The foot of a $55kg$sprinter is on the ground for $0.25s$while her body accelerates from rest to $2m/s.$

a. Is the friction between her foot and the ground static friction or kinetic friction?

b. What is the magnitude of the friction force?

The foot of a $55kg$sprinter is on the ground for $0.25s$while her body accelerates from rest to $2m/s.$

Here, at the initial stage i.e., when the sprinter starts from rest, the friction between foot and ground is static . There is no slippage between the ground and the foot of the sprinter , so there is no relative motion between two surfaces . Hence static friction will act in this case .

The friction between sprinter's foot and ground is kinetic.

The foot of a $55kg$sprinter is on the ground for $0.25s$while her body accelerates from rest to $2m/s.$

We know, friction force is responsible for the acceleration of the sprinter .

To find acceleration (a) of sprinter :

By the kinematic equation,

$$\begin{gathered} v=u+at \\ \Rightarrow a=\frac{(r-u)}{t}\cdots \left(2\right) \end{gathered}$$

Substituting equation (2) in equation (1)

$\Rightarrow F=m_1\frac{(v-u)}{t}\text{... (3)}$

Substitution the values, we get

localid="1649255385611" $$\begin{gathered} F=55kg\times \frac{(2m/s-0)}{0.25s} \\ \Rightarrow F=440\mathrm{N} \end{gathered}$$

The magnitude of friction force is$440\mathrm{N}.$