Which one of the following is not used to reduce friction?

$A$ car of mass $1250 \ kg$ is moving with a velocity of $30 \ m/s$. $A$ resistive force of $750 \ N$ acts on it. If the engine produces a power of $30 \ kW$,what is the acceleration of the car in $m/s^2$?

Fill in the blanks:
$(a)$ The friction acting on both wheels of a moving bicycle is in the ..... direction.
$(b)$ Friction depends on the ........... and ........ of the contact surfaces.
$(c)$ The necessary condition for parking a vehicle on a banked,curved road is ...........
$(d)$ The time rate of change of momentum gives the physical quantity ...........

Give the magnitude and direction of the net force acting on a stone of mass $0.1\; kg$,
$(a)$ just after it is dropped from the window of a stationary train,
$(b)$ just after it is dropped from the window of a train running at a constant velocity of $36\; km/h$,
$(c)$ just after it is dropped from the window of a train accelerating with $1\; m/s^2$,
$(d)$ lying on the floor of a train which is accelerating with $1\; m/s^2$,the stone being at rest relative to the train.
Neglect air resistance throughout.

$A$ block placed on a rough inclined plane of inclination $\theta = 30^{\circ}$ can just be pushed upwards by applying a force $F$ as shown. If the angle of inclination of the inclined plane is increased to $\theta = 60^{\circ}$,the same block can just be prevented from sliding down by application of a force of the same magnitude $F$. The coefficient of friction between the block and the inclined plane is:

Two carts are pushed apart by the explosion of a charge placed between them. $A$ cart of mass $100 \ kg$ stops after traveling a distance of $18 \ m$. What distance will a cart of mass $300 \ kg$ travel before stopping? The coefficient of friction $\mu$ between the carts and the ground is the same for both.