Question #ec3a7

1 Answer
Jul 8, 2015


Maximum height: 20 m
Initial velocity: 20 m/s


When an object is thrown straight up, meaning that its angle of launch forms a #90^@# angle with the horizontal, the time it needs to reach maximum height will be equal to the time it needs to return on the ground.

#t_"up" = t_"down"#

Since you know tha it takes 4 seconds for the ball to return to the thrower, it follows that

#t_"up" = t_"down" = "2 s"#

To determine the initial velocity of the ball, use the fact that, at maximum height, the velocity of the ball is equal to zero.

#underbrace(v_"max h")_(color(blue)("=0")) = v_0 - g * t_"up"#

This implies that the ball was thrown with an initial velocity of

#v_0 = g * t_"up" = 10"m"/"s"^cancel(2) * 2cancel("s") = color(green)("20 m/s")#

To get the maximum height of the ball, use the equation that describes its vertical displacement

#h_"max" = v_0 * t_"up" - 1/2 * g * t_"up"^2#

#h_"max" = 20"m"/cancel("s") * 2cancel("s") - 1/2 * 10"m"/cancel("s"^2) * 2^2cancel("s"^2) = color(green)("20 m")#