Find the net force on the runner using the alternate form of Newton's second law.

Question

A runner with a mass of 85 kg accelerates from 0 to 6 m/s in 9 s. Find the net force on the runner using the alternate form of Newton's second law.

Answer 1

Newton's 2nd law says F=ma

We also know that F is N, which is kg*m/s/s, m is mass in kg, a is acceleration, which is usually represented as Δv/Δt or alternatively as (v₂-v₁)/(t₂-t₁) where v is velocity in m/s and t is time in seconds

There are many alternative ways to express this equation. In the case of this question one way to express it is as: F= m * (v₂-v₁)/(t₂-t₁)

putting numbers in:

F = 85kg * (6m/s-0m/s)/(9s-0s)

F= 85kg * 6m/s / 9s

F = 56.6666666 kg/m/s² or 56.66666666 N

However, we cannot leave our answer with a repeating decimal, we need to round our answer to only 1 significant digit, since 6m/s and 9s both only have 1 significant digit and we cannot calculate a net force with higher resolution than the measurements we used to calculate it.

So our final answer should be 60 N