|What causes a rolling ball to stop?|
|Question Date: 2020-03-01|
First consider friction. If the motion is pure rolling of a perfectly smooth ball on a perfectly flat surface, then the friction between the surface and the ball does not do any work in slowing the ball. This is likely counterintuitive, but realize that work requires that the acting force (which here would be due to friction) cause relative motion between the bodies (that is, force from friction would need to cause the ball to move sideways relative to whatever it is rolling on). In pure rolling, the ball is rotating around an axis, but NOT translating relative to the point of contact between the ball and the surface (in pure rolling that point of contact is instantaneously stationary). BUT no surfaces are perfectly flat, and any roughness leads to some amount of sliding and therefore some frictional dissipation of energy. Furthermore, unless this experiment is conducted in a complete vacuum, the ball will be moving through an atmosphere of gas and microscopic particles. As these contact the ball and are pushed out of the way, there is relative motion of the ball surface and those particles. As a result, there is friction between the ball and the atmosphere (called drag ) which slows the ball.
Now consider inelastic deformation. When a force is applied to an object made of a real material, that object deforms. When the force is removed, the object returns at least partially to its original shape. In the case of a rolling ball, the weight of the ball leads to forces acting on both the ball and the surface (by Newton's laws, must have a force in both bodies). This means that both the ball and the surface deform. One consequence of this deformation is to cause friction force that does oppose the motion of the ball. Another is that the ball and surface contact over an area and the force across that area is non-uniform (because the return to the original shape is non-uniform), which ultimately produces a force that opposes the rotation of the ball. (Basically, deforming the surfaces involves motion of the material of the ball and the surface, meaning is work being done, which further means that some energy is transferred out of the ball.)
One might also wonder where the energy "goes" once it leaves the ball. After all, total energy of a system is conserved; it cannot simply disappear. In this case, the system is not the ball alone; it is both the ball and the surface it is rolling on (and the atmosphere, if one is present). The friction and deformation processes discussed above cause random motion of the individual particles in all of these, effectively meaning that they increase in temperature. Thus, the kinetic energy of the ball is transformed into thermal energy.
[Most information taken from Physics4Kids , BCcampus physics , Prof. (emeritus) Simanek of Lockhaven Univ. of PA , and this article from CSU Physics . The last two are more suitable for readers with at least some physics background.]
Newton's first law states that an object in motion will remain in motion unless acted upon by a force. This may at first seem nonsensical, because when you roll a ball, it eventually stops rolling. Your observation is true, so does this mean Newton's first law must be false? Not necessarily. Let us examine the consequences if both your observation AND Newton's first law are true: if a rolling ball eventually stops rolling, according to Newton, this means it must be acted upon by a force. The force in question is friction. A rolling ball stops rolling for the same basic reason that if you slide a book across the floor, it will eventually stop: there is friction between the floor and the book.
There is also friction between the ground and part of the ball that touches the ground as it rolls. The friction force acts in the opposite direction to the motion of the ball, slowing it and eventually stopping it.
There are three laws in physics that tell us how objects we can see move in space. These are called Newton's Laws of Motion.
The first law tells us that objects in motion will stay in motion, and object at rest will stay at rest, unless they are acted on by an outside force. If we saw a ball rolling, and it had no forces acting on it, it would keep rolling in a straight line forever! However, we know that on Earth, we experience many forces on us. The ball, for instance, will feel the force of gravity pulling it downward, the force of the ground pushing it upward direction it is rolling. Since the force of friction goes against the direction that the ball is moving, that is the force that will cause it to slow down. The force from gravity and the force from the ground only act straight up and down, so they will not affect the ball's speed on a horizontal surface.
Hello Rana, your question is an interesting one because there is more than one possible explanation. If we consider a ball rolling on a flat surface that we push a bit and watch as it slows down, the answer to your question is friction. Friction happens because part of the kinetic energy of the ball (energy from moving) is given away in the form of heat to the surface on which it is rolling and in the form of kinetic energy to molecules in the air that it is pushing away. To think about both of these types of friction consider the following:
1. Friction with the surface: if I rub my hand on an oily surface, my hand will easily slide over it. Conversely, if I rub my hand on a rough surface, my hand will have much more trouble and will feel hot afterwards. This is because in the case of the oily surface, there is much less friction due differences in interactions between the molecules in my hand and those in the oil on the surface. The friction that my hand experiences on the rough surface makes it much harder to rub on there. As I rub it, the molecular interactions between my hand and the surface will lead to heating for both my hand and the surface and therefore, a transfer of energy. The same occurs for the ball, transferring its kinetic energy to heat, slowing it down as it rolls on a rough surface.
2. Friction with the air: if you have ever put your hand outside of a car while driving, you know that you will experience drag from the air. This is because as you are moving, you have to push the air molecules away as your hand starts to occupy the same space as the air molecules previously were. Energy has to be given to the air molecules in order to make them move. For a ball rolling on a surface, the same applies. As it rolls, it has to give up some of its energy to the air molecules that it must push away. Theoretically, if we rolled the ball on a frictionless table in air, no energy would be given away to friction with the table but the ball would still slow down as it gives its energy to the air molecules that it pushes as it rolls forward.
Friction - as the ball rolls, the ball loses its energy to heat and sound. As the energy is lost, the ball slows down and eventually stops.
Friction - the force that is preventing the ball from moving on and on and on.
"When you roll a ball on the ground, the electrons in the atoms on the surface of the ground push against the electrons in the atoms on the surface of your ball that is touching the ground. A rolling ball stops because the surface on which it rolls resists its motion. A rolling ball stops because of friction." ScienceLine.
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