- Review energy and discuss how energy is conserved and changes forms - Investigate how conserved energy works in pulley systems - Study the force of friction on a wooden block TODAY’S OUTCOMES: FORCE, MOTION AND ENERGY

Kinetic energy = ½ mass × (velocity) 2 = 0.5 × 100,000 kg × (10 m/sec) 2 = 5,000,000 Joules Suppose a barge carrying 100,000 Kg of coal (a bit more that 100 tons) is moving down the Ohio river at 10 m/sec when it is notice that there is a fishing boat in the channel, 100 meters away. The boat guiding the barge goes into reverse and stops the barge. What is the kinetic energy of the moving barge? What force does the tugboat have to exert, to remove this much energy while the barge moves 100 m? Energy = Force × distance Force = Energy / distance = 5,000,000 Joules / 100 m = 50,000 Newtons This is exactly the same answer you got in the previous activity using F = m × a ; there you had to explicitly determine the acceleration; here you didn’t.

- In this (and other examples), we saw the energy at the start was the energy at the finish. - Another way to state this is to say energy was conserved. You applied this principle in the last lab, when raising and dropping a metal ball. Changing direction of a cart 2 meters A kg (0.05 N) ball resting on the ground has zero energy.

You applied this principle in the last lab, when raising and dropping a metal ball. Changing direction of a cart 2 meters Raising the ball 2 meters gives the ball stored energy energy = force × distance = 0.05 N × 2 m = 0.1 Joules - In this (and other examples), we saw the energy at the start was the energy at the finish. - Another way to state this is to say energy was conserved.

You applied this principle in the last lab, when raising and dropping a metal ball. Changing direction of a cart 2 meters As the ball accelerates, the stored energy becomes kinetic energy Halfway down, the distance of the ball is half what it started, so it has only half its stored energy - the rest is now kinetic energy (½mv 2 ). - In this (and other examples), we saw the energy at the start was the energy at the finish. - Another way to state this is to say energy was conserved.

You applied this principle in the last lab, when raising and dropping a metal ball. Changing direction of a cart 2 meters At the bottom, the stored energy is again zero - it has all become kinetic energy. So, kinetic energy = ½mv 2 should be equal to the stored energy of 0.1 Joules. - In this (and other examples), we saw the energy at the start was the energy at the finish. - Another way to state this is to say energy was conserved.

Changing direction of a cart Which cart requires more force to lift? IDENTICAL CARTS CLIMBING INCLINES What if the cart follows a crazy ramp up the same block? Does this change the energy stored? A B Which cart travels a longer distance? B When the carts reach the top, what quantity is equal for both, despite the different forces and distances? A The force × distance = energy is the same for both No!

Changing direction of a cart IDENTICAL CARTS CLIMBING INCLINES A B If both carts were released from the top, how would their kinetic energies compare when they reached the bottom? How would their speeds compare? They would be the same, since the (equal) stored energies are converted to kinetic energy. Kinetic energy depends on velocity (or speed) [ Kinetic energy = ½ mass × velocity 2 ] so the speeds would be the same, too.

You’ve heard since you were a young child in science class: ENERGY CANNOT BE CREATED OR DESTROYED. However - is energy always conserved within the system you are measuring? Changing direction of a cart This example assumes energy is conserved - is energy always conserved? (That is, can you create or destroy new energy?)

What happens AFTER the ball hits the ground? Does it still have stored energy or kinetic energy? Changing direction of a cart 2 meters Back to the falling ball example: The ball bounced a bit, so it had some kinetic energy left. What about when the ball comes to a complete rest? Where did the energy “go”? Some went into sound, some went into heat. Energy in the whole room is conserved, but things like friction can cause energy to leave the system you are measuring.

- Stored energy is given by force × distance, and (in the absence of friction) does not depend on the path taken - Stored energy can be changed into kinetic energy - Solve problems involving force, mass and distance using kinetic and potential energy WHAT YOU ARE EXPECTED TO KNOW:

- Review energy and discuss how energy is conserved and changes forms ✓ - Investigate how conserved energy works in pulley systems - Study the force of friction on a wooden block TODAY’S OUTCOMES: FORCE, MOTION AND ENERGY