Transcript Hooke\'s Law Moments.ppt

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 length
Extended length- total length with load
applied
Natural length
Extended length
Force
Applied
Extension
Elasticity and Plasticity
Force
2.
1.
Elastic Limit
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
10
cm
4 cm
4 cm
4 cm
2N
4N
6N
Force Applied
(N)
Extended Length
of spring (cm)
Extension of
spring (cm)
0
10
0
2
14
4
4
18
8
6
22
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
limit 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 factors1. Size of the force acting on the object
2. 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 leverEffort
Fulcrum/Pivot
Load
Effort
Lever
Moment
Pivot
Load
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 formulaEffort or Load
Moment = Force x Distance (from force to pivot)
Units
M
Nm
=
=
F
N
x
x
d
m
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 forces from smaller ones
(opening a tin of paint with a screwdriver)
(Effort)
30cm
5N
Moment = Force x (perpendicular) distance
= 5 N x 0.3 m
= 1.5 Nm
(Clockwise Direction)
See-saw
Principle of Moments
 For a lever to be balanced…
the clockwise turning effect M must equal the
anti-clockwise turning effect M
OR
there must be no resultant moment
Written as an equationM=M
FA x dA = FB x dB
Principle of Moments
Word Equation
ForceA x DistanceA = ForceB x DistanceB
dB
dA
FB
FA
Practice Question
What force would be needed to balance the
beam shown below?
3m
2m
600N
FA
ForceA x DistanceA = ForceB x DistanceB
F
x
2
= 600
x 3
F
= 1800 / 2
F
= 900 N