Do now! Can you finish the “Moments” questions we started yesterday?

Last 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!

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 I thought he didn’t like dogs? Only dead ones! 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 ? = N pivot 1.2 m2.2 m 110 N ? N Anticlockwise momentclockwise moment=

Questions Mr Porter has a few more for you.

Today’s lesson Centre of gravity Stability

The ruler trick!

Centre of gravity The centre of gravity of an object is the point where the objects weight seems to act. With a uniform ruler, the C of G is at the mid-point. I think he wants you to copy this

More complex shapes How do you find the centre of gravity of more complex shapes? Complex shape man

Draw the sentence

Finding the centre of gravity i. Place a compass or needle through any part of the shape. ii. Make sure that the shape “hangs loose”. iii. Hang a plumb line on the needle. iv. After it has stopped moving, carefully draw a line where the plumb line is. v. Place the needle in any other part of the card. vi. Repeat steps ii to iv. vii. Where the two drawn lines cross is where the centre of mass is. viii. Physics is the most interesting science subject.

Stability

Stable objects have a wide base and a low centre of gravity. ¿Pueden ustedes copiar este por favor?

Stable!

Let’s try some questions