Why do things roll?

Question

Why is it so easy to roll a ball or a wheel and yet so much more difficult to slide a block along the floor? What is the difference?

A lot of people are saying surface area and friction. The thing is, is if you have a mass and shape it into a cube and slide it and then shape it into a rectangle with more surface area, both will have the same friction force. Sliding friction is proportional to normal force, not surface area.

Answer 1

i know u want a logical explanation...after all what are the different things ,factors,,,what happens...isnt it?
yeah...here it is then

a very good thing u said just now,...friction is dependant on normal force ...but the question that u asked says that why is it so easy to roll a ball than to slide a block?...
ok,get ready...
why do u think do we feel the need to apply force?
its because another opposing force is there,whixh directly tries to cancell ur motion---the normal force---the force which acts directly opposite to ur force---a force which cancells ur own applied force directly...
now the next question,...why is this force there? between the surfaces in interaction?
it is because of the irregularities in both's surfaces...they interlock....using newton's 3rd law of motion we can say that when we push the body with some force,the body's interlocks exert against one another with the same force....
when our applied force becomes so MUCH that these interlocks GIVE WAY...thats when we overcome friction!

more the surface involved,more are the intelocks involved...BUT,,,the equal action reaction force that U applied is the SAME...ok?

when we SLIDE something,we are having to BREAK these interlocks....when we ROLL something...we are NOT BOUND to BREAK these interlocks....

then now u might say.....why after all..doe sthen friction not become 0 if we do not have to overcome interlocks at all...
my answer is that....there is a COLD WELD or surface electrostatic attraction that is acting like small points on the interacting surface being 'GUMMED OR ADHERED' together...u have to break them too!...breaking them will always be there...whether sliding or rolling.....

here i might also mention where do these come in the formula for friction
formula= mew.mass.gravity
its the mew which is decided by the amount of cold weld power and the amount of interlocks...
i think i have given a ligical explanation for u now...good question...keep thinking that way....do not just go on formulas but solve them out logically....thats the way to physics

Answer 2

Imagine a rectangular block of mass m resting on a horizontal surface. Let us apply a force F in the middle of the vertical face on right. What are the forces acting on the block. downward force, W due to earth's attraction, upward force, R due to reaction, .applied force and force due to friction, f. There is no movement/acceleration possible so R and W always balance. f is a self adjusting force and keeps on balancing the applied Force as long as it is less than cR where c is called coefficient of friction. The box starts sliding when F > cR with an acceleration = [F- cR/m]. If we consider the turning effect of all these forces before F reaches the value of f. we find that turnig would tend to raise many particles up andlower less number of particles of teh block. This requires extra energy. Now imagine the same block of same mass in the form of a ball. turning effect by F will be raising some mass particles up and at the same time lowering almost same number of partcles down. Hence very liitle enrgy or little work is done, the applied force has to do. Also the applied force acts almost on all partcles of the ball whereas frictional force acts only on the partcles in contact which are very very few.

Answer 3

Lower friction is part of the story, but there is another piece as well. Geometrically, the ball has constant width, and symmetry around the center. So its height above a flat surface stays constant the whole time that it moves. Therefore, if you set it in motion it will not move up and down as much as a block, giving the sensation of rolling.

Central symmetry is very critical. There are other shapes in three dimensions with constant width, but which are not centrally symmetric. They sort of bounce rather than rolling smoothly.

Answer 4

Answer: The fact that the friction between the surface of a ball and the ground while it moves is minimum compared to a block.

Answer 5

It has to do with Gravity, friction and what is called "The Moment of Inertia" - Picture, if you will a steel ball. It's center of gravity and its moment of inertia are in the same place - the center of the ball. However if we were to tilt the surface the ball is resting on the moment of inertia shifts to the lower side - this coupled with gravity provides a disporportionate force downwards along one side of the ball. Provided that disproportionality is greater than the coefficient of friction supplied to the ball by the surface it is resting upon, it will cause the ball to roll...

See, simple!

Answer 6

The frictional force opposing movement is porportional to the area in contact. The ball has little surface contact

Answer 7

Contact between the object and the floor. Friction must be overcome for objects to start moving by the application of work. Less contact area means less friction.

Answer 8

surface friction, plus gravity gets on a roll with kinetic energy.

Answer 9

gravity and friction

Answer 10

i don't know try ur self