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VELOCITY

VELOCITY. Remember we mentioned the word ‘Velocity’ before? What does ‘Velocity’ mean?. Is this runner leaning? Which way?. Leaning means Acceleration. So this runner is accelerating. But is he ‘speeding up’ or ‘slowing down’?. Neither. He is ‘changing direction’.

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VELOCITY

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  1. VELOCITY Remember we mentioned the word ‘Velocity’ before? What does ‘Velocity’ mean?

  2. Is this runner leaning? Which way?

  3. Leaning means Acceleration. So this runner is accelerating. But is he ‘speeding up’ or ‘slowing down’?

  4. Neither. He is ‘changing direction’.

  5. A runner leans forwards to speed up:

  6. A runner leans backwards to slow down:

  7. A runner leans sideways to CHANGE DIRECTION:

  8. So, acceleration is a change of speedand/ora change of direction.Can we simplify this definition?

  9. We can.If we use a new quantity: VELOCITY,that specifies a speed in a particular direction

  10. These are speeds:50 mph, 60 mphThese are velocities:50 mph northbound, 60 mph eastwards(speed AND direction)

  11. Would a police car travelling at 120 mph catch a speeding granny going at 71 mph?

  12. Would a police car travelling at 120 mph northbound from Junction 7 catch a speeding granny going at 71 mph southbound from Junction 7?

  13. Direction can matter.That is why we use the quantity velocity as well as the quantity speed.You have met other quantities that use direction…

  14. Other quantities that use direction… Force. The direction of a force matters as much as the strength of a force. Imagine trying to push open a door marked ‘pull’…

  15. Other quantities that use direction… Acceleration Acceleration in the direction you are travelling speeds you up… Acceleration in the direction you are NOT travelling slows you down… its called deceleration.

  16. So, direction matters.That is why we use the quantity velocity as well as the quantity speed, because when we use velocity we are saying that we are interested not just ‘how fast’ but also ‘in what direction’.And if you use velocity, the definition of acceleration become a lot simpler…

  17. Acceleration isa change of velocity (this could be a change in the value of the velocity or a change in the direction of the velocity)

  18. First they accelerate away from the start (leaning forward) : changing speed but not direction. A 200 metre runner Then they accelerate around the bend (leaning sideways) : changing direction but not speed.

  19. Now, from Newton’s 2nd Law we know that accelerations are caused by the application of Forces. And depending on the direction of the force they can cause speeding up, slowing down or a change of direction.

  20. He’s running at top speed. • Other players could apply force to him to: • Help push him over the line • Slow him down • Push him into touch Take Jonah.

  21. Oops…

  22. If there is no force,there is no acceleration,so the velocity DOES NOT CHANGE. Does that make sense?

  23. It made sense to Newton as well. It’s Newton’s 1st Law.

  24. If there is no forceThe velocity does not changeBut stays at whatever value (and direction)it started at. It makes perfect sense when the starting velocity is 0 (standing still). But can you believe it when the starting velocity is some value like 8 mps rightwards?

  25. Moving objects stop eventually… don’t they?They don’t keep going for ever… do they?

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