Moments, Levers and Gears

As well as causing objects to speed up, slow down, change direction and deform, forces can also cause objects do what?

What can also do this?

• rotate

• a system of forces

What is a pivot?

When a force on side of a pivot causes rotation, what directions could this rotation be?

• a fixed point that the object can rotate around

• clockwise or anticlockwise

A force on one side of a pivot is one example of rotation, name 3 more

• a child on a see-saw

• turning the handle of a spanner

• a door opening and closing

What can still happen is two forces act on an object without passing through the same point?

What is it called if the forces are equal and opposite?

• The object can still rotate

• Known as a couple

What is a moment defined as?

The turning effect of a force about a pivot

What equation is used to calculate the size of the moment?

• M = F x d

• M = moment in newton metres (Nm)

• F = force in newtons (N)

• d = perpendicular distance of the force to the pivot in metres (m)

Why is the door handle placed on the opposite side to the hinge for example?

Why would opening a door with a handle close to the pivot be much harder?

• The perpendicular distance from the pivot (hinge) is larger, so this is a larger moment (turning effect) making the door easier to open

• It would require a lot more force

What are the units for moments?

Nm

Why would an object be balanced?

The total clockwise moment about a pivot = the total anticlockwise moment about that pivot

What does the principle of moments state?

If an object is balanced the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot

What can both simple lever and gear systems be used to do?

What concept can this be achieved with?

• Transmit and amplify the rotational effects of forces

• Can be achieved using concept of moments

What do levers do?

Where must the force be applied to a lever?

What 2 things can you do to make a lever work better?

• Increase the size of the force acting on an object to make the object turn more easily

• The force applied to a lever must act further from the pivot than the force has to overcome

• Increase size of force applied

• increase distance of the force from the pivot

Give one example of a lever

How does the … use a lever and how does it work?

• a bottle opener

• It uses a lever to amplify the small force upwards applied by the person to create a large force upwards on the bottle top to remove it

• The line of action of the small force is much further from the pivot than the large force that is needed at the edge of the cap to remove it

• The bottle opener (lever) makes use of moments to act as a force multiplier

Give another example of a type of lever?

What does it do?

How does it work?

• crowbar

• used to exert a large force to a narrow opening which helps lift heavy objects

• the small force acting downwards applied by a person is far away from the pivot

• this creates a large force upwards on the heavy object, making it easier to lift

What do gears do?

similar to levers, multiply the effect of a turning force using moments

What do gears consist of?

How do they work?

• wheels with toothed edges that rotate on an axle or shaft, which acts as a pivot

  • the teeth of one gear fit into the teeth of another gear

  • this lets one gear turn the other, meaning one axle or shaft can be used to turn another shaft

As one gear turns, what must happen to the other?

Where the gears meet, what direction will the teeth then move in?

What happens after this?

• it must also turn

• The same direction

• one of the gears will then move clockwise, and the other anticlockwise (in opposite directions)

Will the moment be the same on both gears?

What will it depend on?

• no

• the size of the gear, which changes the distance of the teeth to the pivot (axle)

If a large gear is driven by a smaller gear, what will happen and what gear is this similar to on a bike or car?

What about the other way around?

Why are these the case?

• the large gear will rotate slower than the smaller gear but will have a greater moment, low gear on bike or car

• the smaller gear will rotate quicker than the larger gear but will have a smaller moment, e.g. a high gear on a bike or car

• because the turning force on the larger gear wheel acts further from its pivot than the turning force of the smaller gear wheel acting on its own pivot