Turning Forces and Levers

Junior Science

Lesson Objectives• Understand the terms - Lever, Fulcrum, Turning effect, Moment

• Recall the formula M= Fd and be able to calculate the moment of a force

• Be able to explain why an object with forces acting on it isn’t turning

• Be able to explain why an object at rest doesn’t turn.

• Be able to calculate the size of a force (or it’s perpendicular distance from

a pivot) acting on an object that is balanced

• Explain, in terms of forces, why objects with pivot points appear at rest or are moving in various situations.

• calculate the size of a force (or it’s perpendicular distance from a pivot) acting on an object that is balanced in a number of contexts.

• Understand equilibrium and the Law of the Lever.

• Be able to carry out simple calculations using a lever.

Turning Effects

When we apply a force, it sometimes causes an object to turn or twist.

• This turning effect of a Force is called the

Moment (M)

i.e. opening a bottle, turning a spanner

• The turning effect can be increased by

- increasing the size of the force

- increasing the distance from the turning point

Turning Effect (Moment)

Moment = Force x perdendicular Distance

A Lever is a rigid body free to move about a fixed point called the fulcrum.

• Levers are machines used to increase force. They are

called "simple machines" because they have only two parts

- the handle and the fulcrum.

• In a lever a force called the EFFORT is used to overcome a resisting force called the LOAD.

• The pivotal point is called the FULCRUM.

What is a Lever?

Examples of Levers

Can you locate the Effort, Load and

Fulcrum on each of the following levers?

Scissors

Effort

Load

Fulcrum

Wheelbarrow

Effort

Load

Fulcrum

The Human Arm

Effort

Load

Fulcrum

Spanner

Effort

Fulcrum

Types of Levers

• There are three main types of levers

– First Class Lever

– Second Class Levers

– Third Class Levers.

• In each type the fulcrum, effort and load are located at difference positions.

A First Class LeverThe First class lever is a very common type of lever.

The fulcrum is in the centre with the effort and load on the outside.

Examples of first class levers include: A seesaw, pliers, scissors.

LoadEffort

Fulcrum

Second Class Lever

A second class lever is one in where the load is in the centre and the effort and fulcrum on the outside.

Examples include a Wheelbarrow, nutcracker, and bottle opener.

Load

Effort

Fulcrum

Third Class Lever

Third class levers are hard to find. They have the effort in the centre and fulcrum and load on the outside.

Examples include Hammers and fishing rods.

Load

EffortFulcrum

Law of Moments

The Turning effect or ‘Moment’ of a force depends on the size of the force and the

distance from the fulcrum.

Moment = Force x Perpendicular distance (from the fulcrum)

M = F(N) x d(m)

The Law of the Lever

F1

F2

F1d1 = F2d2

When a body is in equilibrium the sum of the clockwise moments is equal to the sum of the anti-clockwise moments

about the same point.

When a body is in equilibrium the sum of the clockwise moments is equal to the sum of the anti-clockwise

moments about the same point.

Weight = Force

W = F

The prove the law of the lever

1. Draw diagram of equipment in copy

2. Copy table below

3. Carry out three calculations to test the law of the lever

F1 d1 F2 d2F1d1 F2d2

(N) (m) Nm (N) (m) Nm

Law of the Lever

25cm

20cm

20N F

F1 d1 = F2 d2

20N X 0.25m = F2 x (0.20m)

5Nm = F2 x (0.20m)

5Nm / 0.20m = F2

25Nm = F2

Law of the Lever

25cm

10cm 30cm

10N20N W

(0.25m x 20N) + ( 0.10m x 10M) =

5Nm + 1Nm

6Nm

6Nm / 0.30m

20N =

(W x 0.30)

W

Law of the Lever40cm

d

20N 50N

(20N x 0.4m) =

8Nm

8Nm/50N =

0.16m =

Position is at the 66cm Mark

(50N x d)

d

d

Law of the Lever

5 cm 60 cm

10N

F

(10N x 0.45cm) =

4.5Nm =

4.5Nm / 0.1m

45N =

(F x 0.10m)

F

F

Summary• This turning effect of a Force is called the Moment (M)

Moment = Force x Perpendicular distance (from the fulcrum)

• The turning effect can be increased by increasing the size of the force or the distance from the turning point.

• A Lever is a rigid body free to move about a fixed point called the fulcrum.

• Levers are machines used to increase force. They are called "simple machines" because they have only two parts the handle and the fulcrum.

• In a lever a force called the EFFORT is used to overcome a resisting force called the LOAD. The pivotal point is called the FULCRUM.

• The Law of the lever states thatWhen a body is in equilibrium the sum of the clockwise moments is equal to the sum of the anti-clockwise moments about the same point.

F1 d1 = F2 d2