The diagram above illustrates a body of mass 5.0 kg being pulled by a horizontal force F. If the body accelerates at 2.0 ms-2 and experiences a frictional force of 5 N, calculate the:
- net force on it;
- magnitude of F:
- coefficient of kinetic friction
[g = 10 ms-2 ]
Part (a) Several candidates rather gave the name of the objects performing the specified motion in place of the description of motion made by the objects that answers the question. For example, candidates named the earth instead of movement of the earth about its axis; fan instead of movement of the blades of an electric fan; car instead of a car moving on a straight road etc.
Performance was average.
Part (b). Many candidates attempted this question. The basic skill in the laboratory description of an experiment to determine the density of an irregularly shaped solid is lacking in most candidates. Performance was poor.
Part (c) Performance was average though some candidates missed out the fact that the ‘rate’ is a ‘time rate’ and it takes place in the direction of the applied force. Few candidates defined it without including time hence they lost marks.
Part (d):- Performance was fair. Some candidates however defined inertia rather than explaining it as demanded by the question. Some merely quote Newton’s first law of motion. This is not the case.
Part (e): Performance was fair. Though some candidates confused the solution of (i) for (ii) and calculate static friction in place of the required kinetic friction.
The expected answers are:
8. (a)(i) Rotational Motion
e.g - movement of a wheel about its axis
- movement of the earth about its axis
- movement of the blades of an electric fan ( accept fan)
- motion of a turn table/disc.
- motion of a wind mill.
(ii) Linear Motion
e.g - a car moving on a straight road
- a ball rolling on a (level) ground
- an athlete running on a straight track
(b) The mass (m) of the solid is determined using a chemical/beam balance . A graduated measuring
cylinder is partially filled with water and the initial volume V1 is recorded. The solid is completely
immersed in the water and the final volume V2 is recorded .
Volume of the solid = V2 – V1
Density of the solid = =
OR
The solid is weighed using a chemical/ beam balance and its mass,m, recorded. An Eureka / displacement can is completely filled with water and a graduated cylinder is brought under the sprout. The solid is tied to a thread and gently lowed into the water . The water displaced is collected and its volume measured and recorded as V. The density of the solid is calculated using =
(c) Newton’s second law of motion states that the time rate of change of momentum of a
body is directly proportional to the applied force acting on it and takes place in the
direction of the force.
(d) Inertia is the reluctance of a body to move if it is at rest or to stop if it is already in
motion.
The more mass a body has the greater is its inertia
(e) (i) Net force = F – F1 = ma
= 5.0 x 2.0
= 10.0N
(ii) F = ma + F1
= 10 + 5
= 15 .0 N
(iii) µ = OR
=
= 0.3
