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 Force Applied Natural length Extended length Extension

Elasticity and Plasticity Elastic Limit 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 10 cm 2 N 4 N 6 N Force Applied (N) Extended Length of spring (cm) Extension of spring (cm)

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 elastic limit If a material is loaded beyond its elastic limit then Hooke’s Law no longer applies. Pg 46 Q

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 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 lever- Fulcrum/Pivot Load Effort Pivot Effort Load Lever Moment

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… large forces produce large 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 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 equation- M = M F A x d A = F B x d B

Principle of Moments Word Equation Force A x Distance A = Force B x Distance B dBdB FBFB FAFA dAdA

Practice Question What force would be needed to balance the beam shown below? Force A x Distance A = Force B x Distance B Fx2 =600 x 3 F = 1800 / 2 F = 900 N 3m 600N FAFA 2m