# Feedback Comments on Motion

by Ron Kurtus

A total of **228** comments and questions have been sent in. They are listed according to date.

## List of next 10 letters

## Topic |
## Title |
## Country |

Motion |
Difference between a vector and scalar quantity | RSA |

Newtons Cradle |
Want to buy grandson a Newton's Cradle | USA |

Motion |
Relative velocity of two objects | India |

Motion |
Equations of uniform accelerated motion | USA |

Motion |
Does a compressed spring have more mass? | Pakistan |

Motion |
Can an object at constant velocity be accelerating? | Pakistan |

Pendulum |
How grandfather clock works | USA |

Motion |
Relative motion and reference point | India |

Pendulum |
How long will a Foucault pendulum swing? | Croatia |

Periodic Motion |
Why not express harmonic motion in two variables? | India |

## Difference between a vector and scalar quantity

Topic: **Motion**

### Question

February 15, 2012

i want to know the difference between a vector and scalar quantity

sinenhlanhla - **RSA**

22422

### Answer

A vector has both magnitude and direction, while a scalar only has magnitude. For example, when you say an automobile is traveling at 25 km/hr toward the east, it is a vector. But when you only say it is traveling at 25 km/hr, it is a scalar quantity.

For an illustration, see: Vectors in Gravity Equations.

I hope that helps.

## Want to buy grandson a Newton's Cradle

Topic: **Newtons Cradle**

### Question

December 2, 2011

I would like to buy my 11year old grandson a newton's cradle for Christmas, but am confused on what to buy, so many choices and prices. What do you suggest? Thank you

anita - **USA**

22233

### Answer

Yes, there is a range of prices and type. I've seen a 3 1/2 inch one for $4 up to a 7" for $15. Some executive versions are up to $40.

I have a $15 7" version, and it seems to work well with reasonable quality.

One thing to consider is that although it is an interesting device, it isn't something that is used much after the first few times. It might be good as part of several scientific gadgets.

Best wishes for a Merry Christmas for you and your family.

## Relative velocity of two objects

Topic: **Motion**

### Question

November 19, 2011

if a body moves from point A to B with a velocity of 10 m/s and returns from B to A with velocity 20 m/s. what is the relative velocity? please explain.

MITHILESH - **India**

22178

### Answer

If you see the body move from A to B and then back again, the body is moving relative to you as the observer. However, the assumption is that both A and B are not moving with respect to you. If one is moving, with respect to the other, you could then determine the relative velocity between them.

## Equations of uniform accelerated motion

Topic: **Motion**

### Question

November 1, 2011

please i need explanations of the equations of uniform accelerated motions

Emeka - **USA**

22126

### Answer

Uniform acceleration is when the acceleration is constant, as opposed to varying with time. For example, in the equation F = ma, the force will be constant if the acceleration "a" is uniform or constant. Likewise, in the gravity equation, F = mg, the acceleration due to gravity "g" is a constant.

## Does a compressed spring have more mass?

Topic: **Motion**

### Question

October 8, 2011

since mass is a form of energy,can we conclude that a compressed spring has more mass than the same spring when it is not compressed?

another question sir,if the speed of light were infinite,what woulde the equation of special theory of relativity reduce to?

owais - **Pakistan**

22047

### Answer

Mass is really not a form of energy. Instead, in nuclear reactions, some of the mass can be converted into energy in the form of electromagnetic radiation.

Note that radiation energy is different that kinetic energy of a moving mass. When a spring is compressed, its molecules are pushed together, creating a potential energy. Once released, the spring moves in kinetic energy of its mass. The equation is PE = KE, ans no mass is gained or lost.

If c was infinite, some relativity equations, such as E = mc^2 would result in infinite energy. Other equations have a v^2/c^2 factor, which would reduce to zero.

## Can an object at constant velocity be accelerating?

Topic: **Motion**

### Question

October 8, 2011

respected sir ihave a question that.is it possible for a body to have acceleration when it is moving with uniform velocity or with uniform speed ?

explain briefly the answer?

owais - **Pakistan**

22046

### Answer

Acceleration is the change in velocity with respect to time. The only way an object moving at a constant velocity could be accelerating would be if the velocity or speed was with respect to an object that was accelerating.

For example, you could walk down the aisle in a train at a constant speed. However, if the train was accelerating with respect to someone outside, you would also be seen as accelerating. In other words, it depends on the point of view.

## How grandfather clock works

Topic: **Pendulum**

### Question

August 19, 2011

A point on your grandfather clock statement "Every time the pendulum goes back and forth, it moves a gear one notch."

The pendulum causes the escapement to "rock" and releases the timing gear for a beat. The timing gear rotation is a result of the gravitational pull on the clock weight.

Regards,

John - **USA**

21901

### Answer

Thanks for clarifying that point. I'll update the page to add that information.

## Relative motion and reference point

Topic: **Motion**

### Question

August 18, 2011

what ia relative motion ? how it related to reference point

payal - **India**

21899

### Answer

If you are standing still and see an automobile drive by, it is moving relative to you. You are the reference point.

However, to the person in the automobile, you seem to be moving in the opposite direction. Also, some cars are moving faster and some slower to that person, who is a different reference point than you.

## How long will a Foucault pendulum swing?

Topic: **Pendulum**

### Question

July 28, 2011

How long can swiming Foucault pendelums without adding energy.

Milan - **Croatia**

21847

### Answer

Because of the long wire and heavy weight, the Foucault pendulum can swing for a very long time. Air resistance on the weight and wire, as well as friction on the will will slow it down. Unfortunately, that varies from pendulum to pendulum.

## Why not express harmonic motion in two variables?

Topic: **Periodic Motion**

### Question

May 3, 2011

WHY SIMPLE HARMONIC MOTION IS NOT EXPRESSED IN TERMS OF TWO PERIODIC FUNCTIONS? DOES THE PARTICLE IN SIMPLE HARMONIC MOTION UNDERGO DISPLACEMENT IN TWO DIRECTIONS (x and y)?athi

athira - **India**

21544

### Answer

Although you can express periodic motion in the x- and y-coordinates, it is much simpler to use a polar coordinate system, such that you have use the orbital velocity. However, in the polar system, you can also show the harmonic motion with respect to the angle moved.

## Summary

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