World-class sprinters can accelerate out of the starting blocks with an acceleration that is nearly horizontal and has magnitude $15m/s^2$. How much horizontal force must a 55-kg sprinter exert on the starting blocks to produce this acceleration? Which body exerts the force that propels the sprinter: the blocks

or the sprinter herself?

The mass of the sprinter is

$m=55\text{\hspace{0.17em}kg}$

The required acceleration of the sprinter is

$a=15{\text{\hspace{0.17em}m/s}}^{\text{2}}$

According to Newton's third law the force exerted by a body on another body is equal and opposite to the force exerted by the second body on the first body, that is

${\overrightarrow{F}}_{12}=-{\overrightarrow{F}}_{21}.....\left(1\right)$

According to Newton's second law, the force applied on a body is related to mass of the body and acceleration of the body as

$F=ma.....\left(2\right)$

According to equation (1), the sprinter exerts a certain force on the starting blocks and the blocks apply the same force on the sprinter. This force from the blocks propel the sprinter forward. From equation (2), the required force is

$\begin{array}{c}F=55\text{\hspace{0.17em}kg}\times 15{\text{\hspace{0.17em}m/s}}^{\text{2}}\\ =825\cdot (1\text{\hspace{0.17em}kg}\cdot {\text{m/s}}^2\times \frac{1\text{\hspace{0.17em}N}}{1\text{\hspace{0.17em}kg}\cdot {\text{m/s}}^2})\\ =825\text{\hspace{0.17em}N}\end{array}$

Thus, the required force is 825 N and is applied by the blocks.