Lesson 11: The Concept of Force and Newton’s Laws of Motion
Video Lesson
Lesson Objective
Dear Learners,
At the end of this section, you should be able to:
- Define force, and give examples of forces;
- Practically measure the gravitational force on an object.
- Define Newton’s first, second and third laws of motion;
- Explain the dependence of acceleration on net force and mass;
- Explain how gravity affects the motion of objects.
Brainstorming Question
Can a force always cause motion?
key terms and concepts
Simulation
Concept of Force
Force is a push or a pull acting upon an object resulting from its interaction with another object. It is a vector quantity, meaning it has both magnitude and direction. The SI unit of force is the Newton (N).
- Force
- Contact forces
- Newton’s first law
- Newton’s Second law
- Newton’s Third law
Force is a push or a pull acting upon an object.
Contact forces involve physical contact between two bodies and the non contact forces do not involve any physical contact.
Newton’s first law: An object at rest remains at rest as long as no net force acts on it.
Newton’s Second Law: Force equals mass times acceleration (F=ma).
Newton’s Third Law: Every action has an equal and opposite reaction.
- Forces can cause an object to start moving, stop moving, change direction, or alter its shape.
- Examples include gravitational force, frictional force, and applied force.
Forces can be broadly categorized into two main types: contact forces and non-contact forces. Here are some common types of forces within these categories:
- Contact Forces: are forces that arise from the physical interaction between two objects in direct contact with each other.
- Examples: Applied Force (A person pushing a cart), Frictional Force, Normal Force, Tension Force, Air Resistance Force, and Spring Force.
2. Non-Contact Forces: are Forces that act on an object without any physical contact between the objects involved. These forces act over a distance.
- Examples: Gravitational Force, Electromagnetic Force, Strong Nuclear Force, and Weak Nuclear Force.
Newton’s Laws of Motion
Video
1. First Law (Law of Inertia): states that “ An object at rest stays at rest, and an object in motion continues in motion with the same speed and in the same direction unless acted upon by an unbalanced external force”.
- Implication: Objects resist changes in their state of motion. This resistance is called inertia.
Simulation (Newton’s second law)
2. Second Law (Law of Acceleration): states that “ The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass”. The formula is: F = ma
- Implication: The greater the force applied to an object, the greater its acceleration. For a given force, an object with more mass will accelerate less than an object with less mass.
Hence, applying Newton’s second law, the magnitude of the gravitational force on an object is given by:
Fg = mg
This force is directed towards the center of the earth. Fg is also called weight of the object. Weight of a body is represented by ’W’. The above equation can also be written as:
W = mg
Example:
A force of 10N acts on a block of mass 2kg resting on a smooth horizontal surface. What is the acceleration of the block?
Given:
F =10 N, m = 2kg.
Required:
The acceleration ’a’ of the block
Solution:
The unbalanced force acting on the block is 10N. From Newton’s second law of motion we have:
F =ma
Therefore, a= F /m = 10N /2kg = 10kgm/s2 2kg = 5m/s2
3. Third Law (Action and Reaction): states that “ For every action, there is an equal and opposite reaction”.
- Implication: When one object exerts a force on another object, the second object exerts a force of equal magnitude but in the opposite direction on the first object.