Question 8
(a) (i) Define relative motion.
(ii) Two cars A and B approach each other with velocities 120 km hr-1 and 42 km hr-1 respectively. Determine the velocity of A relative to B
(b) A body of mass, m, is placed on a rough wooden board inclined at angle 0 to the horizontal using a labeled diagram, show
(i) the forces acting on the body;
(ii) that the coefficient of friction μ between the surfaces can be expressed as μ = tan ϴ, if the body is in equilibrium.
(c) State two applications of a centrifuge.
(d) A body of mass 17 kg, attached to a string of length 3.4 m is whirled round in vertical circle. If the tension in the string at the lowest point is 300 N, calculate the speed of the body.
Observation
Part (a) Well attempted by most candidates. Performance was average
Part (b) Candidates performance was below average.
Part (c) Candidates performance wasbelow average.
Part (d) Candidates performance was below average.
The expected responses:
(a) (i) Definition of relative motion
Movement of a body with respect to another body
OR
Movement of a body as seen from the position of another body also in motion or at rest.
(ii) Determination of velocity of A relative to B
VAB = VA – (–VB)
=120 – (– 42)
= 162 km hr –1
(b) (i) Diagram showing forces acting on the body on an inclined plane
(c) Applications of centrifuge
- removal of water from wet clothes (in washing machine)
- separation of particles from liquids in which they are suspended
- separation of solid constituents of blood
- separation of two liquids of different densities
(d) Calculation of speed of body
