Module 3 - Section 4 - Hooks Law

Question 1

What does Hooks law state ?

Answer 1

Hooks law states that extension ( x ) is proportional to the load or force ( F ) - this relationship is called hooks law

Question 2

What happens if a metal wire is supported at the top and then a weight is attached to the bottom of the wire ?

Answer 2

The wire stretches - the weight pulls down with force F, producing an equal and opposite force at the support

Question 3

When does a material only deform ( Stretch, Bend, twist etc ) ?

Answer 3

A material will only deform if there is a pair of opposite forces acting on it ( one supporting the material and one attached to the bottom of the material )

Question 4

What is the equation of Hooks law ?

Answer 4

F = kx

F = force ( load ) (N)
k = spring constant ( the stiffness of the spring ) ( Nm*-1 )
x = extension ( m ) - length of stretched spring - original length of spring

Question 5

What do you have to do in order to stretch/compress ( change the length ) of a spring ?

Answer 5

You have to apply a pair of opposite forces

Question 6

What do tensile forces do ? And what do compressive forces do ?

Answer 6

Tensile forces stretch the spring

Compressive forces squash the spring

Question 7

Does Hooks law only apply to tensile forces ?

Answer 7

No hooks law also applies to compressive forces

Question 8

Is the force constant the same for compressive and tensile forces ?

Answer 8

For a spring - the spring constant ‘k’ has the same value whether the forces are tensile or compressive - However that’s no true for all materials

Question 9

What happens to hooks law when the load is great enough ?

Answer 9

Hooks law stops working when the load is great enough

Question 10

What happens to the graph of force against extension when the force ( load ) becomes great enough ?

Answer 10

When the load on the spring or wire becomes great enough the graph start to curve

Question 11

What’s the difference between the limit of proportionality and elastic limit ?

Answer 11

The limit of proportionality is the extension beyond which Hooke’s law no longer applies, i.e. the extension is no longer proportional to the force. The elastic limit is the extension beyond which the object will not return to it’s original shape when the force is removed.

Question 12

what happens if you exceed the elastic limit ?

Answer 12

If you exceed the elastic limit then the material will be permanently stretched and when all force is removed, the material will be longer than at the start

Question 13

What materials only obey hooks law for really small extensions ?

Answer 13

There are materials such as rubber that only obey hooks law for very small extensions

Question 14

What happens if a force is applied to more than on spring ?

Answer 14

Then you can combine the force constants of the individual springs to find the overall force constant of the system - you can then treat the system as one spring with force constant ( k )

Question 15

how can you combine the spring constant of the springs ?

Answer 15

You can do this by finding out how the springs are arranged

This can be either in series or in parallel

Question 16

How do you combine the spring constants if the springs are in parallel and in series

Answer 16

if the springs are in parallel ( attached to each other and hanging from each other ) then you use the equation: 1/k = 1/k1 + 1/k2
You plot the spring constant of each spring into the equation so spring one you would put its spring constant into k1

If the springs are in parallel - then you use the equation - k = k1 + k2

Question 17

what are the two types of deformations ?

Answer 17

Plastic and elastic

Question 18

what’s an elastic deformation ?

Answer 18

If a deformation is elastic then the material returns to its original shape once the forces are removed

Question 19

What type of deformation will be shown when a material is stretched up to its elastic limit ? and then what type of deformation will it show after the elastic limit ?

Answer 19

The material will show elastic deformation up to its elastic limit then show plastic deformation after surpassing the elastic limit

Question 20

What happens subatomic-ally when you deform something elastically ?

Answer 20

• When the material is put under tension, the atoms of the material are pulled apart from one another
• Atoms can move slightly relative to their equilibrium positions ( move slightly out of place ) without changing position in the material ( without changing the materials arrangement of atoms )
• Once the load is removed, the atoms return to their equilibrium distance apart ( return to their original positions )

Question 21

what subatomically happens when you plastically deform something ?

Answer 21

Some atoms in the material move position relative to one another

When the load is removed, the atoms don’t return to their original positions

Question 22

How can you investigate extension ? Practical

Answer 22

Get a clamp stand, a ruler, a spring or a material that you want to test, then get some weights.

Attach the material that you want ton test to the clamp and attach the clamp to the clamp stand - then attach the ruler to the clamp stand

Measure the original length of the material ( spring ) with the ruler - then add weights to the bottom of the spring one at a time - after each weight added measure the new length of the spring and then calculate the extension for each new weight added

Then plot a graph of force ( weight ) against extension for your results - IF the line of best fit is STRAIGHT then the object obeys hooks law and the gradient is ‘k’ - if you have loaded the spring above its limit of proportionality then the graph will start to curve and this is because the limit of proportionality is the point when a material no longer supports hooks law therefore the force is not proportional to the extension

Make sure you are carrying out the experiment safely - you should be standing up so you can get out the way quickly if the weights fall - and wear safety goggles so that if the material snaps it wont hit you eye