. Friction is causing the book to move along with the car and hence force and displacement are in the same direction. Work is positive.

Power Power is the rate at which work is done. It can be thought of as work per second. Power = Work / sec. Power = Work / time P = W / t Question: Can you name 2 units for power? Answer: Horsepower and Watts.

Power Power has the same trade-offs as work. A motor produces the same amount of power. So, you can make a robot that’s fast, but weak. Or you can make a robot that’s slow, but strong. The total power in must equal the total power out (with an exception)...

Mechanical Energy Energy defined

Potential Energy Defined PE = mgh

Concept Question Is it possible for the gravitational potential energy of an object to be negative? 1) No. 2)Yes. 3) Maybe.

Concept Question Is it possible for the gravitational potential energy of an object to be negative? 1) No. 2)Yes. 3) Maybe. Gravitational PE is mghheight h is measured relative to some arbitrary reference level where PE = 0 ceiling is the zero levelbook has negative PE on the tabledifferences Gravitational PE is mgh, where height h is measured relative to some arbitrary reference level where PE = 0. For example, a book on a table has positive PE if the zero reference level is chosen to be the floor. However, if the ceiling is the zero level, then the book has negative PE on the table. It is only differences (or changes) in PE that have any physical meaning.

Kinetic Energy Defined KE = ½ mv 2

Concept Question Is it possible for the kinetic energy of an object to be negative? 1) No. 2)Yes. 3) Maybe.

Concept Question Is it possible for the kinetic energy of an object to be negative? 1) No. 2)Yes. 3) Maybe. In Newtonian mechanics, the answer is no. The kinetic energy is always positive since the velocity squared and the mass are always positive. Curiously, in quantum mechanics and relativity there are no such constraints. We believe it to still be true, but we are open to the possibility that it may not.

Conservation of Energy Defined

Conservation of Energy Con. Dev. 8-1, Short Cut Lab, Ball Toss Lab

Concep Question 4 Three balls of equal mass start from rest and roll down different ramps. All ramps have the same height. Which ball has the greater speed at the bottom of its ramp? 1 4) same speed for all balls 2 3

Concep Question 4 Three balls of equal mass start from rest and roll down different ramps. All ramps have the same height. Which ball has the greater speed at the bottom of its ramp? 1 4) same speed for all balls 2 3 same initial gravitational PE same height same final KEsame speed All of the balls have the same initial gravitational PE, since they are all at the same height (PE = mgh). Thus, when they get to the bottom, they all have the same final KE, and hence the same speed (KE = 1/2 mv 2 ). Follow-up: Which ball takes longest to get down the ramp?

The Rise of the Machines Okay, not these machines!

Simple Machines Define: Examples: lever, pulley, ramp, gears

Mechanical Advantage Understanding the 2 components of Work is the key to understanding mechanical advantage. Question: Where would you hold the wrench for it to be most effective?

Mechanical Advantage If you hold the wrench here, you need a lot of force... If you hold the wrench here, you don’t need as much force... …but you don’t move very far. …but your hand moves a long way.

Mechanical Advantage It takes the same amount of work to turn the bolt. You can opt for a lot of force and little distance. W = F x d Or you can choose a little force but a lot of distance. W = f x D In many of our machines, we want to increase our force, so we don’t mind going the extra distance.

Mechanical Advantage Question: Which ramp would you prefer to use to move a heavy weight to the top of the box?

Mechanical Advantage With our gear box, you were able to create a large torque here... …but you had to turn this handle many times. Remember: W =  x 

Mechanical Advantage Tug-of-war demo. Concept Development 8-2

Efficiency Are machines 100% efficient? Why or why not? Efficiency = Useful work out / Total work in

Friction = Bad Friction is caused by two surfaces rubbing together. Friction in our gear box causes a loss in the input power. It is lost in the form of heat and sound energy.

Friction = Good But friction is also what makes our robot move. The turning wheel produces a frictional force against the ground, which causes the robot to move.

More fun chapter problems Chapter 8 # 1, 2, 3, 5, 6, 8, 9, 10, 14, 15, 16, 18, 22, 24, 25, 26, 27, 33, 34, 35 & 36.