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Answers to Readers' Questions on Physical Science:

Motion Laws

The following 33 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Why does an India rubber ball bounce funny?
• Can a fan be used to rotate a generator?
• Audio not clear enough
• Difference between contact force and normal reaction?
• Does bouncing ball ignore gravity?
• Why should mass be constant?
• Experiment on dropping objects from an airplane
• What is centripetal force?
• Does mass increase close to the speed of light?
• Changing mass or acceleration

Next 10

Why does an India rubber ball bounce funny?

Question

April 20, 2009

I don't understand how the India rubber ball is made (is there lead inside?) and why every other bounce seems to be similar, but not every bounce. It will bounce once, then zing off on bounce 2, then 3 seems ok, then 4 is whacko also. What laws of physics and rubber and gravity coordinate this very strange ball and its weird bounces??

Bill - Canada

17632

Answer

What happens with such a ball is that when it hits the ground any distortion can cause the ball will start to spin. When it hits the ground again, the rubber grips the ground and shoots the ball at an angle. But it also stops the spin for the next bounce. Depending ho the ball hits the ground, spin can increase, such that the next time it goes at even a greater angle.

The fact that the ball bounces so well and the rubber grips the ground makes bouncing such a ball amusing.

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Can a fan be used to rotate a generator?

Question

March 28, 2009

Respected sir,
I have an inquiry. If 3 metal rods could be connected with the 3 blades of a fan which, in turn, would be used to rotate a generator, what would be the result? I believe, we could enjoy the fan while reuse most of the energy used to rotate the fan. Is this correct?

Sagnik - India

17518

Answer

Although it is a good idea, the problem is that friction, heat and electrical forces would cause a loss of energy. In other words, the energy required to turn the generator would be more than the energy used to turn the fan, because of the friction in the generator and the losses due to heat in the wires.

Often it is better to make the fan motor more efficient, so that not as much electrical energy is required to turn the blades.

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Audio not clear enough

Question

June 3, 2008

Listening to audio of the lesson is very effective way of learning but the voice should be more clear

yash - India

16035

Answer

Thank you for the suggestion. We will try to improve the sound of the voice.

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Difference between contact force and normal reaction?

Question

February 9, 2008

What is the difference between contact force and normal reaction?
How do we apply newton's laws of motion in solving problems involving pulleys?

Priyanka - India

15314

Answer

A normal force is perpendicular to the surface. The contact force may be at an angle to the surface, but the corresponding reaction force will be normal to the surface.

A simple pulley changes the direction of the force. Assuming friction is zero, the input energy equals the output energy. The total momentum will be the same, except in a different direction.

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Does bouncing ball ignore gravity?

Question

November 13, 2007

Ref the statement on periodic motion (Shown Below), surely ignores the effect of gravity?

Bouncing ball
If you drop a ball, it will start to bounce in a regular fashion. A good rubber ball or a super-ball will keep bouncing for a long time. Because of internal friction and air resistance, the ball bounces less and less each time, until it finally stops. A perfect ball **without friction** would bounce forever.

Cheers

Nick - Australia

14776

Answer

If you drop a ball, gravity pulls it downward. The elasticity of the ball propels it ward until the force of gravity pulls it down again. The only way the ball will fall and then bounce is if there is gravity.

the point here is when the ball hits the ground and compresses, some energy is lost to internal friction. Thus, its upward motion is slightly slower and it will not bounce as high as the point from which it was dropped.

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Why should mass be constant?

Question

June 20, 2007

Force=ma but in this why should the mass be constant? if the mass is not constant(eg: in case of balloon) than how can we find the force acting?

Mallika - India

13958

Answer

Equations often do not explain things as well as words. F=ma states that if the mass is constant, then the acceleration is proportional to the applied force.

If the situation is where the mass is changing, such as the mass of fuel in a rocket, then the calculation of the force and acceleration requires higher mathematics, where you must integrate over time.

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Experiment on dropping objects from an airplane

Question

April 17, 2007

Hello, My name is Varinder. I am doing my science project on Newton's second law. The topic is Planes dropping food, water and medical supplies to people stranded by floods, earthquakes and othr natural disasters. How do the pilots find their target and drop cargo on the target? This will be my question. I am gonna do same example now with a tennis ball I am gonna drop tennis ball while walking and see how far it travels with me and on the other hand I will be walking and when i drop I will stay there and wont move. So I need help in Hypothesis and so data. It will be real helpful if I get a reply with an helpful answer.
I really appreciate your help, Please reply soon as possible.!

Varinder - USA

13584

Answer

When an airplane is flying at some given speed, everything inside of it is also going at that speed. Thus, when they drop an item, it is flying forward at the speed of the airplane, while it is dropping according to the acceleration of gravity.

If you know the height of the airplane, you can calculate how long it'll take the object hit the ground, according to the effects of gravity. Since you know the forward speed of the object, you can use that time to calculate how far forward it will travel until it hits the ground.

For example, if an airplane is flying at 240 mph, it is going approximately 300 feet per second. If it takes two seconds for the object to hit the ground, the airplane must drop it 600 feet before flying over the target.

Other factors include air resistance and the direction of the wind. For the sake of your experiment, you can be aware of these but not include them in your calculations.

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What is centripetal force?

Question

March 19, 2007

what is "a centriptal force"?

- USA

13397

Answer

The centripetal force is the force that causes an object to follow a circular path. It is the force you would pull on a string when spinning an object around you.

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Does mass increase close to the speed of light?

Question

February 2, 2007

according to einstein's theory of relativity does mass of a moving body actually increase when moving with speed comparable to speed of light

abdul - Pakistan

13036

Answer

This has been verified by accelerating protons close to the speed of light. It was seeing that their mass actually increased as their speed approach the speed of light.

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Changing mass or acceleration

Question

January 10, 2007

1.if you double the mass, what happens to the acceleration?
2.if the acceleration is reduced to one fourth of the original, what is the corresponding change in mass?
3.how does the mass vary with acceleration at constant force?
4.if you double the force, what happen to the acceleration?
5.if you increase the acceleration of five times the original, how does the corresponding force change?
5. if you reduced acceleration to 1/2 of the original, how is the resulting force related to the original force?
6. how does force vary with acceleration at constant mass?
7. what is the relation between force and acceleration when mass is constant??


pls answer my question, i need a answer now because this is my assignment....!!
plss........

cherrylyn - philippines

12789

Answer

The equation is F = m*a, where F is the force, m is the mass and a is the acceleration. Since F stays the same, if you double the mass, the acceleration must be 1/2, since 2 * 1/2 = 1.

Likewise, if a is reduced to a/4, then F = (4*m)*(a/4).

The other problems follow the same logic.

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