Archive for the ‘physics’ Category

Will an object ever stop because of friction

If an object is experiencing some form of resistance, such as friction or air resistance, will this force ever cause it to stop completely. For example when a car does and emergency stop does it in fact stop?

The obvious answer is yes but it would appear that it doesn’t because of the following:
1) The resistive force is some function of the objects velocity and is probably either proportional to its velocity or the square of its velocity.
2)When an object is at rest it experiences no resistive force, if it did it would start moving again.

This means that as a objects velocity approaches 0 the force that is causing it to slow down is also approaching 0. This means that the velocity of the object wont actually reach 0 but will become infinitely small.

You may therefore think that there is some sort of minimum friction, in addition to the component which is proportional to the objects velocity but this would mean that there would still be friction when the object is at rest so the object would then begin to move backwards.

Any comments or further thoughts would be appreciated to help explain this.

By David Woodford


Moments – Turning Forces

March 16, 2009 6 comments

Moments are often called turning forces. They are the product of the distance a force is acting from the point being considered and the component of the force acting perpendicular to the direction.

The most common example of moments used in on a see saw. Here there a two levers, each side of the see saw, acting from the central pivot. if a person sits on the see saw there is a moment about the pivot because of there weight. While the seesaw is still horizontal this moment is the product of their weight (mass times gravity) and the distance they are still from the pivot because the distance a long the see saw to the pivot is perpendicular to their weight.

Like forces if something is in equilibrium, velocity isn’t changing (usually but not always at rest), the moments have to be balanced. Because moments are turning forces this means that all the moments acting clockwise have to equal all the moments acting anti clockwise. This means that if two people are sitting on the seesaw provided the product of their weight and distance is the same the seesaw wont turn, even if one person is much heavier than the other. For example if a heavy person sits close to the centre they can be balanced by a light person sitting further away.

Just to note – moments don’t have to be calculated about a pivot or turning point, they can be found about any point, so we cud take them about one of the people or the end of the seesaw. IF you do this though, remember that the pivot is providing an upward force on the seesaw that is equal to its mass plus that of the two people times gravity. This sort of method can often be useful in more complex systems as by taking moments about a point then we can ignore all forces acting through it (as their distance is 0) and simplify our equations.

However we can also calculate the moments caused by forces that aren’t perpendicular to their distance, for example the moment cause by the person on the seesaw when it is titled or on the ground. Here we have to find the component of the force that is in the direction that is perpendicular to the distance, by resolving using sin and cos.

This is done by multiplying the product either by the sin of the angle between force and the direction of the distance between the force and pivot or the cos of the angle between them force and the direction perpendicular to the distance.

for example if the seesaw is tilted by 30 degrees from the horizontal we can have to multiply by

cos(30)=sin(60) =√3 /2

By David Woodford,

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