Mechanics

Question 1

Explain the modelling assumptions that the string is light and inextensible.

Answer 1

• LIGHT- tension is same throughout the string + mass of string negligible
• INEXTENSIBLE- same acceleration for both objects

Question 2

Van comes to rest after its engine cut out. In the model of the situaton it is assumed that the van is subjected to a resistive force of magnitude 200N.
Comment on the suitability of the modelling assumption.

Answer 2

Air R unlikely to be of constant magnitude.

Question 3

What is Newton’s 3rd law ?

Answer 3

every action has an equal and opposite reaction

Question 4

What is the assumption made when the pulley is smooth ?

Answer 4

tensions in the 2 parts of the string are the same

Question 5

Table pulley question:
The string passes over a pulley P which is fixed at the edge of the table. The other end of
the string is attached to a small ball B hanging freely, vertically below P and with B at a height of 1 m above the horizontal floor.
Suggest two improvements that could be made in the model. (2)

Answer 5

1. include a ↑ accurate value of g
2. include dimensions of ball –> distance falls changes
3. variable acceleration instead of constant
4. include dimensions of pulley –> string x parallel to table

Question 6

A ramp rests on a smooth solid cylindrical drum which is partly under the ground. The point of contact between the ramp and the drum is C.
Explain why the reaction from the drum on the ramp at point C acts in a direction which is perpendicular to the ramp. (1)

Answer 6

drum is smooth