# A ball with a mass of 0.5 kg is released from rest at a height of 15 meters above ground level. A constant air resistance of 0.9 N acts on the ball. What is the acceleration of the ball and what is...

A ball with a mass of 0.5 kg is released from rest at a height of 15 meters above ground level. A constant air resistance of 0.9 N acts on the ball. What is the acceleration of the ball and what is the speed when it strikes the ground.

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### 3 Answers

A ball with a mass 0.5 kg is released from rest at a height of 15 meters above ground level. Air resistance acts on the ball and is assumed to have a constant value of 0.9 N.

The gravitational acceleration of the ball is 9.8 m/s^2. The deceleration due to the force of air resistance is 0.9/0.5 = 1.8 m/s^2. This gives a net downward acceleration of 9.8 - 1.8 = 8 m/s^2.

As the ball is released from rest the speed with which it hits the ground is v^2 = 2*15*8

=> v = `sqrt 240`

=> `4*sqrt 15` m/s

**The ball strikes the ground with a speed of `4*sqrt 15` m/s.**

@qwerty obviously for the question you were supposed to assume that overwise he wouldn't have wrote it

I'm not sure how to solve the math part but you can't assume that air resistance is constant because air resistance increases with speed. A ball moving 1mph will have less air resistance than a ball moving 200mph.