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Physics Paper 2, Nov/Dec. 2010  
Questions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Main
General Comments
Weakness/Remedies
Strength
















Question 12
Part II : Candidates are expected to answer any three form this part

(a) Explain:
                 (i)           Linear momentum.
                  (ii)          Impulse of a force
(b) State three instances where Newton's third law of motion is applicable.

(c) A ball of mass 4.0 kg. hits a smooth platform vertically with a speed of 3 ms-1 and rebounds with a speed of 2 ms-1. Calculate the impulse experienced by the ball.
(d) A rocket of mass 500 kg carrying 4000 kg of fuel is to be launched vertically. The fuel is consumed at a steady rate of 50 kgs-1, calculate the least velocity of the exhaust gases if the rocked will just lift off the launching pad immediately after firing [g = 10 ms-1]

 

_____________________________________________________________________________________________________
Observation

(a) Many candidates rendered linear momentum wrongly. Majority defined 'momentum' instead of 'linear momentum' .
(b) Majority of the responding candidates were able to state the required number of instances where Newton's third law of motion is applicable.
(c) Several candidates failed to apply the formula adequately. Many candidates substituted 2ms·' since in the question they were told the ball rebounced after impact.
(d) Responses was fairly satisfactory.

The expected answers are:
      (a)(i)              Linear momentum of a body moving in a straight line is the product of its mass and its velocity.
Accept product of mass and linear velocity.
Any valid additional information
e.g.
- Its unit is kgms" - Do not accept Ns
- it is a vector quantity.

             (ii)            Impulse of a force is the product of the (large) force acting on a body and the (short)
time during which it acts. Any valid additional information e.g.
Any valid additional information
e.g.
- Its unit is Ns - Do not accept kgms"
- it is a vector quantity.

(b) Instances where Newton's third law of motion is applicable
- recoil of a gun
- jet propulsion/rocket propulsion
- action of sprinkler/lawn spray
- Accept any other valid instance(s)

(c)          Impulse = Ft = m(v- u)
          = 4[2-(-3)]
        =   20Ns

  (d)            Weight, W=mg
          = (5000 + 4000) x 10
          = 90,000N.
Downward action = upward reaction
          90,000 = 50(v-0)
          v = 90,000
                    50
          = 1.8 X 103 ms-1.



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