5.2 Forces and elasticity

Question 1

Changing shape

Answer 1

For stationary objects, more than one force has to be applied to change their shape.
Their shape can change by:
Stretching (forces in opposite directions away from the object).
Bending (forces that distort the object).
Compressing (forces in opposite directions towards the object).

Question 2

Elastic deformation

Answer 2

An elastically deformed object will return to its original shape when the force stops.
A spring is an elastic object.
Springs return to their original shape when forces stop acting on them.

Question 3

Inelastic deformation

Answer 3

An inelastically deformed object will not return to its original shape when the force stops.
A car is an inelastic object.
After a car has crashed into a tree, it will not return to its original shape.

Question 4

Hooke’s law

Answer 4

The extension of an object that obeys Hooke’s law is directly proportional to the force applied until the limit of proportionality.
The higher the spring constant, the “stiffer” the spring and the more force is needed to stretch it.
The limit of proportionality is the point where Hooke’s law breaks down.
If a spring is stretched too much, it will not return to its original length when the force stops acting on the spring.
F = ke

F = force in newtons (N)
k = spring constant in newtons per metres (N/m)
e = extension in metres (m)

Question 5

Work done on a spring

Answer 5

When a spring is stretched or compressed by a force, work is done by the spring.
Work done is the transfer of energy.
The energy is transferred to its elastic potential energy store.
The energy stored in an elastic object when work is done on the object
Provided the spring is not inelastically deformed (i.e has not exceeded its limit of proportionality), the work done on the spring and its elastic potential energy stored are equal.
Ee = ½ke^2

Ee = elastic potential energy in joules (J)
k = spring constant in newtons per metre (N/m)
e = extension in metres (m)

Question 6

Work done

Answer 6

Work is done when an object is moved over a distance by a force applied in the direction of its displacement.
W = Fs

W = work done in Joules (J) or newton-metres (N m)
F = force in Newtons (N)
s = distance in metres (m)

Question 7

Friction

Answer 7

Friction is a force that works in opposition to the motion of an object.
This slows down the motion of the object.
When friction is present, energy is transferred by heating
This raises the temperature (energy is transferred to the thermal store) of the object and its surroundings.
The work done against the frictional forces causes this rise in the temperature
Imperfections at the interface between the object and the surface bump into and rub up against each other.

Question 8

Air resistance

Answer 8

Air resistance is a type of friction that slows the motion of an object.
Particles bump into the object as it moves through the air.
As a result, energy is transferred by heating due to the work done against the frictional forces.

Question 9

Practical 6 (investigating force and extension)

Answer 9

Aim: investigate the relationship between force and extension for a spring.
Procedure - Set up the apparatus as shown in the diagram, initially without any masses hanging from the spring.
Align the marker to a value on the ruler, record this initial length of the spring
Add the 100 mass hanger onto the spring.
Record the mass (in kg) and position (in cm) from the ruler now that the spring has extended.
Add another 100 g to the mass hanger.
Record the new mass and position from the ruler now that the spring has extended further.
Repeat this process until all masses have been added.
The masses are then removed and the entire process repeated again, until it has been carried out a total of three times, and an average length is calculated.