Calculating work done, Hookes law and the limit of proportionality, elastic and inelastic deformation, and the required practical on force and extension.
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1.What does work done mean?
Energy transferred when a force moves an object through a distance.
2.State the equation for work done.
work done = force x distance moved along the line of action of the force
3.What is the unit of work done?
The joule, J. One joule equals one newton metre.
4.A force of 50 N moves an object 4 m. Calculate the work done.
50 x 4 = 200 J.
5.Why does no work get done if an object does not move?
Work requires movement in the direction of the force, so with no distance moved there is no energy transfer.
6.Why do brakes get hot?
Work is done against friction, transferring energy to the thermal store of the brakes.
7.What is needed to change the shape of an object?
More than one force, otherwise the object would simply move rather than deform.
8.What is elastic deformation?
The object returns to its original shape when the forces are removed.
9.What is inelastic deformation?
The object does not return to its original shape when the forces are removed.
10.State Hookes law.
The extension of a spring is directly proportional to the force applied, provided the limit of proportionality is not exceeded.
11.State the equation linking force and extension.
force = spring constant x extension
12.What is the unit of the spring constant?
Newtons per metre.
13.A spring with constant 50 N/m is stretched by 0.2 m. Calculate the force.
50 x 0.2 = 10 N.
14.What is extension?
The increase in length compared with the original unstretched length.
15.A spring is originally 10 cm and stretches to 14 cm. What is the extension?
4 cm.
16.What is the limit of proportionality?
The point beyond which extension is no longer directly proportional to force.
17.What does a force against extension graph look like below that limit?
A straight line through the origin.
18.What happens to the graph beyond the limit of proportionality?
It curves, as the extension increases more than proportionally for each extra newton.
19.What does the gradient of a force against extension graph represent?
The spring constant.
20.A stiffer spring has what kind of gradient?
A steeper one, because a larger force is needed for the same extension.
21.State the equation for energy stored in a stretched spring.
elastic potential energy = 0.5 x spring constant x extension squared
22.Calculate the energy stored in a spring with constant 100 N/m extended 0.2 m.
0.5 x 100 x 0.04 = 2 J.
23.In the required practical, what is the independent variable?
The force applied, usually by hanging masses on the spring.
24.What is the dependent variable in that practical?
The extension of the spring.
25.Why is the length measured before any masses are added?
To find the original length, so extension can be calculated by subtraction.
26.Why should the spring be checked after unloading?
To confirm it has returned to its original length, showing the deformation was elastic and the limit was not exceeded.
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