In April 1974, Steve Prefontaine completed a 10.0 km race in a time of 27 min, 43.6 s. Suppose "Pre" was at the 7.43 km mark at a time of 25.0 min. If he accelerated for 60 s and then maintained his increased speed for the remainder of the race, calculate his acceleration over the 60 s interval. Assume his instantaneous speed at the 7.43 km mark was the same as his overall average speed up to that time.

Question

Physics

Rayna White

18 January, 04:32

In April 1974, Steve Prefontaine completed a 10.0 km race in a time of 27 min, 43.6 s. Suppose "Pre" was at the 7.43 km mark at a time of 25.0 min. If he accelerated for 60 s and then maintained his increased speed for the remainder of the race, calculate his acceleration over the 60 s interval. Assume his instantaneous speed at the 7.43 km mark was the same as his overall average speed up to that time.

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Natalia Rollins · Answer 1

Acceleration, a = 0.101 m/s²

Explanation:

Average speed = total distance / total time.

At the 7.43km mark, total distance = 7.43km or 7430m

Total time = 25 * 60 s = 1500s

Average speed = 7430m/1500s = 4.95m/s

He then covers (10 - 7.43) km = 2.57 km = 2570 m

in t = 27m43.6s - 25min = 2m43.6s = 163.6 s

Then he accelerates for 60 s, and maintains this velocity V, for the remaining (163.6 - 60) s = 103.6 s.

From V = u + at; V = 4.95m/s + a * 60s

Distance covered while accelerating is

s = ut + ½at² = 4.95m/s * 60s + ½ a * (60s) ² = 297m + a*1800s²

Distance covered while at constant velocity, v after accelerating is

D = velocity * time

Where v = 4.95m/s + a*60s

D = (4.95m/s + a*60s) * 103.6s = 512.82m + a*6216s²

Total distance covered after initial 7.43 km, S + D = 2570 m, so

2570 m = 297m + a*1800s² + 512.82m + a*6216s²

2570 = 809.82 + a*8016

a = 809.82m / 8016s² = 0.101 m/s²