Motion and Its Causes

Force and Newton’s Laws of Motion

What happens if an object is not dropped, but is thrown up or down?

Physicists would say that the ball has been given an initial velocity. But this initial velocity doesn’t affect the force of gravity on the ball, so the ball would still gain a downward velocity of 9.8 m/s each second it is in flight.

Suppose the ball is thrown down with a velocity of 2.0 m/s. Then at time t = 0, the time it was thrown down, it would already have a velocity of 2.0 m/s downward. If you use the table above you see that 0.10 s later it would have a velocity of 2 0.98 m/s = 2.98 m/s. Half a second after launch its downward velocity would be 2 4.9 m/s = 6.9 m/s. That is, the initial velocity is just added to the increasing velocity caused by the gravitational force.

Now suppose the ball is thrown upward with the same 2.0 m/s speed. Now we have to be very careful with the sign of the velocity. We have chosen the downward direction as positive. So the initial velocity of the ball is actually –2.0 m/s.

The gravitational force acts in the downward direction. By Newton’s Second Law that means that the ball will slow down at a rate of 9.8 m/s each second. After 0.1 s it would be going -2.0 0.98 = -1.02 m/s. After 0.20 s it would be going -2.0 1.96 = -0.04 m/s. It is almost at rest! But the gravitational force is still acting on it, so it continues to accelerate downward. After 0.30 s it would be going -2.0 2.94 = 0.94 m/s. That is, it would now be moving downward.

When it is at its maximum height does it stop? At an instant in time it is indeed at rest, but it doesn’t stay motionless for any time interval because the gravitational force keeps acting on it.


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