Hooke’s Law and Moments

GCSE Physics

Learning Intentions

By the end of the lesson we will be able to…

Understand the meaning of elastic and plastic behaviour

Investigate the relationship between force applied and the resulting extension

State and use Hooke’s Law to solve problems

Effective Forces

Force on an object can have the following effects-

What happens to the balloon after the force is no longer applied?

Tacoma Narrows Bridge

7 Nov 1940 Washington

Balloon Stretch

Elasticity and Plasticity

All materials will behave elastically or plastically-

For elastic behaviour – when a force is applied the change in length is proportional to the force. The object will return to its original shape when the force is taken away.

For plastic behaviour – the force and the change of length are not linked. A permanent deformation occurs when the force is taken away.

Terminology

Extension- change in lengthExtended length- total length with load

applied

Force Applied

Natural length

Extended length

Extension

Elasticity and Plasticity

Elastic Limit1.

2.

Force

Extension

1.Elastic region (any force applied below the elastic limit)

2.Plastic region (any force applied above the elastic limit)

Fill out the table for the springs

4 cm

4 cm

4 cm

10 cm

2 N4 N

6 N

Force Applied (N)

Extended Length of spring (cm)

Extension of spring (cm)

10

14

18

22

0

2

4

6

0

4

8

12

Hooke’s Law

The extension of the object’s length will be proportional to the load causing that extension provided the elastic limit is not exceeded

E.g. if the force is doubled, the extension is doubled

Hooke’s Law

If a material is loaded beyond its elastic elastic limitlimit then Hooke’s Law no longer applies.

Pg 46

Q 24 - 27

Learning Intentions

By the end of the lesson we will be able to…

Recognise the turning effect caused by a force

Recall the meaning of the term ‘moment of a force’ and the moment equation

Use the moment equation to solve simple problems

Turning Effect of a Force (pg 46)

The turning effect of a force is called a MOMENT

It depends on two factors-1. Size of the force acting on the object2. The distance the force acts from the pivot

The PIVOT is the point at which the rotation or turning effect occurs around (eg. The hinge of a door)

Turning Effect of a Force

The greater the distance from the pivot that the force acts, the greater the turning effect

Levers

Levers are any objects which experience a turning effect or MOMENT

There are three basic parts to the lever-

Fulcrum/PivotLoadEffort

Pivot

Effort

Load

LeverMoment

Turning Effect of a Force

Draw a diagram to represent the object and mark on the pivot, the forces applied ‘load and effort’ and the distance (between the pivot and the force)

- Wheelbarrow

- Scissors

- Tweezers

- Wrench

Learning Intentions

By the end of the lesson we will be able to…

Recognise the turning effect caused by a force

Recall the meaning of the term ‘moment of a force’ and the moment equation

Use the moment equation to solve simple problems

Moment Equation

The size of the turning effect due to a force can be calculated from the formula-

Moment = Force x Distance (from force to pivot)

M = F x d

Nm = N x m

Effort or Load

Units

A moment is a vector quantity. It has both magnitude and direction. A moment can act either in a clockwise direction or

an anti-clockwise direction.

Levers can be used to…

produce large forceslarge forces from smaller ones (opening a tin of paint with a screwdriver)

Moment = Force x (perpendicular) distance

= 5 N x 0.3 m

= 1.5 Nm

30cm 5N(Effort)

(Clockwise Direction)

See-saw

Principle of MomentsFor a lever to be balanced…the clockwise turning effect M must equal the

anti-clockwise turning effect MORthere must be no resultant moment

Written as an equation-

M = MM = M

FFAA x d x dAA = F = FBB x d x dBB

Principle of Moments

Word EquationForceA x DistanceA = ForceB x DistanceB

dB

FB FA

dA

Practice QuestionWhat force would be needed to balance the

beam shown below?

ForceA x DistanceA = ForceB x DistanceB

F x 2 = 600 x 3F = 1800 / 2F = 900 N

3m

600N FA

2m