6) The goalkeeper catches the ball flying by speed 30 m / s and stopped its movement in a period of 0.15 seconds . What force did he act on the ball, if we consider the stopping of the ball for constantly slow motion? Weight of the ball is 400 grams.
a = -30/.15 = -200 m/s^2
F = ma, so
F = .4*200 = 80N
"... if we consider the stopping of the ball for constantly slow motion?"
is pretty sloppy language. Try saying there was a constant deceleration.