Answer:
His verticak acceleration is 10.02m/s²
Explanation:
Given u = initial velocity = 4.2m/s
H = 2.13m and h =1.25m initial height
At the point where he reaches the crossbar he is no more moving upwards and has reach the maximum height where his final velocity V is 0
So usnng the relation V² = U² + 2a(H - h)
0 = 4.2² + 2a(2.13 - 1.25)
1.76a = 17.64
a = 10.02m/s².
Please note that it is the center of gravity of Brian that is used to measure how high he has jumped.