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.