A body needs a ‘push’ or ‘pull’ to move or to bring a moving body to rest or change its velocity. Hence this push or pull is called force. Force has both magnitude and direction. Therefore force is a vector quantity.
Definition of force in physics
Types of Forces
Force is classified into types as:
- Like parallel force – Two or more forces of equal or unequal magnitude acting along the same direction, parallel to each other.
- Unlike parallel force – Two or more forces either equal or unequal forces act in opposite directions parallel to each other.
Unit of Force: SI unit of force is newton(N) and in the C.G.S system its unit is dyne.
Definition of 1 newton (N) – The amount of force required for a body of mass 1 kg to produce an acceleration of 1 kg⋅m/s2.
1 N = 1 kg m/s2
One dyne is the amount of force required for a body of mass 1 gram produces an acceleration of 1 cm/s-2.
1 dyne = 1 g cm/s-2 ; also 1 N = 105 dyne.
The amount of force required to produce an acceleration of 1 ms-2
in a body of mass 1 kg is called ‘unit force’.
Gravitational unit of force
The gravitational unit of force is kilogram-force, represented by kg f in SI units. In the C.G.S system, its unit is gram force, represented by gf.
1 kg f = 1 kg × 9.8 ms-2 = 9.8 N;
1 g f = 1 g × 980 ms-2 = 980 dyne
Issac Newton formulated three laws of motion.
- Newtons 1st Law
- Newtons 2nd Law
- Newtons 3nd Law
These three are discussed later in the notes.
Mechanics is a branch of physics that studies the effect of force on bodies. It is divided into Statics and dynamics.
- Static – Static deals with the bodies that deal with the bodies that are at rest under the action of force.
- Dynamics – Dynamics deals with moving bodies under the action of forces. Dynamics is further divided into Kinematics and Kinetics
- Kinematics – Kinematics studies the motion of bodies without considering the cause of movement.
- Kinetics – Kinetics studies the motion of bodies considering the cause of movement.
When a force is applied to bodies, they resist change in their state. This property is called Inertia.
Example: We tend to move forward when the bus suddenly stops. Or We move backwards when the bike starts suddenly.
Inertia is defined as the interesting property of a body to resist the state of rest state of uniform motion unless it is influenced by an external unbalanced force.
Types of Inertia
- Inertia of rest – The resistance of a body to change its state of rest is known as Inertia of rest.
- Inertia of motion – The resistance of a body to change its state of motion is called Inertia of motion.
- Inertia of direction – It is the resistance of a body to change its direction of motion is called Inertia of direction.
Linear momentum measures the impact of force on a body. The impact of a force is more when the velocity and the mass of the body are more.
The product of mass and velocity of a moving body provides the magnitude of linear momentum. It acts in the direction of the velocity of the object.
Linear Momentum = Mass x Velocity
That is: p = mv. It helps to measure the magnitude of a force.
The unit of momentum in the SI system is kg ms-1and in the C.G.S system, its unit is g cms-1.
Newton’s Laws of Motion
Newton’s First Law of Motion
This law states that “a body continues to be in its state of rest or state of motion along a straight line unless an external force at act on it”.
This gives the definition of force as well as inertia.
Newton’s Second Law of Motion
According to this law,” The force acting on a body is directly proportional to the rate of change of linear momentum of the body. And the change in momentum takes place in the direction of the force.”
This law is used to measure the amount of force. So this is also called as ‘law of force‘.
F = m x a
Force = mass x acceleration
Newton’s Third Law of Motion
This law states that ‘for every action, there is an equal and opposite reaction ‘.
Example: Recoil of Gun, Swiming, Birds flying etc.
Principal of Conservation of Linear Momentum
As per this principle, there is no change in the linear momentum of a system of bodies as long as no net external force acts on it.
Moment of Force
The rotating effect of a force about a fixed point or fixed axis is called the moment of the force about that point or torque (τ). Torque is a vector quantity and its SI unit is N m.
The torque is measured by-product of the force (F) and the perpendicular distance (d) between the fixed point or axis and the line of action of the force.
τ = F × d
When two equal and unlike parallel forces are applied simultaneously at two distinct points, is a couple.
The line of action of the two forces does not coincide and does not produce translator motion. This is because the resultant is zero. But a couple causes the rotation of the body.
This rotating effect of a couple is called the moment of a couple. The unit of the moment of the couple is Newton metre (N m) in the SI system and dyne cm in the C.G.S system.
Example: Turning of Tap, winding a Screw, Spinning of top
Moment of a couple = Force × perpendicular distance between the
line of action of forces
M = F x S
This article is for Tnpsc, for the topic ‘Force‘. This article provides a brief description of basic concepts of force in Physics. For detailed notes please refer to the Samacheer Kalvi book, Science 10th Std, Unit-1, Laws of Motion.