



 
Hi Jessie. The question expects you to assume his legs are massless  not exactly realistic, but still a good question to learn from. When the person is on the roof, his vertical speed is zero, but he quickly accelerates due to gravity until he touches the ground. The formula for this is v^{2} = v_{0}^{2} + 2ad. You know a = g and d = 3.9 m and v_{0} is the starting velocity which is zero m/s, so you can calculate the speed which is the answer to part (a). For part (b), he is decelerating over a distance of 0.7 m from the speed that is the answer to part (a) down to zero. You need to know the deceleration. Again, you can use the same formula, but this time v_{0} is the answer from part (a), v is zero, and d is 0.7 m. Once you calculate this acceleration, you can use it in the F= ma equation to find the average force during the deceleration. Hope this helps,  


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