Question

How can we know a weak nuclear force exists?

Answer 1

Hint: A force is any interaction that, when unopposed, will change the motion of an object. A force can cause an object with mass to vary its velocity (which includes to start moving from a state of rest), i.e., to accelerate.

Complete answer:
The model of the weak force was created to elucidate the radioactive decays. The electromagnetic force doesn't keep together particles within the nucleus (they are all positive or neutral). We also know that the forces are transported by particles that we will describe theoretically.
The weak interaction is carried by the W and Z bosons. These particles were predicted by Nobel laureates Weinberg , Sheldon Salam and Abdus Glashow within the 1960s, and discovered in 1983 at CERN. The Z boson is neutrally charged and carries a weak neutral current. Its interaction with particles is tough to detect.
The strong nuclear force holds the kernels of matter, atomic nuclei, together, and powers the burning of stars. the consequences of the weak interaction were first discovered at the turn of the 20th century, within the place where it's most obviously at work: in radioactive decay .

Note:
The strong nuclear force, also called the strong nuclear interaction, is the strongest of the four fundamental forces of nature. it's 6 thousand trillion (that's 39 zeroes after 6) times stronger than the force of gravity, consistent with the HyperPhysics website.