How Forces Affect Motion
Class 09 ScienceA force can make an object move from rest, change the speed and direction of motion of a moving object, and can even change the shape of an object.
Force is a physical quantity for which we need to specify direction along with its magnitude and unit. The SI unit of force is newton and its symbol is N. The magnitude of the force expresses its strength.
A spring balance can be used to measure not just the weight of an object but the magnitude of the force in general. If you pull on the free end of the spring balance, it measures the force with which you pull on the spring inside the balance.
Balanced and Unbalanced Forces
Usually, there is more than one force acting on an object. For example, when you are pushing a box placed on a surface then, apart from the force with which you are pushing it, the force of friction is also acting on the box in the direction opposite to that of the motion.
Two forces, which are equal in magnitude but opposite in direction are called balanced forces.
If the forces applied on an object are not balanced, a non-zero net force acts on the object. When two forces are opposite in direction but unequal in magnitude, the magnitude of net force is equal to the difference between the magnitudes of two forces and the direction is along the force of the larger magnitude.
The Force of Friction
Suppose an object is kept at rest on the floor and you apply a force on it in the forward direction. Many a times, you might have experienced that on applying a force on an object, it did not move and you had to apply a larger force to move it.
It is due to the force of friction arising between the bottom surface of the box and the floor acting in a direction opposite to the direction of the force applied by you. The box will start moving when the force applied by you is of larger magnitude than the force of friction, so that a net force acts on the box in the direction of its motion.
Newton's First Law of Motion
An object at rest remains at rest and an object in motion continues to move with a constant velocity, unless a net force acts upon the object.
In other words, if the net force acting on an object is zero, the body cannot begin to move or change its velocity. In such a case, its acceleration is zero.
Newton's Second Law of Motion
The relation between force, mass and acceleration is expressed in the Newton's second law.
When a net force acts on an object, the object accelerates in the direction of the net force. The magnitude of the acceleration is proportional to the magnitude of the net force and is inversely proportional to the mass of the object.
$$ a = \frac{F}{m} $$
$$ F = ma $$
One newton of force is defined as the force that produces an acceleration of 1 m s–2 on an object of mass 1 kg.
Under the influence of gravitational force, an object falls towards the Earth. During this motion, the acceleration involved is called the acceleration due to the gravitational force by the Earth and is denoted by g. Its unit is the same as that of acceleration, m s-2.
The gravitational force acting on an object of mass m is F = mg.
The value of acceleration due to gravitational force by the Earth is g = 9.8 m s-2.
Newton's Third Law of Motion
Whenever one object is exerting a force on a second object, the second object is simultaneously exerting an equal and opposite force on the first object.