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MSTC Physics 2

MSTC Physics 2. Chapter 6 Sections 1 - 3. Impulse. Force applied over a period of time I = FΔt where I = impulse F = average force Δt = time interval Measured in Ns Vector quantity (direction same as direction of force). Linear momentum.

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MSTC Physics 2

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  1. MSTC Physics 2 Chapter 6 Sections 1 - 3

  2. Impulse • Force applied over a period of time I = FΔt where I = impulse F = average force Δt = time interval Measured in Ns Vector quantity (direction same as direction of force)

  3. Linear momentum • Product of an object’s mass and velocity • Inertia in motion p = mv where p = momentum m = mass v = velocity Measured in kgm/s or Ns Vector quantity (direction same as velocity)

  4. Impulse-momentum theorem • If a force is applied for a period of time, the impulse applied is equal to the object’s change in momentum I = Δp FΔt = mΔv

  5. Questions • Which has the greater mass, a heavy truck at rest or a rolling skateboard? • Which has the greater momentum?

  6. Questions • When the force of impact on an object is extended in time, does the impulse increase or decrease? • For a constant force, suppose the duration of impact on an object is doubled. How much is the impulse increased? How much is the resulting change in momentum increased?

  7. Questions • In a car crash, why is it advantageous for an occupant to extend the time during which the collision takes place? • If the time of impact in a collision is extended by 4 times, how much does the force of impact change?

  8. Questions • Visualize yourself on a skateboard. When you throw a ball, do you experience an impulse? • Which impulse is greater: the one on you or the one on the ball?

  9. Questions • Why is more impulse delivered during a collision when bouncing occurs than during one when it doesn’t? • In terms of impulse and momentum, why are air bags in cars a good idea?

  10. Sample Problems 1. Suppose a tugboat is trying to slow a huge barge that has a mass of 2 x 107 kg and is moving with a speed of 3 m/s. The tugboat exerts a force of 2.85 x 106 N for 21 s in the direction opposite to that in which the barge is moving. What will be the change in the barge’s momentum? How far will the barge travel during this time interval if it is brought to a stop?

  11. Sample Problems 2. Suppose a meteorite with mass 6.5 x 104 kg enters Earth’s atmosphere with a speed of 1 km/s. What is the change in the meteorite’s momentum if an average constant force of -1.7 x 106 N acts on the meteorite for 30 s? How far does the meteorite travel during this time?

  12. Law of Conservation of Linear Momentum • The total momentum of an isolated system of bodies remains constant ∑po = ∑pf

  13. Sample Problems 3. A 10,000 kg railroad car traveling at 24 m/s strikes an identical car at rest. If the cars lock together, what is their common speed after the collision? 4. Calculate the recoil velocity of a 4 kg rifle which shoots a 0.05 kg bullet at a speed of 280 m/s.

  14. Sample Problems 5. A child throws a 5.4 kg package horizontally from a boat with a speed of 10 m/s. Calculate the resulting velocity of the boat assuming it was initially at rest. The mass of the child is 20 kg and the boat is 80 kg.

  15. Sample Problems 6. A boy of mass 50 kg is initially on a skateboard of mass 2 kg, moving at a speed of 10 m/s. The boy falls off the skateboard, and his center of mass moves forward at a speed of 11 m/s. Find the final velocity of the skateboard.

  16. Sample Problems 7. A skater of mass 80 kg initially moves in a straight line at a speed of 5 m/s. The skater approaches a child of mass 40 kg, whom he lifts on his shoulders. Assuming there are no external horizontal forces, what is the skater’s final velocity?

  17. Sample Problems 8. A missile of mass 100 kg is fired from a plane of mass 5000 kg initially moving at a speed of 300 m/s. If the speed of the missile relative to the plane is 1000 m/s, what is the final velocity of the plane?

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