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Chapter 7 Linear Momentum

Chapter 7 Linear Momentum. Momentum. Momentum is defined as: ‘ Inertia in Motion ’, meaning how much inertia something has when it is in motion · Equation for Momentum is p = mv · where ‘p’ is momentum in kg*m/s · and ‘m’ is mass in kg · and ‘v’ is velocity in m/s.

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Chapter 7 Linear Momentum

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  1. Chapter 7 Linear Momentum

  2. Momentum • Momentum is defined as:‘Inertia in Motion’, meaning how much inertia something has when it is in motion • ·Equation for Momentum is p = mv • ·where ‘p’ is momentum in kg*m/s • ·and ‘m’ is mass in kg • ·and ‘v’ is velocity in m/s

  3. Under what circumstances would the roller skate and the truck have the same momentum ?

  4. MOMENTUM • An object at rest has no momentum, why? • Because anything times zero is zero • (the velocity component is zero for an object at rest) • To INCREASE MOMENTUM, apply the greatest force possible for as long as possible. • Examples : • pulling a sling shot • drawing an arrow in a bow all the way back • a long cannon for maximum range • hitting a golf ball or a baseball . (follow through is important for these !)

  5. Impulse and Momentum • If momentum changes, it’s because mass or velocity change. • Most often mass doesn’t change so velocity changes and that is acceleration. REMEMEBER THAT: • mass x acceleration = force F= m(a)

  6. Impulse and Momentum • F=ma or a = F/m • Remember also that a = (vf – vi)/ t) or a = Δv/t • So basically, • F/m = Δv/t, in other words Ft = mv

  7. Impulse and Momentum • So, applying a force over a time interval to an object changes the momentum • This change in momentum is called an impulse • Impulse = mv or Ft = mv • But what could cause a change in momentum?

  8. Momentum – Impulse Theorem • Imagine a larger, stronger person coming straight at you, to change the momentum of that larger person you would need to change their velocity. But what is required to change the velocity of an object??

  9. Momentum – Impulse Theorem • A large force over a period of time.

  10. Momentum – Impulse Theorem • Not only does the amount of Force matters when changing momentum, but also the amount of time that the force is applied to the other object. • So here is how we come to the equation that relates change in momentum (Δp) to time (t)

  11. Momentum – Impulse Theorem • It is called the momentum – impulse theorem • Ft = Δp = mvf – mvi or simply Ft = mvf – mvi

  12. SummaryVocabulary • impulse : impact force X time (newton.sec) . Ft = impulse • impact : the force acting on an object (N) usually when it hits something. • impact forces : average force of impact

  13. Summary:Formulas to know • p =mv • Ft = mv (Impulse = mv) • Ft = mvf – mvi • Where p = momentum (kg*m/s) m = mass (kg) v = velocity (m/s) F = force (kg*m/s) vf = final velocity (m/s) vi = initial velocity (m/s)

  14. WHAT HAPPENS WHEN MOMENTUM DECREASES?

  15. MOMENTUM • Decreasing Momentum • Which would it be more safe to hit in a car ? • Knowing the physics helps us understand why hitting a soft object is better than hitting a hard one. Ft mv mv Ft

  16. MOMENTUM • In each case, the momentum is decreased by the same amount or impulse (force x time) • Hitting the haystack extends the impact time (the time in which the momentum is brought to zero). • The longer impact time reduces the force of impact and decreases the deceleration. • Whenever it is desired to decrease the force of impact, extend the time of impact !

  17. DECREASING MOMENTUM • If the time of impact is increased by 100 times (say from .01 sec to 1 sec), then the force of impact is reduced by 100 times (say to something survivable). • EXAMPLES: • Padded dashboards on cars • Airbags in cars or safety nets in circuses • Moving your hand backward as you catch a fast-moving ball with your bare hand or a boxer moving with a punch. • Flexing your knees when jumping from a higher place to the ground. or elastic cords for bungee jumping • Using wrestling mats instead of hardwood floors. • Dropping a glass dish onto a carpet instead of a sidewalk.

  18. EXAMPLES OF DECREASING MOMENTUM Ft= change in momentum • Bruiser Bruno on boxing … • Increased impact time reduces force of impact Ft = change in momentum POOF ! CRUNCH !

  19. BOUNCING • IMPULSES ARE GREATER WHEN AN OBJECT BOUNCES • The impulse required to bring an object to a stop and then to throw it back upward again is greater than the impulse required to merely bring the object to a stop.

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