Physics Quiz - Poll

BigCat said:

This is impossible, rotational + tranlational must still equal the original force applied, one can only transfer to the either, not increase rotational and traslational stay the same.

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I believe the argument stems from the fact that the cue stick in this example pushes back against the cue ball due to the off-center hit. This push back force gives us squirt. This sideways force is greater the farther from center you strike the ball provided you don't miscue. When you add in this component of energy, you get more that the centerball hit.

jsp said:

Good answer. This is correct.

Although you're applying the same force impulse (magnitude and time duration) to the CB in both cases, you're actually imparting more energy into the CB for the second case. So Case 2 will have the greater kinetic energy.

So the further you strike the CB away from center ball, provided you apply the same force impulse and you don't miscue, the CB will have greater kinetic energy. Kinda wild, huh?

I'll leave it up to you to figure out why you probably won't notice this in the real world.

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It is impossible to get more energy out than you have energy in...you are just changing the way the energy is transferred...you are not adding any more energy.
Steve

BigCat said:

This is impossible, rotational + tranlational must still equal the original force applied, one can only transfer to the either, not increase rotational and traslational stay the same.

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You're equating energy and force. They are not the same entities.

Jim

Since it is the same stroke the first one is going to have more energy dirested towards forward movement. The second scenario gives some of this energy into making the ball spin unnaturally, therefore using some of this transfered energy. If we ignore friction from the table cloth; both balls will go an infinite distance until stopped by other means, so that doesn't make sense to even contemplate into the equation.

Short answer: Any time energy is transfered into doing other jobs(applying english) there will be less energy transfered into the initial job(forward force). Sooo, the same amount of energy is given but it is wasted more in the application of side spin.

Of course, in the real world this isn't always apparent. Running english reduces friction when it glances an object it appears that more energy was transfered because the ball will go further than if it was given stun or was hit dead center.

all in the wording

First, the situation as stated is impossible. a force impulse through the center of gravity and roll center is not the same as a force impulse delivered off to the side so the cue ball never receives exactly the same force impulse from the two hits.

If we change the wording to the cue ball receiving the same total force then more force will have to be applied to the cue stick to transfer the same amount of force to the cue ball. Far from gaining anything from a less than center hit energy is wasted in things that serve no useful purpose.

If we change the wording to the cue ball receiving equal forward force impulse then it would require yet more force applied to the cue stick than the conditions of the above paragraph.

As an indication of how unworkable this free energy claim is, note how all other sports hit a ball when seeking the maximum distance. Does a home run hitter try for a glancing blow? A field goal kicker striving for a sixty yarder? The soccer goalie sending the ball back deep into the opposing team's territory? How does a trick shot artist, perhaps the most knowledgeable people in the game when it comes to stroke, hit the eight or nine rail kick onto a hundred dollar bill in the tie breakers?

Bottom line, we burn more energy to transfer the same total energy to the cue ball when striking it to achieve side spin. It is anything but free energy. (edited to change "energy" to "total energy" in this last line to remove any doubt what I am talking about.)

Hu

Jal said:

You're equating energy and force. They are not the same entities.

Jim

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Agreed. I should have stated rotational + translational "kinetic energy" must still equal the original "kinetic energy" applied by the original impulse of force.

Good catch. I feel like I'm writing a paper again

Another way of looking at it is that, given the same stick speed, less impulse develops when the cueball is struck off-center than when struck at center. To generate the same impulse, which was Jsp's premise, the cue has to be traveling faster and therefore has more kinetic energy. That's where the additional energy of the cueball comes from.

But why is there less impulse for an off-center hit given the same stick speed?

Impulse (force X time) equals a change in linear momentum (mass X velocity). Newton's third law tells us that during the collision, the impuse acting on the stick is exactly the same as that acting on the ball (same in magnitude but opposite in direction). Compared to a billiard ball, if you strike a ping-pong ball with your cue, the cue's momentum hardly changes. Therefore the impulse which acted on both the stick and the ping-pong ball must be much less. But if you strike a bowling ball, the stick jumps back and undergoes a much larger change in momentum. The impulse on the stick and the bowling ball must therefore be much larger.

When you strike the cueball off-center, it can rotate as well as move forward, ie, it can "get out of the way" easier. In effect, it presents less mass (inertia) to the stick. For the same stick speed then, as per above, less impulse develops.

Don't feel bad if you got it wrong. Despite the fact that nature does a great job in preparing us to negotiate the world, our intuitions often fail us when it comes to physics, even with as simple a thing as poking a ball with a stick. Jsp mentioned how counter-intuitive it is that a tangential acting force accelerates an object the same as if it were directed through its center of mass. My intuition, and probably yours, also tells me that when you apply the same force for the same amount of time, you're exerting the same amount of effort and therefore expending (transfering) the same amount of energy. Not so. In the peculiar world of physics, applying the same force over the same distance, not time, results in the same expenditure of energy. Is this the same world we live in?

Jim

Anybody else need a beer besides me after reading this thread?

Things always seem to make more sense after beer...a couple of beers if need be.