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by Ron Kurtus

A total of 162 comments and questions have been sent in. They are listed according to date.

## List of next 10 letters

### Country

Forces What is the measurement of force? USA

Forces How much work is done when force moves object? USA

Forces Forces and vectors USA

Forces Why do planets stay in orbit? England

Forces Trying to find the acceleration USA

Forces This website is sweet USA

Forces Applying a force through the center of mass India

Forces Finding average acceleration USA

Forces Constructing strong paper arm for experiment USA

Forces Lifting weight with strip of paper USA

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First 10 letters

## What is the measurement of force?

Topic: Forces

### Question

October 23, 2007

in what do you express the measurement of force?

Dominique Anne - USA

14625

Force is usually expressed in newtons in the metric system and pounds in the English system.

## How much work is done when force moves object?

Topic: Forces

### Question

August 25, 2007

How much work (in Joules) is done when a force of 14N (Newtons)moves an object 6 meters?

Nahiara - USA

14255

A Joule is the work done by 1 Newton of force moving an object 1 meter.

Thus 14N * 6m = 84 Joules.

## Forces and vectors

Topic: Forces

### Question

April 27, 2007

why do we represent force with a vector

fariha - USA

13654

Because force is applied in a given direction. Anything that has a direction is a vector.

## Why do planets stay in orbit?

Topic: Forces

### Question

April 18, 2007

I have a homework question that I really can't figure out. Maybe you could help. It says: Why do planets stay up in space and don't fall? Any ideas?

Brooke - England

13598

The centrifugal force caused by the motion of the planets equal to all its the attraction from the gravity of the sun, thus allowing them to stay in orbit. Swing a ball on a string around you and then let it go, and it will fly outward. That is the force that equalizes the attraction toward the sun.

## Trying to find the acceleration

Topic: Forces

### Question

March 5, 2007

Hi. I'm Mel. I'm learning about acceleration right now. Here's the problem :
A box is being pulled along the floor with a pulling force of 55 N. The friction's force is 5 N. The box is 30kg. What is the crate's acceleration, in meters per second?

I know acceleration is the change in velocity over time, but they don't include a time in the problem. I'm busting my head and can't figure this out. Please help.
Thank you,

Mel - USA

13313

Force = mass times acceleration F = ma. Thus, a = F/m.

The force is 55 N - 5 N = 50 N. Thus the acceleration a = 50 N / 30 kg.

## This website is sweet

Topic: Forces

### Question

February 22, 2007

This website is sweet

Brittney - USA

13226

Thanks for the sweet feedback.

## Applying a force through the center of mass

Topic: Forces

### Question

January 15, 2007

sir,

when we apply force on a rigid body did all the points of the body r affected by the applied force?
what r u meant by point of application of force?did it is different from center of mass of a body?
we apply forces by our hands on a body(for eg:on a block).there r many points on our hand.then what is point of application of force on the block.this is confusing me.

john - India

12835

The center of mass of a body is the balance point of the object. When you apply a force on a rigid body on a line that passes through the center of mass (also called the center of gravity), the object will move as if the force was applied at all points in that direction. But if the force is applied at a point away from the center of mass, a torque or turning will result.

Take a ruler or stick and measure its center of gravity. Then place it on the desk and push on it through that center point. The ruler will slide forward. But if you push in the same direction at a point not through the center, the ruler will rotate. This is why a while will roll and not slide when a force is applied on its edge.

## Finding average acceleration

Topic: Forces

### Question

November 1, 2006

a roller coaster is moving at 10m/s at the top of a hill and 22 m/s at the bottom of the hill two seconds later. Explain how to find the average acceleration of the roller coaster, and then find the average acceleration.

Jennifer - USA

12281

Average acceleration is speed at the bottom, minus the speed at the top, divided by the time.

## Constructing strong paper arm for experiment

Topic: Forces

### Question

October 15, 2006

Dear Mr. Kurtus,

I've wasted a full pack of copy paper trying to find a solution to the project: Construct the lightest and longest possible paper arm that lifts a 200 gram mass. I am exhausted and can do nothing but wait for your answer desperately.

Thanks.

George - USA

12168

The strongest arm would probably a triangular tube. A good size would be to take a sheet of paper and cut off a sheet lengthwise, about 1/8 of the width. fold that paper into a long triangular shaped tube, gluing or taping the sides.

The point where the tube would bend upon adding a weight at its end would probably be near the place you fix or hold the tube. You can test this first 1-sheet tube to see how much weight it would take to bend it.

When you add end-to-end to increase the length, you are also adding the weight of the paper to the 200 grams. This determines the limitation of the length.

Making the tube smaller in diameter cuts the weight and can actually strengthen the tube. But it also becomes difficult to make. Experiment with a single tube to find the optimum diameter.

I hope those ideas help.

## Lifting weight with strip of paper

Topic: Forces

### Question

October 13, 2006

Please give me some clues to resolve the following question:
"Consturct the lightest & longest possible paper arm that lifts a 200 gram mass up to at least 30cm high."
1)The arm may only be made from 8 1/2x11 white copy paper (20Ib. weight). No tape, glue or adhesive.
2)The contestant must remain in contact with the floor at all times and behind one end of the table.
3)A 200 gram standard cylindrical brass weight with (large paper clip) hook on tip will be placed on a lab table at a distance from the end of the table determined by the contestant (Minimum distance=50cm)
4)The contestant must position himself so that no part of his body breaks the vertical plane of the end of the table.

Thank you very much in advance.

George - USA

12153

I just answered your second email before reading this one. Let's start over with the new rules:

At first it seems like the arm must be a single lever where the weight is placed at its end. But the hook on the weight implies that you could hold a long strand of paper on two ends and loop it around the hook and lift the weight. Although you body cannot lean over to pick up the weight, I see nothing against the weight swinging toward you when you lift it.

You can carefully cut a piece of paper in sort of zig-zag pattern to make it into a long thin strip. Then you could hold both ends and loop the paper around the hook to lift the weight. The limitation of the thickness of the strip would be the strength of the paper. You might use less than a sheet of paper with a very thin and long strip.

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