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Momentum

Momentum. Momentum. What is the difference between kicking a: stationary ball? ball travelling towards you at 30 mph? Newton solution: moving inertia. Momentum. vector. ( direction is important). M omentum. Let’s try it: momentum of a walking person m omentum of a speeding bullet.

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Momentum

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  1. Momentum

  2. Momentum • What is the difference between kicking a: • stationary ball? • ball travelling towards you at 30 mph? • Newton solution: moving inertia

  3. Momentum vector (direction is important)

  4. Momentum • Let’s try it: • momentum of a walking person • momentum of a speeding bullet which would you stand in front of?

  5. Physics of “soft” • Why does bullet have more effect? • energy of a walking person • energy of a speeding bullet energy!

  6. Work Examples • Let’s look at the following scenarios: • Work or not? • Lifting a box above your head • Holding that box there for 2 hours • Sliding a box across a frictionless surface at constant speed work not work not work

  7. Energy Equations: Work • Work is the product of the force applied in the directionof motion and the distance it is applied • When the force and the movement are parallel, work is simply Force (F) θ

  8. Energy Units • Notice: from the work formula, energy units are a combo of Force (Newtons) and distance (m) or Newton•meters(N•m) • The SI units for energy are Joules (J). • So, one Joule is equal to 1 Newton•meter.

  9. Energy Equations: GPE • For GPE, we still have force x distance, but this time the force is the objects weight, mg • This gives us the equation: • We use h instead of d since it will always be height for GPE F=mg m

  10. Energy Equations: KE • Now let’s throw a block • The work is done while the block is being accelerated by the hand a distance of d

  11. Energy Equations: EPE • EPE is trickier than GPE • force changes depending on how much you stretch the object • This force depends on both the distance stretched (x) and a spring constant (k) • This equation is known as Hooke’s Law

  12. Energy Equations: EPE • This k comes from how much force is needed to stretch a spring per a certain distance • What is the k for this spring?

  13. Energy Equations: EPE • Since the force at the beginning of the stretch is different than the end, we use an average to calculate the EPE: • Since we usually start the stretch from rest:

  14. Power • In physics, power just means the rate of doing work • So, faster work means more power • The units come out to Joules per second. • We call this a Watt (W)for short

  15. Check Yourself Go to pg. 445

  16. Conservation of Energy • Energy cannot be created nor destroyed, but only changed from one form to another • What does this mean?

  17. Conservation of Energy • All of the energy that you start with… • you end with! • initial energy = final energy • Total energy at top • equals • Total energy at bottom • Total energy anywhere

  18. Conservation of Energy: Example rewrite and expand solve for v

  19. Conservation of Energy: Example plug and chug

  20. Time to practice Turn to pg. 456

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