Final answer:
The child's speed is 3 m/s when they have a mass of 35 kg and a kinetic energy of 157.5 J, calculated by rearranging the formula for kinetic energy and substituting the given values.
Step-by-step explanation:
To calculate the speed of a child with a mass of 35 kg and a kinetic energy of 157.5 J, we use the formula for kinetic energy:
KE = 0.5 × m × v^2, where KE is kinetic energy, m is mass, and v is velocity (speed).
Rearranging the formula to solve for v, we get v = √(2 × KE / m).
Substituting the given values, we have v = √(2 × 157.5 J / 35 kg).
After calculation, we find that the child's speed is 3 m/s, which corresponds to option A).