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Force and Laws of Motion - Balanced and Unbalanced Forces

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Force is a push or pull that can change the state of rest, motion, or the shape of an object. Its SI unit is Newton (NN).

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Balanced Forces: If the resultant of all the forces acting on a body is zero, i.e., ∑F⃗=0\sum \vec{F} = 0, the forces are called balanced forces.

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Effects of Balanced Forces: They do not change the state of rest or uniform motion of an object, but they can change the shape or size of the object (e.g., compressing a spring from both sides).

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Unbalanced Forces: If the resultant of all the forces acting on a body is non-zero, i.e., ∑F⃗≠0\sum \vec{F} \neq 0, the forces are called unbalanced forces.

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Effects of Unbalanced Forces: They produce a change in the speed or direction of motion, meaning they cause acceleration (a≠0a \neq 0).

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Frictional Force: To move a heavy object, the applied unbalanced force must be greater than the force of static friction (fsf_s) acting between the surfaces.

📐Formulae

Fnet=F1+F2+...+FnF_{net} = F_1 + F_2 + ... + F_n

If F1 and F2 act in opposite directions: Fnet=∣F1−F2∣\text{If } F_1 \text{ and } F_2 \text{ act in opposite directions: } F_{net} = |F_1 - F_2|

For balanced forces: Fnet=0\text{For balanced forces: } F_{net} = 0

For unbalanced forces: Fnet=m×a≠0\text{For unbalanced forces: } F_{net} = m \times a \neq 0

💡Examples

Problem 1:

A wooden block is placed on a horizontal table. Two strings are attached to its opposite faces. If a force of 10 N10\,N is applied to the left and 10 N10\,N is applied to the right simultaneously, what is the net force and the type of force?

Solution:

Fnet=Fright−Fleft=10 N−10 N=0 NF_{net} = F_{right} - F_{left} = 10\,N - 10\,N = 0\,N

Explanation:

Since the net force is 0 N0\,N, the forces are 'Balanced Forces'. The block will remain at rest and will not move.

Problem 2:

A box is pushed with a force of 25 N25\,N towards the east, while a frictional force of 10 N10\,N acts towards the west. Calculate the net force.

Solution:

Fnet=Fapplied−ffriction=25 N−10 N=15 NF_{net} = F_{applied} - f_{friction} = 25\,N - 10\,N = 15\,N

Explanation:

The net force is 15 N15\,N towards the east. Since Fnet>0F_{net} > 0, these are 'Unbalanced Forces', and the box will accelerate in the direction of the applied force.

Balanced and Unbalanced Forces - Revision Notes & Key Formulas | CBSE Class 9 Science