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Warm-up

Warm-up. What is the mass of an object weighing 90 N on Earth? On Earth, what is the weight of a 7 kg object? What is the weight of a 14 kg box on the moon (a g on the moon = 3.7 m/s 2 ). You are outside a store, moving a loaded g rocery cart down the street on a very

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Warm-up

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  1. Warm-up • What is the mass of an object weighing 90 N on Earth? • On Earth, what is the weight of a 7 kg object? • What is the weight of a 14 kg box on the moon (ag on the moon = 3.7 m/s2)

  2. You are outside a store, moving a loaded grocery cart down the street on a very steep hill. It is difficult but you are able to pull back on the handle and keep the cart moving down the street in a straight line and at a constant speed. For this situation, the forces on the cart are Unbalanced in the direction of the movement. Balanced with a net force of zero. Equal to the force of gravity acting on the cart Greater than the frictional forces opposing the motion of the cart.

  3. Page 52 Energy : Energy is defined as the ability to do work. One way of measuring the energy of something is to see how much work It can do. There are many forms of energy. Mechanical Energy is our focus today!! The Law of Conservation of Energy

  4. The Law of Conservation of Energy says that Energy cannot be created or destroyed; it is transferred.

  5. Energy is measured in Joules While work is done only when an object experiences a change in motion, energy can be present in an object or a system when nothing is happening at all.

  6. Kinetic Energy • If an object is moving, it has energy. (Be careful, the converse of this statement is not always true!) • This energy is called kinetic energy - the energy of motion.

  7. Kinetic Energy • An object’s kinetic energy depends on: • the object’s mass. • Kinetic energy is directly proportional to mass. • the object’s speed. • Kinetic energy is directly proportional to the square of the object’s speed.

  8. Kinetic Energy The greater the mass or velocity of a moving object, the more kinetic energy it has.

  9. Kinetic Energy • In symbols: 1 KE = mv2 2

  10. Kinetic Energy • Kinetic energy is a scalar quantity. • Common units of kinetic energy: Joules • An object with mass of 1 kg, moving at 1 m/s, has a kinetic energy of 0.5 Joule.

  11. Work • When the kinetic energy of an object changes, work has been done on the object. • Units of work: Joules • Work is a scalar quantity.

  12. Work • Work depends on: • The amount of force applied to the object. • The distance that the object moves while the force is applied. • The direction of the force with respect to the direction the object moves.

  13. Work • If the force on the object is in the direction the object moves, the work done is: W = Fx F x

  14. Work and Kinetic Energy • The work done on an object by the net force equals the object’s change in kinetic energy. Wnet = DKE

  15. Potential Energy • Sometimes work is not converted directly into kinetic energy. Instead it is “stored”, or “hidden”. • Potential energy is stored energy or stored work.

  16. Potential Energy • What do we mean by your potential? • It is what you are capable of doing, not what you are actually doing. • Maybe you are getting a C+ in Physics, but you have the potential to get a B or even an A

  17. Potential Energy Potential energy doesn’t only have to come from Height. • Electrical potential energy • Elastic potential energy • Chemical potential energy

  18. Potential Energy • Potential energy is energy that an object (system) has due to its position or arrangement.

  19. Calculating Potential Energy PE = m g h

  20. Mechanical Energy • Mechanical Energy = PE + KE

  21. Conservation of Energy • If no external forces act on a system, the total energy of the system will remain constant.

  22. Power • Power is the rate work is done. DWork Power = time W P t

  23. Power • Units of power: 1 Joule/sec = 1 Watt • 1000 Watts = 1 kilowatt • Power is a scalar quantity.

  24. Which Newton’s Law? For every action, there is an equal and opposite Reaction An object at rest remains at rest unless an unbalanced Force acts on it Force = mass x acceleration

  25. Does it take more force to accelerate a tennis ball or a bowling ball? You drop a large bowling ball on your toe, And break your toe. You push very hard against a table to move it

  26. Mechanical Energy

  27. Work and Kinetic Energy • The work done on an object by the net force equals the object’s change in kinetic energy. Wnet = DKE

  28. You drop a tennis ball from a height Of 3 feet. It accelerates in the negative Direction to the floor. The ball hits the floor and bounces About 1 foot high. Identify potential energy of the ball Kinetic energy of the ball Work on the ball Conservation of energy taking place

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