If you spin a water-filled cup horizontally on a flat table, did you exert force on the water?
Yes, when you spin a water-filled cup horizontally on a flat table, you do exert a force on the water. To understand why, let's break down the concept:
When you spin the cup, you apply a force to it by pushing it with your hand. According to Newton's third law of motion, every action has an equal and opposite reaction. So, as you exert a force on the cup, the cup exerts an equal and opposite force on your hand.
But what happens to the water inside the cup? It doesn't stay still due to its inertia. Inertia is the tendency of an object to stay at rest or remain in motion unless acted upon by an external force.
As you spin the cup, the water inside it also wants to stay at rest due to its inertia. However, the friction between the cup and the water causes the water to move along with the cup. The force you exert on the cup is transferred to the water, and it starts rotating along with the cup.
So, although you didn't directly push or pull on the water, your force applied to the cup gets transferred to the water due to the friction between the cup and the water.