A__________between two objects causes force.

Explanation of Force:

1. Force is defined as the product of mass $m$ and acceleration $a$ $\left(F=ma\right)$.
2. According to Newton's second law of motion, force is proportional to the rate of change in momentum $\left(F\propto \frac{dp}{dt}\right)$.
3. Therefore, we can see that any object with mass that undergoes a change in velocity or momentum experiences a motion.
4. This change in a steady body only occurs when there is some sort of interaction between objects.
5. The interaction can be directed by contact between two bodies, which are also called contact forces.
6. The interaction can be non-contact in nature, for example gravitational, magnetic and electric forces.
7. In all cases, we see that for a body to experience some force, it must undergo an interaction between two or more bodies.

Hence, an interaction between two objects causes force.