Mechanics and Material Questions

Question 1

Define the moment of a force about a point (2)

Answer 1

• force x perpendicular distance
• between the line of action of force and the point

Question 2

State and explain one reason why the core of an optical fibre is made as thin as possible (2)

Answer 2

to reduce multipath or multimode dispersion

2 (which would cause) light travelling at different angles to arrive at different times/pulse broadening/merging of adjacent pulses/’smearing’/ poor resolution/lower transmission rate/lower bandwidth/less distance between regenerators

or to prevent light/data/signal loss (from core or fibre)

(which would cause) signal to get weaker/attenuation/crossover/data to be less secure

Question 3

Explain, without doing a calculation, how the motion of two trucks immediately after a head on collision would be for a collision that is perfectly elastic. (2)

Answer 3

• Trucks move in opposite directions/rebound
• the direction of momentum is conserved so if the trucks travel at different speeds or are different masses, they wont rebound with the same speed/momentum.

Question 4

The student uses the relationship in question 02.1 to predict frequencies for tensions (in a staionary wave on a string of fixed length) that are much larger than those used in the original experiment. Explain how the actual frequencies produced would be different from those that the student predicts. (2)

She uses a formula.

Answer 4

appreciation of reducing diameter when string is stretched.

lower mass per unit length so (constant of proportionality and hence) frequency is higher (than would be predicted)