Homework Help: physics/dynamics

Posted by justin on Wednesday, January 12, 2011 at 2:16pm.

The position of a ball as its kicked upwards is measured as
y(t) =−(4.9 t^2 + 20t + 0.50)m,
where t is measured in seconds. Find the vertical component of velocity and acceleration of the ball as a function of time. If the ground is located at yg = 0, ﬁnd the time at which the ball hits the
ground.

physics/dynamics - drwls, Wednesday, January 12, 2011 at 3:14pm
Your equation is wrong with a minus sign parentheses outside the paretheses. Please verify that you copied it right.

It should be
y(t) = −4.9 t^2 + 20t + 0.50 (meters)

Zero vertical velocity is attained when
dy/dt = -9.8 t + 20 = 0
t = 20/9.8 = 2.04 seconds

It hits the ground when
−4.9 t^2 + 20t + 0.50 = 0

I suggest you use the quadratic equation and take the positive root.

You should get 4.016 s

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