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Potential and Kinetic

Potential and Kinetic. Energy. Please review the presentation and do challenge problems at the end. Work Review. Work is done when a force is exerted over a distance Which is work? When you drive into a wall and smash your fender A man walking at a steady pace carrying a briefcase

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Potential and Kinetic

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  1. Potential and Kinetic Energy

  2. Please review the presentation and do challenge problems at the end

  3. Work Review • Work is done when a force is exerted over a distance • Which is work? • When you drive into a wall and smash your fender • A man walking at a steady pace carrying a briefcase • When you spend an hour in a futile effort to move a large boulder • Why? • W = F X d • 1 joule of Work=1 newton of Force X 1 meter of Distance

  4. Challenge • A 50-Pound crate is pushed across the floor by a 20-pound horizontal force. Aside from the pushing force and gravity, there is also a 50-pound force exerted upward on the crate and a 10-pound frictional force. • Construct a free body diagram • Which of these forces does no work? • Which does positive work? • Which does negative work?

  5. Working Against Gravity • How much work do you do when you lift a 12-kilogram suitcase 0.75 meters off the ground? • What to consider • Calculate the force needed to lift first • What is the equation for force? What is the unit? • (F = m X g) • What is the equation for work? What is the unit? • (W = F X d)

  6. Energy/ Power Review • Energy is the ability to do work • Joules, Power X Time • If a system is capable of exerting a force over a distance, then it possess energy • Power is defined as the rate at which work is done (or energy that is expended) • Watts, Work/Time = Energy/ Time • 1 Watt of power = 1 Joule of Energy/ 1 second of TIme • Is expending energy the same thing as doing work?

  7. Kinetic Energy • The energy of motion • Two Factors govern the amount of K energy in an object • Kinetic energy is proportional to mass • If you double the mass, you double the kinetic energy • Kinetic energy increases as the square of its velocity • If you double the speed, you increase the kinetic energy 4 times • Mathematically, kinetic energy equals the mass of the moving object times the square of that objects velocity, multiplied by the constant ½ • KE (in joules) = ½ X mass (in kg) X (velocity (in m/s/s) • KE = ½ mv²

  8. Potential Energy • Contains the potential to do work (motion) • Gravitational Potential Energy of any object equals its weight (the force of gravity exerted on the object) times its height above the ground. • Gravitational potential in joules = Mass (kg) x g (m/s/s) x height (meters) • PE = mgh

  9. Interchangeable Forms of Energy • Rollercoaster • As the car is lifted up to the top- force is equal to? • F of g times the mass plus passengers over a distance h, where h is the height of the track • At the top • Maximum PE, PE = mgh • As the car begins descent • Acquires KE • Looses height and PE • Propose what will happen in terms of E at the very bottom • Propose what will happen in terms of E as the car begins second hill

  10. Conservation of Energy Law • In a closed system, neglecting any frictional forces, the total amount of kinetic and gravitational potential energy is conserved

  11. Energy of a Falling Body • The fact that the sum of kinetic and potential energies must be conserved provides us with an easy way to a analyze the fall of an object • What would be a good statement that describes the KE and PE energy in terms of a falling object • KE at the end of a fall is equal to PE at the beginning • Initial potential energy = final kinetic energy • Equation- mgh = ½ mv² • Cancel mass- gh= ½v², or v = square root of 2gh* • Therefore the speed of the objects at the end of the fall is independent of the object’s mass • More explanation on next slide.

  12. Continued • Equation from falling body

  13. Challenge 1 • Would you rather be hit by a 1 kilogram mass traveling 10 meters per second or a 2 kilogram mass traveling 5 meters per second? • Hint-do the math

  14. Challenge 2 • A woman weight lifter can lift a 150-lb weight from the floor to a stand 3.5 feet off the ground. What is the total work done by the woman in joules?

  15. Challenge 3 • You throw a softwall (250 g) straight up into the air. It reaches a maximum altitide of 15 meters and then returns to you (assume a departure and return to ground level). • What is the GPE in joules of the ball at its heights position? • What is the KE of the ball as it leaves your hand? (assume there is no E losses while it is in the air) • What is the KE of the ball as it returns to your hand? • From the KE, calculate the velocity of the ball.

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