Contact Forces

Contact forces are forces that arise when objects are in physical contact with each other. These forces can be categorised into several types, each with distinct characteristics and effects. Below, we’ll delve into the various types of contact forces, providing detailed explanations and examples for each.

1. Frictional Force

Definition:

Frictional force is the force that opposes the relative motion or tendency of such motion of two surfaces in contact. It acts parallel to the surface of contact.

Types of Friction:

• Static Friction: Prevents an object from moving when a force is applied.

• Kinetic Friction: Acts on a moving object.

Example: Walking
Friction is essential for walking as it provides the necessary grip between our feet and the ground, preventing slipping and enabling forward movement. When we push our foot backward, friction generates a forward force that propels us ahead. This force also ensures stability and balance, as it keeps our feet from sliding while we walk. Friction allows for efficient energy transfer from our muscles to the ground, facilitating smooth and controlled motion. Additionally, it enables us to start, stop, and change direction effectively, making walking possible on various surfaces.

2. Normal Force

Definition:

Normal force is the support force exerted upon an object in contact with another stable object. It acts perpendicular to the surface of contact.

•  Normal force is a contact force exerted by a surface perpendicular to an object resting on it.

•  Acts perpendicular to the surface with which an object is in contact.

•  Magnitude is usually equal to the weight of the object in the absence of other vertical forces.

•  On flat surfaces without other vertical forces, calculated as  FN = mg 

•  On inclined planes, the normal force is less than the object’s weight.

•  Directly proportional to frictional force

•  Reaction force to the object’s weight acting on the surface, according to Newton’s third law.

•  Varies with different scenarios, such as on a flat surface, inclined plane, or with additional vertical forces (e.g., an accelerating elevator).

•  For an object at rest on a flat surface with no vertical acceleration, the normal force balances the object’s weight, maintaining equilibrium.

Example: A Book on a Table
A book resting on a table experiences a normal force from the table equal to the gravitational force acting on the book. This normal force prevents the book from falling through the table.

3. Tension Force

Definition:

Tension force is the pulling force transmitted along a rope, string, cable, or similar object when it is pulled tight by forces acting from opposite ends.

Example: Tug of War

In a game of tug of war, the tension force in the rope is created by the teams pulling at both ends. This tension is what transmits the force from one end to the other.

4. Spring Force

Definition:

Spring force is the force exerted by a compressed or stretched spring upon any object that is attached to it. 

Example: A Stretched Rubber Band
When you stretch a rubber band, it exerts a force pulling your fingers back together. This force increases as the rubber band is stretched further.

This force follows Hooke’s Law, which states that the force exerted by the spring is proportional to its change in length.

Hooke’s law is also an important concept that will be discussed in detail soon!

5. Applied Force

Definition:

Applied force is the force that is applied to an object by a person or another object. It can be in any direction and of any magnitude, depending on the situation.

Example: Pushing a box
When you push a box, the force you exert is an applied force. The direction and magnitude of the force depend on how hard and in which direction you push the box.

6. Air Resistance Force

Definition:

Air resistance force, also known as drag, is a type of frictional force that acts upon objects as they travel through the air. It opposes the motion of the object.

Example: A Parachute
When a parachute is deployed, air resistance acts against the downward motion of the skydiver, slowing their descent.

7. Buoyant Force

Definition:

Buoyant force is the upward force exerted by a fluid that opposes the weight of an immersed object. This force is responsible for making objects float or sink in a fluid.

Example: A Boat on Water
A boat floats because the buoyant force exerted by the water balances the weight of the boat. If the boat is heavy enough to displace an amount of water equal to its weight, it will float.

Non-Contact Forces

Non-contact forces are forces that act on an object without any physical contact between the source of the force and the object. These forces act over a distance and can affect objects even when they are not in direct contact. Below, we will discuss various types of non-contact forces in detail, including their definitions, principles, and examples.

1. Gravitational Force

Definition: Gravitational force is the attractive force between two masses. According to Newton’s law of universal gravitation, every particle of matter in the universe attracts every other particle with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

• F is the gravitational force.

• m1 and m2​ are the masses of the two objects.

• r is the distance between the centres of the two masses.

Example: Earth’s Gravity Objects on Earth experience a gravitational pull towards the centre of the Earth. This force gives objects weight and causes them to fall when dropped.

2. Electromagnetic Force

Definition: Electromagnetic force is the force between charged particles. It includes both electric forces (between stationary charges) and magnetic forces (between moving charges). This force is described by Coulomb’s law for electric charges and by the Lorentz force law for magnetic fields.

Example: Repulsion and Attraction

• Repulsion: Two like charges (both positive or both negative) repel each other.

• Attraction: Two unlike charges (one positive and one negative) attract each other.

3. Magnetic Force

Definition: Magnetic force is the attraction or repulsion that arises between electrically charged particles due to their motion. This force is part of the electromagnetic force.

Example: Magnets Two magnets can attract or repel each other depending on the orientation of their poles (north and south). A compass needle aligns with Earth’s magnetic field due to magnetic forces.

4. Nuclear Forces 

Definition:

Forces between particles in an atom’s nucleus, encompassing the strong nuclear force and the weak nuclear force.

4.1. Strong Nuclear Force

Binds protons and neutrons in the nucleus, exceptionally potent over short distances.

4.2. Weak Nuclear Force

Facilitates radioactive decay and specific nuclear reactions, less powerful than the strong nuclear force.