Which of the following forces can only be applied when in contact with an object?

To determine which force can only be applied when in contact with an object, we can classify the forces listed into two main categories: contact forces and non-contact (action-at-a-distance) forces.

1. Contact Forces:
These are forces that require physical contact between two or more interacting objects to be exerted. Examples include friction, tension, normal force, and applied force.

2. Non-Contact (Action-at-a-Distance) Forces:
These are forces that can act over a distance without physical contact between the interacting objects. Examples include gravitational force, electrostatic force, and magnetic force.

Let's analyze the given options based on these definitions:

• Frictional force: This force opposes the relative motion between two surfaces in contact. It cannot exist without physical contact between the surfaces. Thus, it is a contact force.
• Gravitational force: This is the attractive force between any two masses (for example, the Earth pulling down on an apple even before it hits the ground). It operates over a distance without requiring physical contact.
• Electrostatic force: This is the force of attraction or repulsion between electric charges. For example, a rubbed balloon can attract small pieces of paper from a distance without touching them.
• Magnetic force: This is the force exerted by magnets on other magnets or magnetic materials (like iron nails). A magnet can pull an iron nail towards itself from a distance.

Therefore, the only force among the choices that requires physical contact to be applied is the Frictional force.