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

## Presentation on theme: "Do now Can you read the “impulse” sheet you stuck in last lesson?"— Presentation transcript:

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

Last lesson 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

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

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

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?

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

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

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

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

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

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

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

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

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

Moments – Turning forces

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

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

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

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!

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

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

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

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

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

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

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

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

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=

Anticlockwise moment = clockwise moment ? X 1.2 = 110 x 2.2 ? X 1.2 = 242 ? = 242/1.2 ? = 201.7 N pivot 1.2 m2.2 m 110 N ? N Anticlockwise momentclockwise moment=

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

Whew! We seemed to have survived the power point pivot

Don’t speak too soon! pivot BOMB

pivot BOMB

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pivot

Nice one!

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

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