Why Does a Glass Move Outward When Opening the Fridge Door?

[Ibix]

No force can be exerted without resistance.

Do you think the other planet is massless and has no inertia?

 

[ALBAR]

You mean motion is impossible? Can't be. A force can result in acceleration! Try kicking a football.

This is exactly why we need to differentiate cause from effect. The football must push back 100% at every instant force is applied, but it undergoes acceleration as an effect of the force being applied. The acceleration triggers the resistive force per the second law resulting in a symmetric pair of opposing forces.

 

[jbriggs444]

The acceleration triggers the resistive force per the second law

Correlation, not causation. Whether the ball strikes your foot or your foot strikes the ball, a strain arises, a stress arises and a pair of forces arises.

Normally we consider that the relative motion of ball and foot is the "cause" and the ensuing contact deformation, force pair and acceleration are "effects".

However, the second law is neutral on the matter. It only assures us that when a net force exists, so will an acceleration. And conversely, if there is an acceleration, there must be a net force.

Same for the third law. It only assures us that a force from A on B in one direction will be accompanied by an equal force of B on A in the opposite direction. It is silent about the question of causation.

 

[Ibix]

The acceleration triggers the resistive force per the second law resulting in a symmetric pair of opposing forces.

But your foot accelerates, so that must be the effect and the ball the cause!

 

[sophiecentaur]

If the gravitational forces are reacting to something, then what are the other two forces that the third law requires doing?

Whaaaat? You now have three forces?? Going completely classical, there's a force acting on the CM of Earth - just like a piece of string and a ball whirling round . The string pulls against the ball and the ball pulls against the string. Also the string pulls on your hand as you pull it. That's N3 happening at each end of the string. Connect the ball directly to your hand (throw away the string) and the direct force is the same as the tension in the string. The gravitational attraction between Earth and Moon is the equivalent to the tension in the string.

 

[ALBAR]

But your foot accelerates, so that must be the effect and the ball the cause!

We are getting into collision physics now. Actually the foot decelerates due to the counter force it needs to exert any force at all on the football.

 

[jbriggs444]

We are getting into collision physics now. Actually the foot decelerates due to the counter force it needs to exert any force at all on the football.

The decelleration of the foot (times the mass of the foot) matches the contact force of ball on foot. That's Newton's second law.

No need to invoke magical counter forces. Or anthropomorphic "needs".

 

[A.T.]

Actually the foot decelerates ....

Whether the speed to the foot increases or decreases is just a matter of the chosen reference frame. If your "causation logic" depends on that arbitrary choice, then it is equally arbitrary, and not physically relevant.

 

[Ibix]

Actually the foot decelerates due to the counter force it needs to exert any force at all on the football.

The foot decelerates because the ball exerts a force on it. That's Newton's second law.

 

[ALBAR]

Whether the speed to the foot increases or decreases is just a matter of the chosen reference frame. If your "causation logic" depends on that arbitrary choice, then it is equally arbitrary, and not physically relevant.

Please, PLEASE let's stay in the good old Newtonian inertial reference frame when discussing the most basic principles of mechanics.

 

[jbriggs444]

Please, PLEASE let's stay in the good old Newtonian inertial reference frame when discussing the most basic principles of mechanics.

There is more than one Newtonian inertial reference frame. Newton's laws do not pick out a preferred frame. The first law picks out a class of inertial reference frames, not a single frame.

 

[Orodruin]

Please, PLEASE let's stay in the good old Newtonian inertial reference frame when discussing the most basic principles of mechanics.

Nobody said anything about non-inertial frames. Just two different inertial frames. All you are really doing in this thread is showing off a seemingly quite lacking grasp of classical mechanics discussing with people who do understand it in quite great detail.

 

[Ibix]

Please, PLEASE let's stay in the good old Newtonian inertial reference frame when discussing the most basic principles of mechanics.

There's an inertial frame where the foot is at rest the instant before contact and another where the ball is at rest. And another where the ball is at rest after contact and another where the foot is at rest. And an infinity of frames where neither is ever at rest. There are infinitely many where the foot slows down and infinitely many where it speeds up.

Nobody needs to invoke non-inertial frames to pick a frame where the foot increases or decreases speed.

 

[PeterDonis]

Thread closed for moderation.

 

[Dale]

After discussion this thread will remain closed