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Understanding Momentum and Impulse in Physics

In today's lesson, students will explore the concepts of momentum and impulse. They will learn the definition of momentum (p = mv) and its applications in collisions, illustrated by examples like a lorry and a truck. We will discuss the conservation of momentum and how to calculate joint velocity post-collision. Lastly, students will understand impulse as the change in momentum with practical examples, including calculating the force of a punch. Homework includes reading pages 44 and 45 of the textbook, due next lesson, Monday, October 12th.

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Understanding Momentum and Impulse in Physics

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  1. Do now! Can you write in your planners that today’s homework is to read pages 44 and 45 of your new text book! Due next lesson Monday 12th Oct

  2. Last lesson • Momentum YouTube - Spectacular 100mph Train Crash Test

  3. Momentum • Momentum is a useful quantity to consider when thinking about "unstoppability". It is also useful when considering collisions and explosions. It is defined as Momentum (kg.m/s) = Mass (kg) x Velocity (m./s) p = mv

  4. An easy example • A lorry has a mass of 10 000 kg and a velocity of 3 m.s-1. What is its momentum? Momentum = Mass x velocity = 10 000 x 3 = 30 000 kg.m.s-1.

  5. Conservation of momentum • In a collision between two objects, momentum is conserved (total momentum stays the same). i.e. Total momentum before the collision = Total momentum after Momentum is not energy!

  6. A harder example! • A car of mass 1000 kg travelling at 5 m/s hits a stationary truck of mass 2000 kg. After the collision they stick together. What is their joint velocity after the collision?

  7. A harder example! Before 2000kg 1000kg 5 m/s Momentum before = 1000x5 + 2000x0 = 5000 kg.m/s Combined mass = 3000 kg After V m/s Momentum after = 3000v

  8. A harder example The law of conservation of momentum tells us that momentum before equals momentum after, so Momentum before = momentum after 5000 = 3000v V = 5000/3000 = 1.67 m/s

  9. Momentum is a vector • Momentum is a vector, so if velocities are in opposite directions we must take this into account in our calculations

  10. An even harder example! Snoopy (mass 10kg) running at 4.5 m/s jumps onto a skateboard of mass 4 kg travelling in the opposite direction at 7 m/s. What is the velocity of Snoopy and skateboard after Snoopy has jumped on? I love physics

  11. Because they are in opposite directions, we make one velocity negative An even harder example! 10kg -4.5 m/s 4kg 7 m/s Momentum before = 10 x -4.5 + 4 x 7 = -45 + 28 = -17 14kg v m/s Momentum after = 14v

  12. An even harder example! Momentum before = Momentum after -17 = 14v V = -17/14 = -1.21 m/s The negative sign tells us that the velocity is from left to right (we choose this as our “negative direction”)

  13. Today’s lesson Impulse

  14. Let’s follow Mr Porter

  15. Impulse Ft = mv – mu The quantity Ft is called the impulse, and of course mv – mu is the change in momentum (v = final velocity and u = initial velocity) Impulse = Change in momentum

  16. Units Impulse is measured in N.s (Ft) or kg.m/s (mv – mu)

  17. 5 m/s Impulse -3 m/s Note; For a ball bouncing off a wall, don’t forget the initial and final velocity are in different directions, so you will have to make one of them negative. In this case mv – mu = 5m - -3m = 8m

  18. Example • Dylan punches Henry in the face. If Henry’s head (mass 10 kg) was initially at rest and moves away from Dylans fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch?

  19. Example • Dylan punches Henry in the face. If Henry’s head (mass 10 kg) was initially at rest and moves away from Dylans fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch? • m = 10kg, t = 0.2, u = 0, v = 3

  20. Example • Dylan punches Henry in the face. If Henry’s head (mass 10 kg) was initially at rest and moves away from Dylans fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch? • m = 10kg, t = 0.2, u = 0, v = 3 • Ft = mv - mu

  21. Example • Dylan punches Henry in the face. If Henry’s head (mass 10 kg) was initially at rest and moves away from Dylans fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch? • m = 10kg, t = 0.2, u = 0, v = 3 • Ft = mv – mu • 0.2F = 10x3 – 10x0

  22. Example • Dylan punches Henry in the face. If Henry’s head (mass 10 kg) was initially at rest and moves away from Dylans fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch? • m = 10kg, t = 0.2, u = 0, v = 3 • Ft = mv – mu • 0.2F = 10x3 – 10x0 • 0.2F = 30

  23. Example • Dylan punches Henry in the face. If Henry’s head (mass 10 kg) was initially at rest and moves away from Dylans fist at 3 m/s, and the fist was in contact with the face for 0.2 seconds, what was the force of the punch? • m = 10kg, t = 0.2, u = 0, v = 3 • Ft = mv – mu • 0.2F = 10x3 – 10x0 • 0.2F = 30 • F = 30/0.2 = 150N

  24. Now let’s try some fun questions!

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