P2.1.5 Forces and Elasticity

P2 Physics

KS4 ADDITIONAL SCIENCEMr D Powell

Mr Powell 2012Index

Connection

• Connect your learning to the content of the lesson

• Share the process by which the learning will actually take place

• Explore the outcomes of the learning, emphasising why this will be beneficial for the learner

Demonstration

• Use formative feedback – Assessment for Learning

• Vary the groupings within the classroom for the purpose of learning – individual; pair; group/team; friendship; teacher selected; single sex; mixed sex

• Offer different ways for the students to demonstrate their understanding

• Allow the students to “show off” their learning

Activation

• Construct problem-solving challenges for the students

• Use a multi-sensory approach – VAK• Promote a language of learning to

enable the students to talk about their progress or obstacles to it

• Learning as an active process, so the students aren’t passive receptors

Consolidation

• Structure active reflection on the lesson content and the process of learning

• Seek transfer between “subjects”• Review the learning from this lesson and

preview the learning for the next• Promote ways in which the students will

remember• A “news broadcast” approach to learning

Mr Powell 2012Index

P2.1.5 Forces and Elasticity

a) A force acting on an object may cause a change in shape of the object.

b) A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.

c) For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape.

d) The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded:

F = ke

F, is the force in Newtons, Nk, is the spring constant in Newtons per metre, N/me, is the extension in metres, m

Mr Powell 2012Index

a) A force acting on an object may cause a change in shape of the object C

Task: Think about loading each of these objects with weight and seeing how they extend. We can plot graphs to show the loading and then unloading curves.

1. Draw them in your book.

2. Pair up with another person and discuss why this happens?

3. Write down your findings.

Mr Powell 2012Index

b) A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy. A A spring by it’s nature wants to be

coiled.

If you apply a force using a weight it works against the structure of the spring

The more force applied the more it will extend.

If we release the spring the “work” that we have “done” (WD = F x d) is recovered as it is “stored” inside the spring.

The sankey diagram shows what occurs when a spring is released.

Kinetic

Thermal (wasted)

EPE Spring

Mr Powell 2012Index

c) For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape.

A When you load a wire and stretch in

it behaves in a proportional way.

If you double the force you also double the extension.

This only happens till you get to the “elastic limit”

If you go beyond this the wire will permanently deformed

Below this the wire acts like a spring and you recover the “Work Done”

Mr Powell 2012Index

Rubber and Polythene....

Explaining stiffness and elasticity

Polythene

polythene is along flexiblechain moleculewhich folds up

bondrotates

stretching can rotate some bonds,making the folded chain longer

Young modulus

~108 — 109 Pa

chains arefolded

bondrotates

Stretching polythene rotates bonds

Elastic extensibility ~ 1%

Rubber

Rubber stretches and contracts by chains uncoiling and coiling up again. Rubber is elastic, not plastic.

In stretched rubber, the chain bonds rotate, and chainsfollow straighter paths between cross-links. When let go,the chains fold up again and the rubber contracts.

In unstretched rubber, chains meander randomlybetween sulphur cross-links.

sulphur cross-linkssulphur cross-links

Elastic extensibility > 100%

Rubber

Rubber stretches and contracts by chains uncoiling and coiling up again. Rubber is elastic, not plastic.

In stretched rubber, the chain bonds rotate, and chainsfollow straighter paths between cross-links. When let go,the chains fold up again and the rubber contracts.

In unstretched rubber, chains meander randomlybetween sulphur cross-links.

sulphur cross-linkssulphur cross-links

Elastic extensibility > 100%

Mr Powell 2012Index

d) The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded: ASpring Extension... Hookes Law

We need to think about the form

F = k∆L

Where the force is related by a constant for the spring.

Mr Powell 2012Index

Example Results for one or two springs (in parallel)

Parallel Results

Extension (m) Force (N)

0.000 00.005 10.017 20.035 30.050 4

0.000 0.010 0.020 0.030 0.040 0.050 0.0600

0.51

1.52

2.53

3.54

4.5

Force (N)

Extension (m)

Forc

e (N

)Series Results

Extension (m) Force (N)

0.000 0

0.035 1

0.090 2

0.175 3

0.254 4 0.000 0.050 0.100 0.150 0.200 0.250 0.3000

1

2

3

4

5

Force (N)

Extension (m)

Forc

e (N

)

P2.1.5 Forces and Elasticity

a) A force acting on an object may cause a change in shape of the object.

b) A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.

c) For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape.

d) The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded:

F = ke

F, is the force in Newtons, Nk, is the spring constant in Newtons per metre, N/me, is the extension in metres, m

P2.1.5 Forces and Elasticity

a) A force acting on an object may cause a change in shape of the object.

b) A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.

c) For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape.

d) The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded:

F = ke

F, is the force in Newtons, Nk, is the spring constant in Newtons per metre, N/me, is the extension in metres, m

P2.1.5 Forces and Elasticity

a) A force acting on an object may cause a change in shape of the object.

b) A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.

c) For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape.

d) The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded:

F = ke

F, is the force in Newtons, Nk, is the spring constant in Newtons per metre, N/me, is the extension in metres, m

Mr Powell 2012Index

human scale

Looking inside metals and ceramics

stiffnesselectricalopticalthermal

yield strength(metals)

yield strength(metals)

fracturestrength(ceramics)

XXXX

crack

1 m

1 mm

1 m

1 nm

1 pm

precipitates

arrays of dislocations

alloying element

dislocation

atoms and electrons

Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.

human scale

Looking inside polymers

electrical

stiffnessthermaloptical

flow

strength

XXX X

H

C

H

H

C H

C

H

H

C

H

H

C

H

H

C

C

H

H

C

H

H

C

H

H

C

H

C

C

C

1 m

1 mm

1 m

1 nm

1 pm

craze

tangled molecules

molecules

atoms

Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.