Is it possible to trigger a nuclear reaction with physical force? In Mission Impossible Fallout, they're dealing with 3 plutonium cores. If one of those cores was thrown against a wall by Ethan Hunt, could it start a chain reaction and explode? For that matter, could any blunt force cause the nuclear reaction to initiate or would it require a radio active trigger?
 A: Yes and no. Yes to "phyisical force" construed generally, no to "thrown by a human".
The movie is referencing something legitimate, assuming that it has been relayed accurately here in that the core of the weapon is plutonium, and that is that to cause plutonium to detonate, you have to apply force and compress it.
However, here's the catch: to cause plutonium to detonate, you have to both compress it a lot AND the compression must be virtually perfectly symmetrical.
Why is this? The answer is that it's not so much the "mass" of the core that counts, but the density. Even better, it's the probability for a neutron to strike another suitable nucleus, which is determined by mass (total number of available nuclei), density (proximity of nuclei to each other/crowding of the space), impurities (unsuitable nuclei that waste neutrons) and more. Remember that the fission chain reaction works by having neutrons, released from the breakup of one nucleus, strike other nuclei with enough energy, as well as the fact of the absorption of the neutron itself changing the stability, to cause them to fission as well. The less dense the fuel, the further apart the nuclei, and the lower the chance of a neutron striking in any given length. Think about a crowd of people, and you are in around the center of that crowd, and you decide to strike out in some direction perfectly randomly. If the crowd is very sparse, and/or small, you will have much less chance of bumping into someone than if it is packed around you. A small crowd needs to be pushed closer together to get a strong chance of you to encounter another in your outward movement, than a larger one.
When the bomb is sitting inert, the "crowd" is large, but not dense enough. That's a good thing, as otherwise the bomb would have detonated at the instant of its creation. It has to be made denser. And how does that happen, that it gets dense enough? The only thing that can provide the raw strength required is chemical high explosives. Moreover, they have to be shaped and timed extremely precisely to get a nearly perfectly spherical, ingoing shock wave around the plutonium core. Even the slightest (at least for a very good strong value of "slightest") imperfection will turn your fission bomb into a fizzle bomb or even a dirty bomb - the latter being much more likely with even "modestly large" imperfections - because now the density won't straightforwardly increase, and moreover, material will even be "squirted" uselessly out the sides. The early developers of the first nuclear bombs in the Manhattan Project (can't remember and don't remember the names) once compared this to "trying to crush a beer can full of beer without spilling any of the beer". And yes, that's also who it took to figure out how to do it: some of the best scientific geniuses in the world, recruited for a purpose that we can only in all truest fairness say has not yet killed us all.
A human thrower's arm has nowhere near either the power, nor the symmetry, to produce such a crush. Think about it: can you take a ball of solid metal in your hand and squeeze it smaller like a ball of rubber? Throwing the core at the wall might smash a hole through the wall given they're kinda heavy (my rough guesstimate recall says around 10 kg or so), at least if it's a wooden wall and not a concrete wall, but that's about it. You might dent it, too (I think plutonium is kind of soft stuff, like lead, but I don't know off the top of my head), but that's about it. Note that "denting" doesn't change density, but rather it just shifts material around (plastic deformation).
ADD: Upon seeing this question pop up again, I wonder if not OP's question was whether that it would explode with physical force or if this would be a good way to actually stop the bomb from exploding. In the latter case, the answer is yes, provided that there isn't a failsafe that sets off the core in the "correct" way upon tampering. If the core is damaged and rendered asymmetrical, then if/when the explosives go off, the "beer" will spill and the nuclear explosion will not ignite. Indeed, you could just hit it with a hammer, even - again, provided there is not a failsafe and/or you've managed to disable that first (which is a military science question, not physics).
A: Blunt force over a limited area of fissionable material does not cause a nuclear explosion.  Over the years there have been two main ways to detonate a fission bomb, neither of which requires a radioactive trigger.
The first way is to bring two sub-critical masses together to form a single critical mass, and that's all it takes to get a nuclear explosion.
The other way is to pack high explosives around a sub-critical mass of fissionable material and design it so that it burns evenly so that it compresses the material to a critical mass.
There are techniques of a radioactive nature that can improve the yield of the explosion, but they are not required for detonation.
One of the ways used to abort a nuclear launch from a nuclear detonation is to detonate the high explosive at one point which will of course cause blunt force over a portion of the nuclear material, but it will blow up the warhead without causing a nuclear explosion.
A: As written in direct comments to the question: in the case of fission bomb designs the critical mass state is achieved by setting off a chemical explosion. .
As written in comments: the problem is to design the chemical explosion in such a way that you get a significant yield of nuclear chain reaction.
By contrast:
If there is some mishap in which two lumps of nuclear bomb grade material are brought together accidentally then the immediate heat from the first fission events will drive those lumps away from each other. 
I think that means that accidentally bringing lumps of nuclear fission material together will result in the tiniest nuclear explosion, causing radiation poisoning of all the people in the same room, but not much else.
Bringing lumps together on purpose, but with only the force of a human throwing the stuff will have a similar result; radiation poisoning of the people in the room, not much else.
To get a significant yield you have to do something extremely violent and extremely precise. The lumps of fissile material must be brought in contact faster than the speed at which the nuclear chain reaction developes. That is very, very difficult.
A: It is possible for good bomb from enriched uranium if you are OK with high probability of fizzle.

A weapon constructed from 40 kg of 93.5% HEU, with a 10 cm tungsten carbide reflector would produce a full yield of >10 kt. The required assembly time for a 50% chance of complete assembly is some 48 milliseconds, equal to a velocity of only 9 m/sec. This can be achieved by simply dropping the bullet 4.4 meters!
link

Moving part must be somewhat lighter than 40 kg, so it guess if you raise it above head and throw down full force then you can get about 10 m/s velocity.
A: If I have not missed something, nobody here has discussed the following scenario: a person runs up and hits a barely subcritical mass of plutonium (or enriched uranium) with their body. Could the mass become critical as a result?
I believe so. And the reason for criticality is not the modest force applied to the plutonium by the human body, but the fact (Criticality Safety Basics,
A Study Guide, INEEIJEXT-98-O0895, Rev. 1) that

The human body, with its high content of water and carbon compounds,
is all too often an excellent reflector [and moderator of neutrons].

Also there:

Neutron reflection with humans as reflectors was a factor in the
January 2, 1958 accident at the Mayak Enterprise...
...authorities established a critical experiments facility on site.
Designed to measure critical parameters for highly enriched uranium
solutions, equipment included a tank bolted to structural members, a
neutron source, neutron detectors, a control rod, and small-diameter
connecting lines.
On January 2, 1958, four facility staff members completed an
experiment and decided to accelerate solution draining by violating
procedures. They placed favorable-geometry vessels nearby and unbolted
the tank. Three people tipped the tank to pour solution into these
safe vessels.
Solution geometry in the tipped tank became optimal with effective
neutron reflection by three humans, resulting in a power excursion. A
single spike of about $2.3\cdot 10^{17}$ fissions occurred, ejecting
part of the solution from the tank.

Another quote from the same document to provide some balance to the above:

A supercritical condition normally does not last long. Out-of-reactor
it usually releases enough energy to displace or boil or melt material
into a subcritical configuration within seconds.

