# Will anyone please explain this problem using 4 fundamental forces?

If there are only four fundamental forces (gravity,e.m.,strong,weak) in the universe and all other forces can be expressed as a function of them then consider this problem. Suppose I throw a particle 'A' such that it hits another particle 'B' and thus inertia of B has changed. Now what force out of the four fundamental forces is experienced by the particle B? (A and B can be any day-to-day object and are completely uncharged)

• Do you mean an elementary particle, e.g. as in the LHC, or a macroscopic particle as in colliding pool balls? – John Rennie Mar 29 '18 at 14:45
• If B has inertial motion, no force at all. – Stéphane Rollandin Mar 29 '18 at 15:59

I think your main doubt is described by "A and B can be any day-to-day object and are completely uncharged". Yes even when two day to day objects are completely neutral, when they come close (in contact), the electrons of two objects that are closest to the contact point repel each side electrons which causes change in momentum of colliding objects. In the vicinity of contact point, electrons from both sides are closer to one another as compared to electrons from one side and protons from other side. This is because electron cloud surrounds the nucleus and comes closer first. Thereby in case of collision, repulsion is more effective than attraction even though the objects are uncharged. This is general description, Pauli's exclusion principle is likely proper theoretical explanation of this general description.

As Jens already pointed out in his answer, for everyday objects this is nearly always electromagnetism. In the case of solid objects this is usually the electromagnetic repulsion supported by the Pauli exclusion principle keeping matter stable (again, dmckee pointed at an excellent answer on the topic). In some cases (liquids, gases) there are thermodynamic forces like pressure and internal degrees of freedom like liquid flow that resist the deformation and actually change the internals, but the interaction is again electromagnetic.

Throwing a ball up in the air is likely the simplest case where you actually get a case where electromagnetism is not the explanation: in this case object B is the Earth. The gravitational attraction between the ball and the Earth changed the momentum of both to the same degree. Of course the Earth is not noticeably moving (especially since the momentum is returned when the ball comes to rest on the ground) but it was a purely gravitational interaction changing the ball's direction. In gravity assist maneuvers in space spacecraft act as thrown objects that change their and a planet's momentum using gravity.

As far as I can imagine the only "everyday" experience of weak or strong force moving objects all involve particles.

Seeing the microscopic flashes of fluorescence on a radium watch is not due to the nuclear forces, just the electromagnetic interaction between a charged alpha particle and a fluorescent material. The cause of the radium decay was due to the residual strong nuclear force (the energy of an alpha particle plus a radon nucleus is lower than the energy of a radium nucleus) - the actual strong nuclear force only acts on small distances compared to an atomic nucleus, but there is a "tiny" residual force transmitted through meson exchanges that keeps nuclei together.

So basically you need to do particle physics with a neutron or neutrino beam to actually "push" something using the other two forces.

I think it is mostly the electro-magnetic force that causes every-day (macroscopic) objects to bounce off each other. You can watch the Vsauce video "You Can't Touch Anything" for that. Basically the electrons surrounding all the atoms will repell.

When it comes to individual particles, "hitting" each other, then the question becomes more interesting. Then, this really becomes a quantum field theoretical question. It depends on which particles are interacting because there is a vast zoo of different particles, each interacting differently with each other. For example, a photon does not interact via the strong force.

If you take the question at face value- so A and B are everyday objects (cellphone, brick, bust of Heisenberg in bronze, etc), and they are thrown by a human rather than some device that takes them to near light speed, then the answer is that B experiences the electromagnetic force during the collision. Although you state that the objects carry no net charge, if they are everyday objects they will be composed of a (more or less) equal number of positive and negative charges. When they collide the interaction of their constituent atoms and molecules is EM in nature.

There will be a gravitational force between the two objects, but it will be strictly negligible.

The strong and weak forces operate at very short range. Given that you are talking about throwing one object at the other, the electric forces between them will ensure that none of the nuclei involved ever come sufficiently close together for the weak or strong forces to play any part.

## protected by Qmechanic♦Mar 29 '18 at 17:57

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