Force

# Force

## Introduction

Force is a fundamental concept in physics that plays a crucial role in understanding the behavior and interactions of objects. It is a vector quantity, meaning it has both magnitude and direction. Forces are responsible for causing changes in the motion or shape of an object. In this note, we will explore what force is, its effects, the formula for calculating force, the unit of force, different types of forces, and some solved word problems related to force.

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## What is Force?

Force can be defined as a push or a pull that can cause an object to accelerate, decelerate, or change its shape. It arises from the interaction between two objects and is characterized by its magnitude and direction. Forces can act through direct contact between objects or through fields, such as gravitational or electromagnetic fields. The concept of force is a cornerstone of Isaac Newton’s laws of motion and is essential for understanding the dynamics of objects.

### Effects of Force

Forces can produce various effects on objects, including:

• Change in motion: Forces can cause objects to start moving, stop moving, or change their velocity. This effect is described by Newton’s second law of motion, which states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.
• Change in shape: Some forces can deform or change the shape of objects. For example, when a compressive force is applied to a spring, it gets compressed, and when a tensile force is applied to a rubber band, it elongates. These effects are governed by Hooke’s law, which states that the deformation of an object is directly proportional to the force applied to it.

## Force Formula

The formula to calculate force is derived from Newton’s second law of motion. It can be stated as:

F = m × a

where F represents the force, m is the mass of the object, and a is the acceleration produced by the force. This formula indicates that the force acting on an object is directly proportional to its mass and the acceleration it experiences.

### Unit of Force

The standard unit of force in the International System of Units (SI) is the newton (N). One newton is defined as the force required to accelerate a mass of one kilogram by one meter per second squared (1 N = 1 kg × m/s²). Other commonly used units of force include pounds-force (lbf) and dynes.

## Types of Force

Forces can be broadly categorized into two main types: contact forces and non-contact forces. Let’s explore each of these types in more detail.

### Contact Forces

Contact forces are forces that act on an object through direct physical contact. They arise from the interaction between two objects that are in direct contact with each other. Some common examples of contact forces include:

1. Friction: Friction is a contact force that opposes the motion or attempted motion between two surfaces in contact. It can be helpful, such as when walking or driving, or detrimental, like the friction that can slow down a moving object.
2. Normal Force: The normal force is the contact force exerted by a surface to support the weight of an object resting on it. For example, when you sit on a chair, the chair exerts an upward normal force to balance your weight.
3. Applied Force: Applied force is a contact force that is directly applied to an object by another object. For instance, pushing a cart or lifting a book are examples of applied forces.

### Non-Contact Forces

Non-contact forces, also known as field forces, act on an object without the need for physical contact. These forces can act over a distance through a field created by one object, influencing another object. Some common examples of non-contact forces include:

• Gravitational Force: The force of gravity is a non-contact force that attracts objects with mass towards each other. It is responsible for keeping planets in orbit around the Sun and objects on Earth’s surface.
• Electromagnetic Force: The electromagnetic force is a non-contact force associated with electrically charged particles. It includes the forces of attraction and repulsion between charged objects and is responsible for the behavior of magnets.
• Magnetic Force: Magnetic force is a non-contact force exerted by magnets or magnetic fields. It can attract or repel magnetic objects without direct contact.
• Electric Force: The electric force is a non-contact force that acts between charged particles. It can be attractive or repulsive, depending on the charges involved. Understanding the different types of forces, whether contact or non-contact, is essential for comprehending the interactions and behaviors of objects in various scenarios. These forces govern the motion, shape, and stability of objects, contributing to the fundamental laws of physics.

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### Solved Examples on Force

Example 1: A box with a mass of 20 kg is being pushed along a horizontal surface with a constant acceleration of 2 m/s². Determine the net force acting on the box.

Solution: To find the net force, we can use Newton’s second law of motion, which states that the net force is equal to the product of mass and acceleration.

Given:

Mass of the box (m) = 20 kg

Acceleration (a) = 2 m/s²

Using the formula: F = m × a

F = 20 kg × 2 m/s²

F = 40 N

Therefore, the net force acting on the box is 40 N.

Example 2: A car of mass 1,500 kg is traveling at a constant speed of 20 m/s on a level road. Determine the force of friction acting on the car.

Solution: When a car is moving at a constant speed, the net force acting on it is zero. Therefore, the force of friction must be equal in magnitude and opposite in direction to the driving force.

Given:

Mass of the car (m) = 1,500 kg

Speed of the car (v) = 20 m/s

Since the car is moving at a constant speed, the acceleration is zero (a = 0). Thus, the net force is also zero.

Using the formula: F = m × a

F = 1,500 kg × 0 m/s²

F = 0 N

Therefore, the force of friction acting on the car is 0 N.

## Frequently Asked Questions on Force

### What is force?

Force is a physical quantity that represents a push or pull on an object, capable of causing changes in its motion or shape. It is characterized by its magnitude and direction.

### What is the SI unit of force?

The SI unit of force is the newton (N). One newton is defined as the force required to accelerate a mass of one kilogram by one meter per second squared (1 N = 1 kg × m/s²). It is named after Sir Isaac Newton, the renowned physicist who formulated the laws of motion.

### What is the CGS unit of force?

The CGS (Centimeter-Gram-Second) unit of force is the dyne (dyn). One dyne is defined as the force required to accelerate a mass of one gram by one centimeter per second squared (1 dyn = 1 g × cm/s²). While the dyne is not commonly used in everyday measurements, it is still utilized in certain scientific and engineering contexts that utilize the CGS system of units.

### Is force a vector?

Yes, force is a vector quantity. As a vector, it has both magnitude and direction. The direction of the force vector indicates the direction in which the force is applied, while the magnitude represents the strength or intensity of the force. This vector nature of force is essential in accurately describing and analyzing the motion and interactions of objects in physics.

### Is kgf a unit of force?

Yes, kgf (kilogram-force) is a unit of force. It is a gravitational metric unit of force commonly used in some regions, particularly in engineering and mechanical applications. One kilogram-force is defined as the force exerted on a mass of one kilogram due to the Earth's gravitational field. However, it is important to note that the kilogram-force is not a part of the International System of Units (SI) and has been largely replaced by the newton (N) as the standard unit of force.

### What is balanced force?

A balanced force refers to a situation in which the net force acting on an object is zero. When balanced forces are applied to an object, there is no change in its motion or velocity. The forces exerted in opposite directions cancel each other out, resulting in a state of equilibrium. In other words, the forces are balanced when their vector sum adds up to zero, leading to a stable or stationary condition.

### Can force be negative?

Yes, force can be negative. In physics, force is considered a vector quantity, which means it has both magnitude and direction. When a force is negative, it indicates that the force is acting in the opposite direction of a chosen reference or coordinate system. The negative sign signifies the opposite direction of the force vector. For example, if a force of -10 Newtons is acting in the negative x-direction, it means the force is directed opposite to the positive x-axis. The negative sign does not imply a weaker force, but rather a force acting in the opposite direction as compared to the chosen reference.

### What is the relation between SI and CGS unit of force?

The relationship between the SI (International System of Units) unit of force, which is the newton (N), and the CGS (Centimeter-Gram-Second) unit of force, which is the dyne (dyn), is as follows: 1 newton (N) = 100,000 dynes (dyn) In other words, one newton is equal to 100,000 dynes. This conversion factor allows for the conversion of force values between the two systems of units. The SI system, with the newton as its unit of force, is widely used in scientific and engineering contexts, while the CGS system, with the dyne as its unit of force, is less commonly used in everyday measurements but still finds application in certain scientific fields.

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