Which one of the following is not a contact force?
Gravitational force
Understanding Contact and Non-Contact Forces
Forces are interactions that can cause an object to change its state of motion. They can be broadly classified into two main types: contact forces and non-contact forces.
What are Contact Forces?
Contact forces are those that require physical contact between the interacting objects for the force to be exerted. Examples include pushing a door, friction between surfaces, or the tension in a rope.
What are Non-Contact Forces?
Non-contact forces, also known as field forces, are those that act on an object without direct physical contact. These forces act through a distance. Examples include the force of gravity, magnetic force, and electrostatic force.
Analyzing the Given Options
Let's examine each option to determine whether it is a contact force or a non-contact force.
- Push force: When you push an object, your hand or another object must be in direct physical contact with it. Therefore, push force is a contact force.
- Gravitational force: This is the force of attraction between any two objects with mass. For example, the Earth exerts a gravitational force on the moon, keeping it in orbit, even though there is no physical contact between them. Similarly, Earth pulls objects towards its center without touching them. Thus, gravitational force is a non-contact force.
- Frictional force: Friction arises when two surfaces are in contact and rub or tend to rub against each other. It opposes relative motion. This force requires physical contact between the surfaces, making it a contact force.
- Strain force: Strain refers to the deformation of a material caused by stress (applied force). The internal forces resisting this deformation or causing the object to return to its original shape are related to the material's properties and the applied external forces which are typically in contact with the object. These internal forces act within the material itself as a result of external contact or pressure. Therefore, it is considered in the context of contact or internal material forces which resist external contact.
Based on this analysis, gravitational force is the only force among the options that does not require physical contact between objects to act.
| Force Type | Requires Contact? | Classification |
|---|---|---|
| Push force | Yes | Contact Force |
| Gravitational force | No | Non-Contact Force |
| Frictional force | Yes | Contact Force |
| Strain force | Yes (usually as a result of external contact forces causing internal strain) | Contact Force (or related to internal forces from contact) |
Therefore, the force that is NOT a contact force is the gravitational force.
Revision Table: Forces in Physics
| Force Category | Definition | Examples |
|---|---|---|
| Contact Forces | Forces that act when objects are in direct physical contact. | Push, Pull, Friction, Normal Force, Tension, Air Resistance, Strain Force (as a reaction to contact) |
| Non-Contact Forces | Forces that act through a distance without physical contact. | Gravitational Force, Magnetic Force, Electrostatic Force |
Additional Information on Non-Contact Forces
Non-contact forces are often described using the concept of a field. For instance:
- Gravitational Field: Any object with mass creates a gravitational field around it. Another object with mass placed in this field experiences a gravitational force.
- Electric Field: Charged objects create electric fields. Other charged objects in this field experience an electrostatic force.
- Magnetic Field: Magnets and moving charges create magnetic fields. Magnetic materials or moving charges in this field experience a magnetic force.
These fields extend through space, allowing forces to be transmitted over a distance without the need for physical touching.
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