Newtons First Law of Motion?

Question

I was asked at work from a co-worker this question.

If a ball is dropped from a 50 story building. When the net force acting down upon it (Mass x Acceleration) is equal to the Drag (Force acting up on it) Will the acceleration be equal to zero?

I see where he is coming from..But doesnt the net force include all forces?

If someone could explain.

Thanks!

Answer 1

If it is travelling at constant speed then it is not accelerating

Answer 2

when the speed or velocity constant means zero acceleration..

and if a ball is dropped means it free fall AND it does have acceleration which is gravitational acceleration.

9.81ms^-2

Answer 3

If the ball was falling in a vacuum, it would accelerate at 9.8 m/sec^2   until it hit the ground. There would be no maximum velocity.

When it falls in air, at the instant it is released, the acceleration is 9.8 m/sec^2, but then the air resistance force, acting upwards, starts to have an effect.

As the body falls faster, the air resistance force increases, the nett force downwards on the body decreases, and the acceleration decreases.

Eventually, the air resistance force acting upwards, is equal to the weight of the body, the nett force is zero, and the acceleration is then zero.

The body then moves at a constant speed. This is called the terminal velocity.

Theoretically, a body takes infinite time to reach its' terminal velocity, but can in practice be so close to it that the difference can be ignored.

Some bodies, e.g. a feather, reach their low terminal velocities very quickly, whereas others take much longer.

The terminal velocity  of a parachutist if his parachute does not open is about 120 m.p.h.

During World War 2, there were bombs whose terminal velocities were supersonic, and therefore could not be heard arriving.

If you could throw the ball down at a speed faster than its terminal velocity, it would then slow down to its terminal velocity, as for a time, the drag of the air upwards would be greater than the weight downwards.

Answer 4

When the ball is first dropped it starts to accelerate under the influence of gravity. It accelerates at 9.8 metres per second per second due to the gravitational field of the Earth.

As it's velocity increases it starts to experience resistance to the air through which it is travelling. The faster it travels the more air resistance it experiences.

There comes a point at which the force due to air resistance is equal to the force from gravity pulling it down. At this point the velocity will stop increasing and it's acceleration will be zero. This is called terminal velociy.

The ball still experiences both gravity and air resistance at this point. However, the two forces are in equilibrium, they are equal in size and directly oppisite in direction.