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Exploring Energy Conservation and Forces in Motion

This text discusses the concept of energy conservation and its transformation in different forms. It also explores the force of friction on a wooden block and investigates how conserved energy works in pulley systems.

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Exploring Energy Conservation and Forces in Motion

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  1. TODAY’S OUTCOMES: FORCE, MOTION AND ENERGY - 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

  2. 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? Kinetic energy = ½ mass × (velocity)2 = 0.5 × 100,000 kg × (10 m/sec)2 = 5,000,000 Joules 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.

  3. - 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. 2 meters A 0.005 kg (0.05 N) ball resting on the ground has zero energy. Changing direction of a cart

  4. - 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. 2 meters Raising the ball 2 meters gives the ball stored energy energy = force × distance = 0.05 N × 2 m = 0.1 Joules Changing direction of a cart

  5. - 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. 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 (½mv2). Changing direction of a cart

  6. - 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. 2 meters At the bottom, the stored energy is again zero - it has all become kinetic energy. So, kinetic energy = ½mv2 should be equal to the stored energy of 0.1 Joules. Changing direction of a cart

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

  8. They would be the same, since the (equal) stored energies are converted to kinetic energy. How would their speeds compare? 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? Kinetic energy depends on velocity (or speed) [ Kinetic energy = ½ mass × velocity2 ] so the speeds would be the same, too. Changing direction of a cart

  9. This example assumes energy is conserved - is energy always conserved? (That is, can you create or destroy new energy?) 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

  10. Back to the falling ball example: What happens AFTER the ball hits the ground? Does it still have stored energy or kinetic energy? 2 meters 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. Changing direction of a cart

  11. - 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:

  12. TODAY’S OUTCOMES: FORCE, MOTION AND ENERGY - 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

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