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Work and Energy

Work and Energy. Work and energy Work vs. energy Work definition 2 important points about work Work and energy examples. Work and Energy. Modified 3 rd Equation Multiply by ½ m ma = Force Work equals change in Kinetic Energy All scalars , use only magnitudes!

elizabeth-cochran
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Work and Energy

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  1. Work and Energy • Work and energy • Work vs. energy • Work definition • 2 important points about work • Work and energy examples

  2. Work and Energy • Modified 3rd Equation • Multiply by ½ m • ma = Force • Work equals change in Kinetic Energy • All scalars, use only magnitudes! • Units N-m, or kg m2/s2 Joules (J)

  3. Work vs. Energy • Product of force, distance, and how they’re working together increases or decreases the magnitudeof v. • How force and distance work together is very important. • If f and d inline, magnitude of v increases. • If f and d partially inline, magnitude of v increases a little. • If f and d perpendicular, magnitude of v remains constant. • If f and d partially opposed, magnitude of v decreases a little. • If f and d opposed, magnitude of v decreases. • If f but no d v remains constant.

  4. Work Definition • Definition F . x .cos(θ) • Cos(θ) extracts F and x working together • +1 when together • -1 when opposed • -1 to +1 when in between • 0 when perpendicular • Work is a scalar quantity F x

  5. Two important things • Total Work is • The work of the sum of all forcesΣFi x distance • or • The sum of the individual works of all forces. Σ(Fi x distancei) • Individual Work is • Force component in direction of displacement. • or • Displacement component in direction of force.

  6. Examples of Work and Energy • Example 6.5 – Work to increase car speed • Problem 18 – Work to stop car • Example - Work of Moon going around Earth • Problem 23 - Air resistance on baseball • Example 6.8 – Falling baseball • Use 2nd law • Use work • Example 6-9 - Roller coaster • Use work • Couldn’t do easily by 2nd law! • Vertical circle example (use work) • Note how you “mix up” dimensions!

  7. Examples of Work and Energy - I • Example 6.5 – Work to increase car speed • Problem 18 – Work to stop car • Work of Earth on Moon magnitude of v unchanged, ΔKE = 0 force perpendicular to distance Work = 0

  8. Examples of Work and Energy - II • Problem 23 - Air resistance on baseball • Softball mass 0.25 kg, speed 95 km/h (26.4 m/s) • Slows to 90% speed (23.75 m/s) • Does this in 15 m (pitcher to home) • What is average air resistance? (Like car slowing)

  9. Examples of Work and Energy III • Example 6.8 – Rock falling 3 m - Kinematic (down positive) • Rock falling 3 m – Work/Energy Same thing really!

  10. Examples of Work and Energy IV • Example 6-9 - Roller coaster • Forces mg, normal force • Normal force does no work! • Work of mg • (mgx’) x distance • mg x height • 2nd way much easier, just mgh! • Note how you “mix up” dimensions! vo v

  11. Examples of Work and Energy V • Vertical circle • Forces mg, Tension • Circle radius r • Tension does no work! • Work of mg • (mgx’) x distance • mg x height • 2nd way much easier, just mg2r! • Note how you “mix up” dimensions!

  12. Work/Energy advantage • Work done my gravity • And the string tension does no work! But this is the path you can use for calculating the work! This is the path the ball takes

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