Transcript Slide 1
P2.1.5 Forces and Elasticity
P2 Physics
Mr D Powell
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
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
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
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
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, N k, is the spring constant in Newtons per metre, N/m e, is the extension in metres, m
a) A force acting on an object may cause a change in shape of the object
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.
b) A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.
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.
EPE Spring Kinetic Thermal (wasted)
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.
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”
Rubber Rubber and Polythene....
sulphur cross-links sulphur cross-links sulphur cross-links sulphur cross-links
Explaining stiffness and elasticity
between sulphur cross-links.
Polythene
In stretched rubber, the chain bonds rotate, and chains follow straighter paths between cross-links. When let go, the chains fold up again and the rubber contracts.
In unstretched rubber, chains meander randomly between sulphur cross-links.
Elastic extensibility > 100% chains are folded bond rotates follow straighter paths between cross-links. When let go, the chains fold up again and the rubber contracts.
Elastic extensibility > 100% stretching can rotate some bonds, bond rotates polythene is a long flexible chain molecule which folds up Elastic extensibility ~ 1% Stretching polythene rotates bonds
Young modulus ~10 8 — 10 9 Pa
d) The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded: Spring Extension... Hookes Law
We need to think about the form
F = k∆L
Where the force is related by a constant for the spring.
Example Results for one or two springs (in parallel)
4,5
Parallel Results
4 3,5
Extension (m)
0.000
0.005
0.017
0.035
Force (N)
0 1 2 3 3 2,5 2 1,5 1 0,5 0 0,000 0.050
4 0,010
Series Results
Extension (m) Force (N) 0.000
0.035
0 1 0.090
0.175
0.254
2 3 4 5 4 3 2 1 0 0,000 0,050
Force (N)
0,020 0,030
Extension (m)
0,040
Force (N)
0,100 0,150
Extension (m)
0,200 0,050 0,250 0,060 0,300
P2.1.5 Forces and Elasticity
a) b) A force acting on an object may cause a change in shape of the object.
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, N k, is the spring constant in Newtons per metre, N/m e, is the extension in metres, m
P2.1.5 Forces and Elasticity
a) b) A force acting on an object may cause a change in shape of the object.
A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.
c) d) 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.
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, N k, is the spring constant in Newtons per metre, N/m e, is the extension in metres, m
P2.1.5 Forces and Elasticity
a) b) A force acting on an object may cause a change in shape of the object.
A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.
c) d) 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.
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, N k, is the spring constant in Newtons per metre, N/m e, is the extension in metres, m
Looking inside metals and ceramics
human scale 1 m X X X X
Looking inside polymers
XXX X craze 1 mm fracture strength (ceramics) crack strength 1 m precipitates arrays of dislocations yield strength (metals) alloying element dislocation yield strength (metals) 1 nm stiffness electrical optical thermal atoms and electrons Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.
1 pm flow 1 m human scale 1 mm 1 m 1 nm tangled molecules stiffness thermal optical H C H C H C H C H H C H H C H H C H H C H H C H H C H 1 pm molecules C electrical C C atoms
Source: MF Ashby and HR Shercliff, Cambridge University Engineering Department.