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Force and Laws of Motion - The Concept of Force

Grade 9CBSE

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

🔑Concepts

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A force is a push or a pull upon an object resulting from its interaction with another object. It is a vector quantity, meaning it has both magnitude and direction.

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Forces can change the state of rest or motion of an object, change the speed of a moving object, change the direction of motion, or change the shape and size of an object.

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Balanced Forces: When the resultant of all forces acting on a body is zero (Fnet=0F_{net} = 0), the forces are called balanced forces. They do not change the state of rest or motion but can change the shape of an object.

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Unbalanced Forces: When the resultant of all forces acting on a body is greater than zero (Fnet>0F_{net} > 0), the forces are unbalanced. These forces cause a change in the speed or direction of motion (acceleration).

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The SI unit of force is the Newton (NN). One Newton is defined as the force required to accelerate a mass of 1 kg1 \text{ kg} at a rate of 1 m/s21 \text{ m/s}^2.

📐Formulae

F=m⋅aF = m \cdot a

Fnet=F1+F2 (if acting in the same direction)F_{net} = F_1 + F_2 \text{ (if acting in the same direction)}

Fnet=F1−F2 (if acting in opposite directions)F_{net} = F_1 - F_2 \text{ (if acting in opposite directions)}

1 N=1 kg⋅m/s21 \text{ N} = 1 \text{ kg} \cdot \text{m/s}^2

💡Examples

Problem 1:

A wooden block is pulled from the left with a force of 125 N125 \text{ N} and from the right with a force of 82 N82 \text{ N}. Calculate the net force acting on the block and determine its direction.

Solution:

Given: Force from left F1=125 NF_1 = 125 \text{ N}, Force from right F2=82 NF_2 = 82 \text{ N}. Since the forces act in opposite directions, the net force FnetF_{net} is the difference between them: 125−8243\begin{array}{r} 125 \\ - 82 \\ \hline 43 \end{array} Fnet=43 NF_{net} = 43 \text{ N}.

Explanation:

Because the force from the left is greater than the force from the right, the net force of 43 N43 \text{ N} acts towards the right (in the direction of the larger force).

Problem 2:

What is the force required to accelerate an object of mass 5 kg5 \text{ kg} at 4 m/s24 \text{ m/s}^2?

Solution:

Using the formula F=m⋅aF = m \cdot a: m=5 kgm = 5 \text{ kg} a=4 m/s2a = 4 \text{ m/s}^2 F=5×4F = 5 \times 4 F=20 NF = 20 \text{ N}

Explanation:

According to the second law of motion, force is the product of mass and acceleration. By substituting the given values, we find the total force needed.