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Springs

Springs. And pendula, and energy. Elastic Potential Energy. What is it? Energy that is stored in elastic materials as a result of their stretching. Where is it found? Rubber bands Bungee cords Trampolines Springs Bow and Arrow Guitar string Tennis Racquet. Hooke’s Law.

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Springs

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  1. Springs And pendula, and energy

  2. Elastic Potential Energy • What is it? • Energy that is stored in elastic materials as a result of their stretching. • Where is it found? • Rubber bands • Bungee cords • Trampolines • Springs • Bow and Arrow • Guitar string • Tennis Racquet

  3. Hooke’s Law • A spring can be stretched or compressed with a force. • The force by which a spring is compressed or stretched is proportional to the magnitude of the displacement (Fa x). • Hooke’s Law: Felastic = -kx Where: k = spring constant = stiffness of spring (N/m) x = displacement

  4. Hooke’s Law Felastic = -kx k = spring constant = 10 (N/m) x = displacement = 0.2m F = - (0.2m)(10 N/m) = -2N Why negative? Because the direction of the Force and the displacement are in opposite directions.

  5. Hooke’s Law – Energy • When a spring is stretched or compressed, energy is stored. • The energy is related to the distance through which the force acts. • In a spring, the energy is stored in the bonds between the atoms of the metal.

  6. Hooke’s Law – Energy • F = kx • W = Fd • W = (average F)d = d(average F) • W = d*[F(final) – F(initial)]/2 • W = x[kx - 0 ]/2 • W = ½ kx2 = D PE + D KE

  7. Hooke’s Law – Energy • This stored energy is called Potential Energy and can be calculated by PEelastic = ½ kx2 Where: k = spring constant = stiffness of spring (N/m) x = displacement • The other form of energy of immediate interest is gravitational potential energy • PEg = mgh • And, for completeness, we have • Kinetic Energy KE = 1/2mv2

  8. Simple Harmonic Motion & Springs • Simple Harmonic Motion: • An oscillation around an equilibrium position in which a restoring force is proportional the the displacement. • For a spring, the restoring force F = -kx. • The spring is at equilibrium when it is at its relaxed length. • Otherwise, when in tension or compression, a restoring force will exist.

  9. Restoring Forces and Simple Harmonic Motion • Simple Harmonic Motion • A motion in which the system repeats itself driven by a restoring force • Springs • Gravity • Pressure

  10. Harmonic Motion • Pendula and springs are examples of things that go through simple harmonic motion. • Simple harmonic motion always contains a “restoring” force that is directed towards the center.

  11. Simple Harmonic Motion & Springs • At maximum displacement (+ x): • The Elastic Potential Energy will be at a maximum • The force will be at a maximum. • The acceleration will be at a maximum. • At equilibrium (x = 0): • The Elastic Potential Energy will be zero • Velocity will be at a maximum. • Kinetic Energy will be at a maximum

  12. Simple Harmonic Motion & Springs

  13. The Pendulum • Like a spring, pendula go through simple harmonic motion as follows. T = 2π√l/g Where: • T = period • l = length of pendulum string • g = acceleration of gravity • Note: • This formula is true for only small angles of θ. • The period of a pendulum is independent of its mass.

  14. Example 3 Changing the Mass of a Simple Harmonic Oscilator 10.3 Energy and Simple Harmonic Motion A 0.20-kg ball is attached to a vertical spring. The spring constant is 28 N/m. When released from rest, how far does the ball fall before being brought to a momentary stop by the spring? What about a 0.4 kg ball?

  15. Simple Harmonic Motion & Pendula • At maximum displacement (+ y): • The Gravitational Potential Energy will be at a maximum. • The acceleration will be at a maximum. • At equilibrium (y = 0): • The Gravitational Potential Energy will be zero • Velocity will be at a maximum. • Kinetic Energy will be at a maximum

  16. Conservation of Energy & The Pendulum • (mechanical) Potential Energy is stored force acting through a distance • If I lift an object, I increase its energy • Gravitational potential energy • We say “potential” because I don’t have to drop the rock off the cliff • Peg = Fg * h = mgh

  17. Conservation of Energy • Consider a system where a ball attached to a spring is let go. How does the KE and PE change as it moves? • Let the ball have a 2Kg mass • Let the spring constant be 5N/m

  18. Conservation of Energy • What is the equilibrium position of the ball? • How far will it fall before being pulled • Back up by the spring?

  19. Conservation of Energy & The Pendulum • (mechanical) Potential Energy is stored force acting through a distance • Work is force acting through a distance • If work is done, there is a change in potential or kinetic energy • We perform work when we lift an object, or compress a spring, or accelerate a mass

  20. Conservation of Energy & The Pendulum Does this make sense? Would you expect energy to be made up of these elements? • Peg = Fg * h = mgh • What are the units?

  21. Conservation of Energy & The Pendulum Units • Newton = ?

  22. Conservation of Energy & The Pendulum Units • Newton = kg-m/sec^2 • Energy • Newton-m • Kg-m^2/sec^2

  23. Conservation of Energy Energy is conserved • PE + KE = constant For springs, • PE = ½ kx2 For objects in motion, • KE = ½ mv2

  24. Conservation of Energy & The Pendulum • http://zonalandeducation.com/mstm/physics/mechanics/energy/springPotentialEnergy/springPotentialEnergy.html

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