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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 Force to stop a truck Ireland

Forces This is a really good website England

Forces Accelerating a crate Australia

Forces and Torque How do super fast trains work without friction? India

Forces and Torque Have to assemble over 20,000 pulleys Brisbane Australia

Forces How do you convert fractions into decimals? UK

Forces 5 main key words in force UK

Forces Could you tell me about air resistance? England

Forces Stopping a heavy truck versus a light truck USA

Forces Comparing punch with fist and boxing glove USA

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First 10 letters

## Force to stop a truck

Topic: Forces

### Question

September 5, 2006

If a truck has a mass of 60tonnes and is travelling at 80k/h what is its kinetic energy and how much force would it take to stop it if its brakes failed?

If the same truck was travelling down a 5degree gradient, what force would then be needed to stop it.

Joseph - Ireland

11849

Kinetic energy = mass x velocity squared, divided by 2. So, you have 60 x 80 x 80 / 2.

Force equals mass times acceleration. In order to know the force required to stop the truck, you need to know how fast it is supposed to stop. It takes less force to stop the truck in 1 minute than in 1 second.

## This is a really good website

Topic: Forces

### Question

September 1, 2006

This is a really good website. Very helpful. I've sent a couple of questions already. I visited once when I was a week away from my sats in yr 6

Brooke - England

11812

Thanks for the feedback. I'm glad you like the site.

## Accelerating a crate

Topic: Forces

### Question

August 13, 2006

Dear Sir

I was wondering, if you apply a constant force to something, like when pulling a crate, does it accelerate? why? How then do you pull something and get it to travel at a constant speed?

Kind regards,

Annette - Australia

11696

When you start from zero speed and go up to some other speed, you are accelerating the object. Acceleration is the change in speed or velocity. If there was no friction, an object would continue to travel at a constant speed. Otherwise you need to apply just enough force to equal the force of friction.

## How do super fast trains work without friction?

Topic: Forces and Torque

### Question

July 29, 2006

hello sir,
SIr we know that friction provides the force for a vehicle to move. But tehn hiow does suoper fast trains move when we know that their wheels arnt in direct contact with the rails?

Ayush - India

11565

In a normal vehicle, such as an automobile, the engine provides a torque on the wheels. Friction resists the torque, causing the vehicle to move forward. Remember that friction is a resistive force.

The super fast trains are often called mag-lev trains, because a magnetic force powers the motion. They go fast because no friction resists the motion.

## Have to assemble over 20,000 pulleys

Topic: Forces and Torque

### Question

May 11, 2006

Good Morning, I have to assemble over 20,000 pulleys with 2 bearings and 1 shaft per pulley, they have to rotate freely about the shaft with a maximum initial torque no greater than .34 n/m or 3 lb.in., is there a device or a machine that i can use to apply and measure this load.
Thanks Mike

Mike - Brisbane Australia

11082

Good golly! It will take you forever to assemble over 20,000 pulleys. I assume your initial torque is for the whole system and not each individual pulley. With so many pulleys, the initial force would be enormous.

The torque divided by the radius of a pulley should give you the force required to turn it. A simple spring scale should measure that force.

## How do you convert fractions into decimals?

Topic: Forces

### Question

April 22, 2006

How do you convert fractions into decimals and decimals into fractions?

From Pooja

Pooja - UK

10911

The "/" sign in a fraction means "divided by." Thus, a fraction such as 12/36 means "12 divided by 36". When you divide it out, you will get a decimal, such as 0.333.

Going the other direction is not as easy. A decimal such as 0.1 equals 1/10, 0.23 = 23/100, and 0.224 = 224/1000. You can simplify the fractions, like 224/1000 = 28/125.

## 5 main key words in force

Topic: Forces

### Question

April 20, 2006

i would really appreciate it if you helped me on this subject so that i could better at science because it is my favourite subject.
what are the 5 main key words in force, i only know two of them

stephanie - UK

10892

I'm sorry, but I'm not sure what your teacher means by that. I guess you could say: mass, push, pull, acceleration, and pressure.

## Could you tell me about air resistance?

Topic: Forces

### Question

April 4, 2006

could you tell me more about air resistance and how this force works?

Brooke - England

10743

When moving an object through a fluid like air or water, the object must move the fluid out of the way. When moving through air, the force required to move the air out of the way is called the air resistance.

If the object is streamlined, then the air easily moves out of the way and the air resistance is small. If you place the palm of you hand out the window of a moving car, you can feel the air resistance on something that is not streamlined.

Air resistance is a force that moves in the opposite direction of motion, slowing things down. In that way, it is similar to the force of friction.

## Stopping a heavy truck versus a light truck

Topic: Forces

### Question

March 23, 2006

does it require more work to stop a light truck or heavy truck moving with the same momentum? i thought it would be the heavy truck because it has more mass.

- USA

10577

This problem requires a logical look at the situation:

Work = force times distance. Force = mass times acceleration. Acceleration = change in velocity. Momentum = mass time velocity.

If the trucks have the same momentum, the heavier truck is going slower. Thus, its acceleration (or deceleration in the case of slowing down) is less. That means the force to stop the heavy truck is less. Since they travel the same distance to the red light, the work is less to stop the heavy truck.

## Comparing punch with fist and boxing glove

Topic: Forces

### Question

March 23, 2006

Why is a punch more forceful with a bare fist than with a boxing glove?

- USA

10574

A boxing glove absorbs some of the force, spreading the energy through the material. The cushioning effect of the boxing glove means that the force of the blow is not as immediate as when hitting with a hard fist.

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