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# 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

### Country

Motion What is angular motion? Sri Lanka

Newtons Cradle Motion when 1st ball half the other balls Serbia

Pendulum Pendulum at the pole India

Motion Laws Why is the opposite force the same? South Africa

Pendulum Equations Why is pendulum equation approximate? Australia

Pendulum What will be the graph of the pendulum? South Africa

Motion A man crosses a road Namibia

Motion Acceleration versus deceleration USA

Motion How do I know which formula to use? USA

Newtons Cradle What would happen if two balls were smaller? Kuwait

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First 10 letters

## What is angular motion?

Topic: Motion

### Question

April 25, 2011

what is angular motion? what are the eqations we must use when calculating angular velocity ?

zeena - Sri Lanka

21510

### Answer

Angular motion is moving in a curved path. Angular velocity is the rate that the angle changes in a circular path. If R is the radius of the circular path and V is the straight line velocity, then W = V/R is the angular velocity.

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## Motion when 1st ball half the other balls

Topic: Newtons Cradle

### Question

April 19, 2011

Hello! If 1st ball was 1/2 mass of other 4 balls will 5th ball move at all if we swing 1st one.If it does will it very depending on the angle we swing 1sd ball?

Despot - Serbia

21500

### Answer

The idea of Newton's Cradle is to show the conservation of momentum. P = m*v.

The angle the 1st ball swings only determines its velocity. If all balls are equal, the velocity of the 5th ball will be the same as the velocity of the 1st ball.

If the 1st ball was 1/2 the mass of any of the other balls, the 5th ball will move slightly.

If the 1st ball was 1/2 the mass of the total of the other balls, the the 4th and 5th balls will at the velocity of the 1st ball.

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## Pendulum at the pole

Topic: Pendulum

### Question

March 22, 2011

Is the pendulum moves in the poles of the earth?And Is the gravity causes the stopping of pendulum.

Raji - India

21365

### Answer

A pendulum will move in the direction you swing it. However, a pendulum at the North or South Pole will seem to change directions as the Earth spins.

Friction in the wire or string and air resistance are what slow down a pendulum.

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## Why is the opposite force the same?

Topic: Motion Laws

### Question

March 13, 2011

In your quez there is a question or a quez it goes like this(if the explosive charge propels a bullet forward with a force of 25 newton,s what is the force of the recoil of the rifle? The answer is 25N so hows that happen can you please explain it to me so i can undstand it better

Mthobisi - South Africa

21322

### Answer

The reason for this is due to Newton's Third Law of Motion that says for every action, there is an equal opposite reaction. That means that if you push on something with a certain force, an equal force is pushing on you.

It is part of the conservation of momentum. The force on the bullet in one direction requires an equal force on the rifle in the other direction, in order to keep the momentum of the combination zero.

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## Why is pendulum equation approximate?

Topic: Pendulum Equations

### Question

March 13, 2011

I would like to know:
1. For small swing angles the relationship between period, length and gravity is (approximately);why is this only spproximate?
2. what values of gravity obtained (from larger anle or smaller angle) and why?

Ava - Australia

21321

### Answer

The derivation of the motion of a pendulum is complex, using differential equations. However, when the swing angle is small, the equations can be greatly simplified. Otherwise complex equations are used.

The acceleration due to gravity is either 9.8 m/s^2 or 32 ft/s^2.

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## What will be the graph of the pendulum?

Topic: Pendulum

### Question

March 12, 2011

what will be the graph of T/L,the slope of the graph and the relationship?

Precious - South Africa

21315

### Answer

I'm sorry, but we do not draw graphs.

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## A man crosses a road

Topic: Motion

### Question

March 9, 2011

1. A man crosses aroad 8.0m wide at speed of 2.0m/s.
How long does the man take to cross the road?

An object is falling freely naer to the earth surface.
Which word describes the acceleration of the object?
1.constant
2.Decreasing
3.Increasing
4. Zero

Hilma - Namibia

21310

### Answer

Distance = speed times time or Time = distance / speed

Time = 8.0 m / 2.0 m/s = 4 s

Acceleration due to gravity is a constant. (See our lessons on Gravity).

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## Acceleration versus deceleration

Topic: Motion

### Question

March 8, 2011

I would like to say that in physical science we do not use deceleration as you would in an English class setting. Since acceleration is a vector quantity then it must use direction and so in the case your site speaks of one should use the opposite direction, not use the word deceleration. When we try to reteach even such common errors it is very difficult.

Dawn - USA

21306

### Answer

Thanks for your comment. You bring up a point of controversy and one of the confusing aspects of scientific terminology, since some refer to both acceleration and deceleration as vectors, which are opposite in meaning.

However, the accepted physics definitions are that acceleration is a vector that is any change in velocity, while deceleration is a scalar indicating a decrease in speed. But an increase in speed is also called acceleration and is a scalar quantity.

It is an unfortunate and confusing standard. But to make sure there is no confusion among students, I'll update the motion material and try to clarify the distinction.

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## How do I know which formula to use?

Topic: Motion

### Question

March 7, 2011

When I read the material I understand it but how do I know which formula to use to get the answer.

Sherry - USA

21303

### Answer

It depends on what you are trying to find or solve for. Some equations and formulas can be rearranged if you are given only certain variables.

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## What would happen if two balls were smaller?

Topic: Newtons Cradle

### Question

February 26, 2011

I have two questions. If you don't mind:
1- What would happen if all balls are in same size and the last two balls have smaller size (for example have a size of half diameter) but all 5 balls are made from same material (steel) and I only raise the 1st ball and then release it?

2- What would happen if I have all 5 balls have the same size and made of steel except the 4th ball is made of copper but has the same size as the other balls and I only raise the 1st ball and then release it?
Thanks in advance

Alanziy - Kuwait

21258

### Answer

Newton's Cradle works on the principle of the Conservation of Momentum, which is mass times velocity. That means that if you lift a certain mass on one side of the cradle, the same mass will move on the other side of the cradle, due to the transfer of momentum.

If the mass of the two smaller balls equals the mass of a regular ball, then when you lift the regular ball on one end, the two smaller balls will move on the other end. However, if the two balls way you'd are more or less than a single ball, you will get an uneven reaction.

You can have balls of different sizes or materials, as long as they have the same mass the cradle will work properly.

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