A sprinter starts from rest and accelerates along a straight track. After 4 seconds, the sprinter reaches a top speed of 10 m/s and then runs at this constant speed for a further 6 seconds before crossing the finish line. Describe the motion of the sprinter throughout the race and explain how the forces acting on the sprinter change between the two phases of the race.

During the first 4 s the sprinter starts from rest and accelerates, so his velocity increases from 0 to 10 m s⁻¹. The motion is uniformly accelerated; the displacement is ½at² with a≈2.5 m s⁻².

After 4 s the sprinter reaches a top speed of 10 m s⁻¹ and then runs at this constant speed for the remaining 6 s. The velocity remains 10 m s⁻¹, so the displacement in this phase is 10 m s⁻¹ × 6 s = 60 m.

In the acceleration phase the forward (driving) force produced by the sprinter’s muscles is larger than the resistive forces (air resistance, friction, etc.), giving a non‑zero resultant force that produces the acceleration.

In the constant‑speed phase the forward force equals the resistive forces, so the resultant force is zero. By Newton’s First Law the sprinter continues to move at 10 m s⁻¹.

Thus the race consists of an accelerating phase followed by a constant‑velocity phase, with the forces changing from a net forward force to a balanced pair of forces.