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Answers to Readers' Questions on Physical Science:

Motion Laws

The following 39 comments and questions have been sent in. They are listed according to date.

List of first 10 items

• Confused about Newton's Third Law
• Meaning of motion and speed
• What is relative motion between two objects in contact?
• Difference between acceleration and deceleration
• Does the moon light depend on the sun?
• How did newton define gravity?
• 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?

Next 10

Confused about Newton's Third Law

Question

June 27, 2010

I have little confusion in Newton's third law of motion. As we know that when we apply the forces to the body which are equal in magnitude but apposite in direction then the forces always cancel there effect and body remains in rest so as Newton says "For every action there is an equal and opposite reaction." SO when I apply a force on my book the book accelerates (why). It must have equal and apposite reaction it must be at rest so why it accelerate....

Yasir - Pakistan

19974

Answer

The statement: "For every action there is an equal and opposite reaction" can be confusing, because it does not clarify when it is applied. The requirement is that two objects be freely moving.

One examples is when you are sitting in a boat and you push against another person's boat, both boats will move apart, according to the force you apply. Your action pushes the other boat, but the reaction pushes on you and moves your boat.

Another example is when a cannon on wheels shoots a cannonball, the cannon also rolls backward.

However, when you push on a book, both you and the book are held back by the force of friction. If you push hard enough, you overcome the book's friction and it accelerates away from you. But your friction is too great and you don't move.

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Meaning of motion and speed

Question

June 10, 2010

I have some problem about the meaning of word example motion, speed...Can you help me ?

oulaypheth - Laos

19878

Answer

When anything moves from one position to another, it is in motion. For instance, when you walk from one place to another, you are in motion.

However, with speed, you need to know how far the object has moved within a given time. The speed of the object is the distance it moved divided by the time it took to go that distance.

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What is relative motion between two objects in contact?

Question

May 24, 2010

The definition of Friction is - It is the force that opposes relative motion between two objects in contact.
What is Relative motion between two objects in contact.
Please give the answer in class 8 level.

Shiv - India

19781

Answer

Our definition is similar: Friction is a force that resists the motion of an object that is in contact with another object or material. See the lesson on friction at: http://www.school-for-champions.com/science/friction.htm

What they mean by relative motion is that it tries to cover all possibilities. The friction between object A and object B that are in contact can be caused by A moving and B is still, B is moving and A is still and both are moving at different speeds or directions. From the viewpoint of object A, B is always moving relative to A, no matter which case you consider.

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Difference between acceleration and deceleration

Question

April 11, 2010

dear student helpers
my name is Ahmed Ali and i live in somalia especially lasanod which is the most important town in there,and i'm student.
my aim message is to help me a few questions about the physics:

1-What is the different between acceleration and desseleretin?
2-Give me more information about the linear motoin.
3-What is the exact meaning of pysical sceince?

ahmed - Somalia

19527

Answer

Acceleration is the change in velocity over a period of time. It can mean increasing velocity or decreasing velocity. Often when the velocity is decreasing, it is called deceleration so there is no misunderstanding.

Linear motion is motion in a straight line. Often it is also at a constant velocity, although it is not a requirement.

Physical Science concerns non-living things. It includes Astronomy, Physics, Earth Science and similar sciences.

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Does the moon light depend on the sun?

Question

February 22, 2010

Q 1- I belive that if you look at the moon from the earth you will find an arabic written word " ALI ". have you observed this?

Q 2- I belive that moon light dose not depend on sun light, it has its own light which is soft and cool.

Nasir - Pakistan

19270

Answer

People have observed in many things when looking at the surface of the Moon from the Earth. One example is the "Man in the Moon" that appears to be a face of the moon, caused by the coloring of the craters. However, it is possible that the Arabic written word "ALI" appears to many people. It could be an allusion, or it could have some special meaning.

The light from the moon is reflected from the sun. Much of it is absorbed, making the colors seem cool and soft. When you see the crescent of the moon, you will notice that the list part always faces where the sun is. For example, the crescent will be facing west after the sun has gone down over the horizon.

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How did newton define gravity?

Question

November 19, 2009

does newton define gravity as a force that acts at a distance and accelerates bodies of matter towerd each other?

hadiyah - USA

18702

Answer

Isaac Newton defined gravity that way. For more information, see:
http://www.school-for-champions.com/science/gravity.htm

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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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