A child of mass 40kg jumps off a wall and hits the ground at 4m/s he bends
his knees to absorb the impact. What is the average force exerted on the child's legs?
The average force exerted on the child's legs can be calculated using the equation Force = Mass x Acceleration. In this case, the acceleration is the change in velocity (4m/s) divided by the time it takes for the child to hit the ground. Assuming the time is 0.5 seconds, the acceleration is 8m/s2. Therefore, the average force exerted on the child's legs is 320N (40kg x 8m/s2).
Assuming the time is 0.5 seconds,
Why would you assume that? You indeed need to know the acceleration but it is not in the question.