8.4 - A Force Can Change the State of Motion

In this section, we will learn about how a force can change the state of motion of an object. The state of motion refers to whether an object is at rest, moving with a constant velocity, or accelerating. The application of force plays a key role in changing an object's motion in different ways.

1. What is Force?

Force is a physical quantity that can change the motion of an object. It is defined as any interaction that, when unopposed, will change the motion of an object. The SI unit of force is the Newton (N).

2. Types of Forces

• Contact Forces: Forces that act on an object when it is in physical contact with another object. Examples include friction, tension, and normal force.
• Non-Contact Forces: Forces that act on an object without direct physical contact. Examples include gravitational force, magnetic force, and electrostatic force.

3. Effects of Force on Motion

Force can change the state of motion of an object in different ways:

• Change in Speed: A force can increase or decrease the speed of an object. For example, when a car accelerates, the engine applies a force that increases its speed.
• Change in Direction: A force can change the direction of motion of an object. For example, when a ball is hit with a bat, the direction of its motion changes.
• Change in Shape: A force can deform an object. For example, when you apply force to a soft rubber ball, it may change its shape temporarily.

4. Newton's First Law of Motion

According to Newton's First Law of Motion, an object will remain at rest or continue to move with a constant velocity unless acted upon by an external force. This property of objects to resist changes in their state of motion is called inertia.

5. Force and Acceleration

When a force is applied to an object, it causes the object to accelerate. The amount of acceleration depends on the mass of the object and the magnitude of the force applied. This relationship is expressed by Newton's Second Law of Motion:

Newton’s Second Law of Motion:

The second law states that the acceleration of an object is directly proportional to the net force acting on the object and inversely proportional to its mass. The mathematical expression is:

$ F = ma $

Where:

• F is the force applied on the object (in Newtons, N),
• m is the mass of the object (in kilograms, kg),
• a is the acceleration produced (in meters per second squared, m/s²).

6. Balanced and Unbalanced Forces

Forces can be categorized as balanced or unbalanced:

• Balanced Forces: When two forces acting on an object are equal in magnitude and opposite in direction, they cancel each other out. The object does not move or continues moving with constant velocity.
• Unbalanced Forces: When the forces acting on an object are not equal, they produce a net force that causes the object to accelerate in the direction of the greater force.

7. Applications of Force in Daily Life

Force plays an essential role in various activities of daily life. Some common examples include:

• Pushing or pulling a cart to move it from one place to another.
• Braking a bicycle, where frictional force slows down the motion.
• Throwing a ball, where the force applied by the hand changes the speed and direction of the ball.

In summary, a force can change the state of motion of an object by altering its speed, direction, or both. The application of force is governed by Newton’s Laws of Motion, which provide a clear understanding of how forces affect objects in motion.