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Feedback Comments on Force

by Ron Kurtus

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

You can read them to further your understanding of the subject.

List of next 10 letters




Fundamental Forces Why can't a sharp tool pry away electrons? India
Force Damage when two cars collide USA
Force Standing on ice and deflects ball USA
Force What is the relative magnitudes of forces in moving car? USA
Force Person flies forward when bus stops USA
Forces and Torque How to convert torque to linear force? USA
Force at Distance Is wind an example of force at a distance? USA
Force Verticle acceleration and calculating loads India
Forces What happens when mud strikes a wall? Pakistan
Forces and Torque Give me an example of torque pakistan

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First 10 letters

Why can't a sharp tool pry away electrons?

Topic: Fundamental Forces


December 16, 2008

Why cannot a very sharp tool driven by a hammer be used to split or prise electrons away from an atom ? Is not this force sufficient ?

Sridhar - India



Although a very sharp tool is able to cut common materials, examining its point under a microscope will show that its diameter is many atoms thick. Thus it with not be effective in breaking away electrons from an atom.

On the other hand, molecular forces like adhesion, which is used in static electricity will easily pry away electrons from a material.

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Damage when two cars collide

Topic: Force


July 28, 2008

Had this question in class the other day and I'm unable to apply the conservation of energy theory on this one. Im in a car and there is an exact same car traveling towards me head on at the same speed, if i swerve I would collide with a massive concrete wall, which one would be less damaging?

Steve - USA



If you shot a solid ball at a solid wall of extreme mass, the conservation of energy would require the ball to reflect back with the same velocity. Likewise, if two solid balls traveling at the same speed--such as on a pool table--hit each other head on, each would bounce back at the same speed. This is due to the conservation of energy and momentum. So in both cases, the force on the one ball to change directions is the same.

The damage done on a car in a collision is caused by the force pushing the car backwards. The example with the solid ball says the force is the same in both situations. The speed that the cars move backwards after the collision is greatly decreased because of the energy expended in damaging the cars. The assumption is that the wall does not become damaged by the collision.

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Standing on ice and deflects ball

Topic: Force


July 28, 2008

So if Im standing stationary on a frozen pond is surprised by quickly approaching heavy ball and I deflect it rather than catch it, what effect will it have on me?

Steve - USA



If you catch the ball of mass "m" and velocity "v", you will slide backwards at velocity "V", where your mass is "M", according to the solution of the equations: mv = (m + M)V and mv^2/2 = (m + M)V^2/2, which is the conservation of momentum and conservations of energy, respectively.

If you deflect the ball at an angle, how much you move back will depend on the angle it is deflected.

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What is the relative magnitudes of forces in moving car?

Topic: Force


July 11, 2008

I have this exercise that says: Susan is driving down the road at a constant speed. Explain the forces acting on the car and their relative magnitudes.
I would like you help me because this is the first time that I have to solve an exercise like this.

Laura - USA



Think of all the things that can slow down a car.

First of all, there is the force of air resistance. It depends on how fast the car is going and on how streamlined the car is. Its magnitude is difficult to estimate because of the differences in streamlining in cars. Just having the windows open would change the values.

Then there is resistance from the tires. This depends on how much air is in the tires and the weight of the car and passengers.

There is some resistance from fiction in the drive train and transmission of the car.

Finally, the force applied by the engine through the tires must equal the sum of the other three resistances, since there is no acceleration. Thus, the relative magnitudes of the resistance and motion forces are equal.

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Person flies forward when bus stops

Topic: Force


April 18, 2008

What is the force of impact to a person, unrestrained, in a wheelchair (weighing 250lbs) riding in a 60' bus traveling at 40 mph that comes to a sudden stop (stopping distance in 40'). The person in the wheelchair is thrown 10' forward onto the floor.

Christine - USA



I assume this is a science question and not something for an insurance claim.

The impact force would be the mass or weight times the deceleration. F=ma.

The time to stop is distance to stop (40 feet) divided by the the speed in feet/second.

The deceleration is the velocity divided by the time.

The size of the bus does not matter, but there are problems which affect the solution: Did the wheelchair roll forward before the person flew out? Did the person slide across the floor? How much friction was there?

If this is for an insurance claim, the actual force does not matter. It is the injury sustained that matters.

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How to convert torque to linear force?

Topic: Forces and Torque


January 16, 2008

I have a question regarding linear force and torque.
How much linear force at a fulcrum point would be needed to lift 8 lbs. at 2 ft.? To my understanding it would need 16 ft./lbs. of torque but how do I convert that to linear force at its fulcrum?

Justin - USA



If you have an axle that you are applying a 16 ft-lb torque to, it would lift 8 lbs on an arm 2 ft from the axle. You could call the axle the fulcrum point, but it usually isn't referred to that. Now the axle has to sit on something, and the force on that would be the 8 pound weight plus the weight of the axle and arm.

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Is wind an example of force at a distance?

Topic: Force at Distance


January 14, 2008

is wind an example of force at a distance? If not, why not?




Wind consists of moving atoms or molecules that are directly striking against an object. Thus, they are not a force at a distance, which is a force caused by some field where matter is relatively far apart.

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Verticle acceleration and calculating loads

Topic: Force


January 8, 2008

Dear sir,
Iwant to know about vertical acceleration.why it is considered while calculating loads.

pratyusha - India



If you calculate a load, you are measuring the effect of gravity on items. Any vertical acceleration will be working against the acceleration due to gravity and must be subtracted in the calculations.

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What happens when mud strikes a wall?

Topic: Forces


January 8, 2008

why Newtons third law is not corrected when we strike the mud to the wall?

Shahid - Pakistan



Most of the energy in the collision is dissipated by the deformation of the mud. Newtons law will work for a solid, hard object. But when something is soft, the energy goes into deforming the material.

Note that if the mud is wet, it will spatter when it strikes the wall.

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Give me an example of torque

Topic: Forces and Torque


October 26, 2007

i m not understanding about torque give me a good example

monna - pakistan



When you push on the outside of a wheel, it will turn on its axle. You provide a force on the wheel and that is converted to a torque on the axle.

Likewise, when you turn a screwdriver, it provides a torque on the screw to turn it.

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