hooke’s law and moments gcse physics. learning intentions by the end of the lesson we will be able...
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Hooke’s Law and Moments
GCSE Physics
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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
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Effective Forces
Force on an object can have the following effects-
What happens to the balloon after the force is no longer applied?
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Tacoma Narrows Bridge
7 Nov 1940 Washington
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Balloon Stretch
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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.
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Terminology
Extension- change in lengthExtended length- total length with load
applied
Force Applied
Natural length
Extended length
Extension
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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)
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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
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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
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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
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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
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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)
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Turning Effect of a Force
The greater the distance from the pivot that the force acts, the greater the turning effect
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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
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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
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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
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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.
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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)
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See-saw
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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
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Principle of Moments
Word EquationForceA x DistanceA = ForceB x DistanceB
dB
FB FA
dA
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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