We know that if something is fixed to the ground and we give force to it, then it will give reaction force in equal amount. How about if "something" isn't fixed, e.g. a toy truck? When we push it for an instant, does the toy truck at the same time give reaction force back to the us? Does it only give reaction back for the time they touch each other? Is my concept correct?

Yes.  Nothing is truly fixed.  The Earth is just a bigger toy truck that would accelerate less because of its mass.

34 minutes ago, Boltzmannbrain said:

Yes.  Nothing is truly fixed.  The Earth is just a bigger toy truck that would accelerate less because of its mass.

So, does reaction force only exist for the moment they touch each other?

F=ma

The force is felt while accelerating the item.

If they move at a constant speed they can be touching but feel no reaction force.

(Imagine two cars moving in line at 100 kph. They could be 10 metres apart. They feel no reaction force. The second car could be "touching" the first, and they'd still feel no reaction force. But if the second car is being used to speed up the first car, or the first is being used to slow down the second, then they feel reaction force.

* ignoring lots of friction and drag and stuff.)

37 minutes ago, kenny1999 said:

So, does reaction force only exist for the moment they touch each other?

It’s present for as long as there is an action force. 

16 hours ago, kenny1999 said:

Does it only give reaction back for the time they touch each other? Is my concept correct?

Well, that's obvious, isn't it? If they are not touching, how can a force be applied in this example? But as    pzkpfw  said, f=ma, so as long a the force is not zero,  the acceleration will not be zero, but as soon as the acceleration drops to zero, the force will be at zero. In other words, there will only be a force while the truck is being accelerated.  (and vice versa)   

I don't want to 'muddy the waters' for Kenny, as I don't know his level of understanding.
He may, however, ask "What is the lunar lander pushing against when it accelerates upwards from the lunar surface ?".
F=ma is not the complete analysis in such cases.

A rocket expells part of itself ( the fuel ), at high speed, and accelerates in the opposing direction, ensuring that momentum is conserved.

20 hours ago, kenny1999 said:

We know that if something is fixed to the ground and we give force to it, then it will give reaction force in equal amount. How about if "something" isn't fixed, e.g. a toy truck? When we push it for an instant, does the toy truck at the same time give reaction force back to the us? Does it only give reaction back for the time they touch each other? Is my concept correct?

It is not necessary for the action - reaction pair to touch.

Note carefully the use of the word 'pair'  -  that is two forces between two objects or bodies.

You should never introduce a third body  directly into this as this complicates matters.

 

If the forces are due to touching, they are called contact forces but here is an example of a non contact action - reaction pair.

Coulomb's Law (dallaswinwin.com)

Something is up with that last link :  

   

 

 

2 hours ago, mistermack said:

Something is up with that last link :  

   

 

 

Well this is really rather distressing.

 

Dallaswin is a bunch of physics teachers trying to develop some physics tutorials and works just fine on my old Windows XP using fox.

 

But windows10 doesn't seem to like it, although I can find the page, it tell me it is not secure, which is not surprising since is is just http not https.

 

Here is a screenshot of part of their explanation on W10.

 

 

Perhaps it's been reported for copyright violation or some such thing, rather than having malicious malware in it. 

15 hours ago, studiot said:

Here is a screenshot of part of their explanation on W10.

Isn't it a fact that nothing is really "touching" when solid objects push against each other?

Modern Physics tells us that there is no actual 'contact' force.
But the classical contact approximation is indistinguishable at macro scales.

Hi, ok, well, I have one more conceptual question.

For example, when I am pushing a toy truck such that it is accelerating, I know that net force on the truck isn't zero, but at this time, is the force my hand gives to the truck equal to the reaction force that the truck gives back to my hand?

6 minutes ago, kenny1999 said:

Hi, ok, well, I have one more conceptual question.

For example, when I am pushing a toy truck such that it is accelerating, I know that net force on the truck isn't zero, but at this time, is the force my hand gives to the truck equal to the reaction force that the truck gives back to my hand?

Yes.

1 hour ago, Genady said:

Yes.

positive?

6 minutes ago, kenny1999 said:

positive?

Yes.