Month: December 1969

Why does a basketball bounce higher than a bowling ball?

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Why does a basketball bounce higher than a bowling ball?

When a ball bounces from a rigid surface, the ball’s surface distorts inward and then pops back outward. During the inward motion, the ball stores energy—pushing its surface inward takes energy. During the outward motion, the ball releases that stored energy. But not all the energy invested in the ball emerges as useful work. Some of that energy is turned into thermal energy and never reappears. A properly inflated basketball returns a good fraction of the energy it receives while other balls may not. In fact, a bowling ball bounces pretty well from a hard surface such as cement. But when it hits a softer surface such as wood, the wood receives much of its energy and wastes that energy as thermal energy.

Why do some objects bounce off the ground (balls) whereas others would break (eg…

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Why do some objects bounce off the ground (balls) whereas others would break (eggs)?

Some objects can deform elastically, storing energy in the process, while others can’t. The surface of a rubber ball is made up of long, flexible molecules called polymers that can bend and stretch without breaking. As the ball’s surface dents during an impact, these polymer molecules move about and begin to exert forces on one another (storing energy in the process). As the ball rebounds, these molecules release their stored energy and push the ball back into the air. An egg, on the other hand, is made of hard, crystalline material that shatters during the deformation. Whole rows of atoms and molecules rip apart from one another and are unable to return. The egg doesn’t store the impact energy. Instead, it turns that energy into thermal energy. The shell just crumbles.

When the falling ball bounced off the rising board, why did the ball go upward v…

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When the falling ball bounced off the rising board, why did the ball go upward very quickly? Because of your frame of reference?

The frame of reference from which you observe the situation doesn’t cause the rebounding ball to move quickly, but it does help you to understand why the ball rebounds so quickly. Instead of describing the ball bounce from the rising board, let’s look at the ball bouncing from a horizontally moving bat. That way, we won’t have to worry about gravity—we can pretend it doesn’t even exist for a moment. Let’s begin from the fan’s inertial frame of reference as a pitched ball heads toward a bat at home plate. As the ball approaches the bat, the bat approaches the ball. Both objects are moving, which makes things complicated. So we’ll now shift to the bat’s frame of reference for a while. In this frame of reference, the bat is stationary and the ball is approaching at high speed. (This rapid approach speed reflects the fact that the two objects are each moving toward the one another in the fan’s reference frame.) The ball now bounces from the bat. Because it approached the bat at such a high speed, the ball rebounded at a high speed, too—it heads away from the bat at high speed. Now we’ll shift back to the fan’s reference frame. The ball is still going away from the bat at high speed, but now we must notice that the bat itself is heading toward the outfield at a high speed, too. So the ball must really be heading toward the outfield fast—it’s outrunning the bat toward the outfield. And that is the case. The ball heads toward the outfield at a much higher speed than it had when it was heading toward the bat originally. In the fan’s frame of reference, there is a large transfer of energy from the bat to the ball

What forces are involved when hitting the sweet spot of a baseball bat?

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What forces are involved when hitting the sweet spot of a baseball bat?

If the ball bounces from the sweet spot, the two push on one another hard. The ball slows to a stop and then reverses its direction, rebounding from the bat at high speed. The bat accelerates in the opposite direction, and begins to rotate slightly about its center of mass. This rotation is just right to keep the bat’s handle from accelerating either toward or away from the ball. That’s why the hit feels so clean and neat. The handle doesn’t accelerate. The force from the ball on the bat also doesn’t cause the bat to vibrate, because the sweet spot is a vibrational node.

If you throw a dead ball at a baseball, would the baseball not roll as far as if…

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If you throw a dead ball at a baseball, would the baseball not roll as far as if you throw a super ball at it?

Your right. The dead ball transfers less momentum to the baseball than the lively super ball does. That’s because the dead ball transfers momentum only one, essentially coming to a stop on the baseball’s surface. The bouncy ball transfers momentum twice because it also pushes on the baseball as it rebounds. Overall the baseball receives more momentum (and also more energy) from the super ball than from the dead ball. The dead ball turns much of the collision energy into thermal energy.

If I’m a WWF Wrestler, and I sling-shot myself off the ropes, and my momentum ca…

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If I’m a WWF Wrestler, and I sling-shot myself off the ropes, and my momentum carries me as I put a flying shoulder block on my opponent, is my momentum conserved and do I feel any momentum against me?

As you bounce off the ropes, you exchange momentum with the ropes (and the earth). As a result, you normally reverse your momentum and head back into the ring. When you hit your opponent, you begin to exchange momentum with him/her. If you hit your opponent feet first and jump backward, you will reverse your direction of travel again and your opponent will receive an enormous amount of forward momentum. All of this transfer of momentum means that your personal momentum will change often but the total momentum of the earth and its population won’t change. That momentum will just be rearranged amount the various objects.

If all the laws of physics always happen the same, then what relevance does the …

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If all the laws of physics always happen the same, then what relevance does the frame of reference have?

If you observe the world from an inertia frame of reference—meaning that you aren’t accelerating—then all of the laws of physics will apply properly to the objects you see. Energy will be conserved during activities, momentum will be transferred between objects without being created or destroyed, and so on. So it’s true that any inertial frame of reference will do. However, there is often a “best” reference frame from which to observe a situation. A good example of this is the situation in which a ball bounces from a bat. The best inertial reference frame from which to watch that bounce is the frame of the moving bat. In that special inertial reference frame, the bat doesn’t move and the ball bounces off the stationary bat.

How do rubber bouncing balls work? Does the table exert more force than is appli…

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How do rubber bouncing balls work? Does the table exert more force than is applied, causing an upward acceleration?

The table never pushes up on the ball harder than the ball pushes down on the table. That would violate Newton’s third law and is just not the way our universe works. As the ball strikes the table, the two objects dent. The ball dents most and has work done on its surface—the table pushes the surface inward and work is force times distance in the direction of that force. The ball stores this work/energy as a deformation of its elastic surface and a compression of the air inside the ball. The ball then rebounds from the table as this stored energy reemerges as kinetic energy in the ball. Throughout the bounce, the upward force that the table exerts on the ball is much larger than the ball’s downward weight. As a result, the ball accelerates upward the whole time. It starts the bounce heading downward and finishes the bounce heading upward.

When you transfer momentum between two objects, why is it that the change in tot…

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When you transfer momentum between two objects, why is it that the change in total momentum is 0?

Suppose you are standing motionless on extremely slippery ice. If you now take off your shoe and throw it northward as hard as you can, you will transfer momentum to it. Since you and your shoe were initially motionless, your combined momentum was 0. Neither of you nor the shoe was moving, so the product of mass times velocity was 0. But after you throw the shoe, both you and the shoe have momentum. Your momentum is equal to your mass times your velocity, so your momentum points in the direction you are going. The shoe also has momentum, equal to its mass times its velocity. But since it is heading in the opposite direction from you, it has the opposite momentum from you. Together, your combined momentum remains exactly 0—it didn’t change. In general, momentum is transferred from one object to another so that any change in momentum in one object is always compensated for by an opposite change in momentum in the other object.