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Do now Can you read the “impulse” sheet you stuck in last lesson?

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Last lesson Impulse

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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

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

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Impulse 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 5 m/s -3 m/s

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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?

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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

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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

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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

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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

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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

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Today’s lesson Know what the turning effect of a force is Know that the moment of a force depends on force and the distance from the pivot Know the principal of moments

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What happens when you try to open a door with one finger? Where do you push? Hi, my name is Mr Stick hinges

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Do you push near the hinges? I can’t do it!

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Do you push far from the hinges? That’s easier!

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Moments – Turning forces

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The turning effect of a force depends on two things; The size of the force Obviously!

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The turning effect of a force depends on two things; The distance from the pivot (axis of rotation) Not quite so obvious! Axis of rotation

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Turning effect of a force The turning effect of a force is called the moment of the force The moment is calculated by multiplying the force by the distance from the pivot

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Turning effect of a force – moment of a force Moment (Nm) = Force (N) x distance from pivot (m) Note the unit is Nm, not N/m!

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A simple example! nut spanner (wrench) 50 N 0.15 m

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A simple example! nut spanner (wrench) 50 N 0.15 m Moment = Force x distance from pivot Moment = 50 N x 0.15 m Moment = 7.5 Nm

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What do you do if the nut won’t move and you can’t push harder?! nut spanner (wrench) 50 N 0.15 m

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Get a longer spanner! nut spanner (wrench) 50 N 0.25 m Moment = Force x distance from pivot Moment = 50 N x 0.25 m Moment = 12.5 Nm

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More than one force Take an uneven see-saw for an example Do you think we’ll be safe in this power point? It’s not looking good! pivot

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If the see-saw is balanced, what must be the weight of the dog on the left? pivot 1.2 m2.2 m 110 N ? N

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The force on the left is trying to turn the see- saw anticlockwise about the pivot pivot 1.2 m2.2 m 110 N ? N

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The force on the right is trying to turn the see-saw clockwise about the pivot pivot 1.2 m2.2 m 110 N ? N

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If the see-saw balances, the turning effect anticlockwise must equal the turning effect clockwise pivot 1.2 m2.2 m 110 N ? N Anticlockwise momentclockwise moment=

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Anticlockwise moment = clockwise moment ? X 1.2 = 110 x 2.2 ? X 1.2 = 242 ? = 242/1.2 ? = N pivot 1.2 m2.2 m 110 N ? N Anticlockwise momentclockwise moment=

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Rotational Equilibrium When there is no resultant moment on an object (when anticlockwise moments = clockwise moments) we say the object is in rotational equilibrium. YouTube - Alan Partridge's Apache office

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Whew! We seemed to have survived the power point pivot

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Don’t speak too soon! pivot BOMB

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pivot BOMB

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pivot BOMB

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pivot BOMB

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pivot

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Nice one!

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pivot You’ll have to do some questions now, or he’ll kill me too!

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pivot Read pages 54 and 55 and make notes. Then try the questions that Mr Porter is giving you.

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