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Force and Pressure - Concept of Force and its Effects

Grade 8CBSE

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

🔑Concepts

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A force is defined as a push or a pull on an object resulting from its interaction with another object. An interaction of one object with another object results in a force between the two objects.

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Force has both magnitude (strength) and direction. It is a vector quantity. If the direction or magnitude of the applied force changes, its effect also changes.

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The SI unit of force is the Newton, represented by the symbol NN.

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Net Force (FnetF_{net}): If two forces act on an object in the same direction, the net force is the sum of the two forces. If they act in opposite directions, the net force is the difference between the two forces.

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Effects of Force: (1) A force can move a stationary object. (2) It can stop a moving object. (3) It can change the speed of an object. (4) It can change the direction of motion of an object. (5) It can change the shape or size of an object.

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Force per unit area is called pressure (PP), which is related to the force (FF) applied perpendicularly to a surface of area (AA).

📐Formulae

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

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

P=FAP = \frac{F}{A}

1 kgf≈9.8 N1 \text{ kgf} \approx 9.8 \text{ N}

💡Examples

Problem 1:

Two boys, A and B, are pushing a heavy box. Boy A exerts a force of 30 N30 \text{ N} and Boy B exerts a force of 20 N20 \text{ N} in the same direction. What is the net force acting on the box?

Solution:

Given F1=30 NF_1 = 30 \text{ N} and F2=20 NF_2 = 20 \text{ N}. Since they are in the same direction, Fnet=F1+F2=30 N+20 N=50 NF_{net} = F_1 + F_2 = 30 \text{ N} + 20 \text{ N} = 50 \text{ N}.

Explanation:

When forces are applied in the same direction, they add up to produce a larger resultant force.

Problem 2:

In a tug-of-war, Team X pulls the rope with a force of 2500 N2500 \text{ N} towards the left, while Team Y pulls with a force of 2300 N2300 \text{ N} towards the right. Calculate the net force and specify the direction of motion.

Solution:

Fnet=2500 N−2300 N=200 NF_{net} = 2500 \text{ N} - 2300 \text{ N} = 200 \text{ N}. The direction of the net force is towards the left (direction of the larger force).

Explanation:

When forces act in opposite directions, the net force is the difference between them, and the object moves in the direction of the greater force.

Problem 3:

Identify the effect of force in the following scenario: A goalkeeper dives to save a football heading towards the goal.

Solution:

The force applied by the goalkeeper stops the moving ball or changes its direction.

Explanation:

This demonstrates that a force can change the state of motion (speed or direction) of an object.