(a)(i) D
The force due to gravity (weight) always acts vertically downwards towards the centre of the Earth. In the diagram, force D is the only arrow pointing straight down.
(a)(ii) A
Air resistance (drag) is a frictional force that opposes the motion of an object moving through the air. Since the cyclist moves in the direction of force C, force A points backwards and represents the air resistance.
(a)(iii) decelerating / slowing down / less speed
Force C is the forward driving force, and force A is the backward resistive force. When force A becomes larger than force C, the resultant force acts opposite to the direction of motion, causing the cyclist to decelerate.
(b)(i) 12 (m/s)
Average speed is calculated by dividing the total distance travelled by the total time taken: $v = \frac{s}{t} = \frac{250\text{ m}}{21\text{ s}} = 11.9\text{ m/s}$, which rounds to $12\text{ m/s}$.
(b)(ii) 9000 J
Work done is the product of force and the distance moved in the direction of the force: $W = F \times d = 36\text{ N} \times 250\text{ m} = 9000\text{ J}$. The unit of work is the joule (J).
