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Physics 9702 Doubts | Help Page 244

__Question 1127:
[Deformation > Hooke’s law]__

For a wire, Hooke’s law is obeyed
for a tension F and extension x. The Young modulus for the material of the wire
is E.

Which expression represents the
elastic strain energy stored in the wire?

A ½ Ex B Ex C ½ Fx D Fx

**Reference:** *Past Exam Paper – November 2008 Paper 1 Q23*

__Solution 1127:__

**Answer: C.**

As Hooke’s law is obeyed, a graph of
tension against extension would be a straight line passing through the origin.

The elastic strain energy is given
by the area under the graph. This would correspond to that of a triangle.

Elastic strain energy = ½ Fx

__Question 1128:
[Work, Energy and Power]__

Which statement is correct?

A A ball lands on the ground and
bounces. The kinetic energy changes sign, because the ball changes direction.

B A car drives up a slope at a
steady speed. The power generated by the engine equals the potential energy
gained per unit time.

C An electric heater can be 100%
efficient.

D It is impossible for momentum to
be conserved in a collision.

**Reference:** *Past Exam Paper – June 2015 Paper 11 Q17*

__Solution 1128:__

**Answer: C. **

Choice A: Kinetic energy is a scalar and does not have a direction.

Choice B: The power from the engine does not just increase the potential
energy of the car. It is also providing for the kinetic energy – even if it is
constant, it is not created in its own. The KE comes from the fuel.
Additionally, there would be a frictional drag on the car and energy is
required to overcome this. This energy comes from the engine.

Choice C: This is correct. Usually, energy is dissipated as heat in
systems but for an electric heater this heat is useful. So, all the output
power is useful in this situation.

Choice
D. This is obviously wrong.
__Question 1129:
[Kinematics > Terminal velocity]__

A stone of mass m is dropped from a
tall building. There is significant air resistance. The acceleration of free
fall is g.

When the stone reaches its terminal
velocity, which information is correct?

**Reference:** *Past Exam Paper – November 2011 Paper 12 Q7*

__Solution 1129:__

**Answer: B. **

At terminal velocity, the speed is
constant. So, there is no acceleration and the resultant force on the stone is
zero.

The force of gravity always acts on
the stone and is equal to mg.

For
the resultant force to be zero, the air resistance should be of the same
magnitude (= mg).