According to Newton’s law of motion, the
force depends on the rate of change of
momentum.
a) State whether the force is external or
internal? Justify your answer.
b) What happens to the linear
momentum when the force is absent?
c) The motion of a particle of mass m is
described y = ut +1 /2 gt^2 . find the
force acting on the particle.
d) Why is it more difficult to rotate a
stone by tying it to a longer string than
a shorter string?

$$\begin{gathered} Dear Student, \\ a) The force is dependent the way we choose the system. But is is external force \\ which acts on the body and is responsible for the momentum change. The moment \\ we choose a system the work done by internal forces is zero. \\ b) When the linear mommentum is conserved when the force is absent. \\ Reason : F = dP/dt \\ where F = External force \\ P = Momentum \\ If F = 0 it means that P = cons\tan t. Or momentum is conserved, \end{gathered}$$

c) The equation is given as y = ut + (1/2)g t², we can compare it with the motion equation that is 
s = ut + (1/2) a t² 
so we get acceleration as a = g
Therefore the force is F = mg

d) We know that the Torque equation is given as 

Torque = R X F
where R = Distance 
and F is the Force.
So for a short string the R is small so torque required is less to rotate it 
and for a long string since R increases the torque required is more.

Regards