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!

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 m 2.2 m

110 N? N

The force on the left is trying to turn the see-saw anticlockwise about the pivot

pivot

1.2 m 2.2 m

110 N? N

The force on the right is trying to turn the see-saw clockwise about the pivot

pivot

1.2 m 2.2 m

110 N? N

If the see-saw balances, the turning effect anticlockwise must equal the turning effect

clockwise

pivot

1.2 m 2.2 m

110 N? N

Anticlockwise moment clockwise 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 m 2.2 m

110 N? N

Anticlockwise moment clockwise moment=

Questions

We then answered 2 sheets of questions on moments. Those who were absent last lesson (PE trip) will have to complete these for homework due FRIDAY.

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.

More complex shapes

How do you find the centre of

gravity of more complex shapes?

Complex shape man

Draw the sentence

1. 2. 3. 4.

5. 6. 7. 8.

Draw the following sentences

i. Place a compass or needle through any part of the shape.

ii. Make sure that the shape “hangs loose”.iii. Hang a pumb 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

Stability

Stable objects have a wide base and a low centre of gravity.

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favor?

Stable!

Let’s try some questions

Forces and springs

Robert Hooke

Investigating forces and springsYou are going to investigate how much a spring stretches when a force is applied to it.

The amount a spring stretches is called its extension. This is the difference in length between the stretched spring and the length of the spring when it was unstretched.(Remember we are looking at the force on the spring. A mass of 100g will have a weight (force of gravity pulling it down) of 1 Newton.

• Add masses to your spring and carefully measure its extension. You can do this until the spring breaks! (but you must wear goggles and be careful during this investigation).

Your experimental report will include the following• A table of results• A graph of your results • A conclusion describing what you have discovered (think about this!)

Table of results

Force (N) Length of spring (cm)

Extension (cm)

0 3.4 0

1 5.4 2

2 7.4 4

3 9.4 6

Table of results

Force (N) Length of spring (cm)

Extension (cm)

0 0

1

2

3

Graph

Force (N)

Extension (cm)

Go!