Hey there... I'm studying for a test tomorrow and there's a problem I can't seem to figure out... I hope you all don't mind me asking here (hey, it's what the forum's for ), and I hope you all can see that I really did attempt to work the problem myself.(adsbygoogle = window.adsbygoogle || []).push({});

Now, when the ball hits the ground, it's VInst is equal to 14m/s. Newton's 3rd law states that at that instant, the same force will be applied upwards towards the ball. A ball of mass 1.7 kg is dropped from a height, h, and hits the ground at a velocity of 14.0 m/s and bounces up to its original height. If the ball is in contact with the ground for 1/27 of a second find the average force the ground applies to the ball. (Use GUESS method.)

Answer 758 N

We know that the acceleration of a falling object is 10m/s^2. Yes, I am aware this is simplified, and would prefer to use 9.81 myself, but the department head for physics at my school rather seems to like rounding it off to a nice number.

If the velocity on impact was 14m/s, and the acceleration is 10m/s^2, the object took 1.4 seconds to hit the ground. We know the average velocity (free fall to 14m/s) is 7m/s.

X = X0 * Vavg * t...

Plugging everything in, the dX is equal to 7m/s*1.4s, leaving us with a distance of 9.8, which is h. I'm not quite sure of the relevence of this, but I figure it wouldn't hurt to find it. :P.

If the ball comes into contact with the ground for 1/27 of a second, then seconds 1.40 to 1.44 is the time in which the ball is on the ground. This is where I am severely confused. What happens here? The ground exerts an equal force of impact on the ball (17N), but doesn't it have to be greater than the force of gravity for it to bounce back up? The weight of the ball is also 16.7N (1.7kg * 9.8m/s), but these two forces can't be the same... acceleration would be zero and the ball would just hit the ground and stop.

So... what happens?

Also, the physics book is... well... for the most part, absolute garbage. I was wondering if any of you could recommend a good physics book or resource, ranging from the mechanics of physics to quantum physics, preferably.

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# Homework Help: Bouncing ball and acceleration

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