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Force and Pressure - Contact and Non-contact Forces

Grade 8CBSE

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

🔑Concepts

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Force is a push or a pull on an object arising from its interaction with another object. It can change the state of motion (speed or direction) or the shape of an object.

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Contact Forces: These forces act only when two objects are in physical contact with each other. Examples include Muscular Force (exerted by muscles) and Frictional Force (which opposes the relative motion between surfaces).

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Non-contact Forces: These forces act even when objects are not in physical contact, acting through a field. Examples include Magnetic Force, Electrostatic Force (exerted by a charged body on another charged or uncharged body), and Gravitational Force.

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

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Pressure is defined as the force acting per unit area of a surface. It is calculated as P=FAP = \frac{F}{A}.

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The SI unit of pressure is N/m2N/m^2, also known as the Pascal (PaPa).

📐Formulae

F=m×aF = m \times a

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

W=m×gW = m \times g

1 Pa=1 N/m21\text{ Pa} = 1\text{ N/m}^2

💡Examples

Problem 1:

A force of 200 N200\text{ N} is applied to an object over an area of 5 m25\text{ m}^2. Calculate the pressure exerted on the object.

Solution:

Given: Force F=200 NF = 200\text{ N}, Area A=5 m2A = 5\text{ m}^2. Using the formula P=FAP = \frac{F}{A}, we get P=2005=40 PaP = \frac{200}{5} = 40\text{ Pa}.

Explanation:

Pressure is the ratio of force to the area over which it is distributed. Here, the force is divided by the surface area to find the pressure in Pascals.

Problem 2:

An object of mass 10 kg10\text{ kg} is resting on the ground. Calculate the force of gravity (weight) acting on it, taking g=9.8 m/s2g = 9.8\text{ m/s}^2. Is this a contact or non-contact force?

Solution:

W=m×g=10 kg×9.8 m/s2=98 NW = m \times g = 10\text{ kg} \times 9.8\text{ m/s}^2 = 98\text{ N}. This is a non-contact force.

Explanation:

Weight is the force with which the Earth pulls an object towards its center. Since gravity acts over a distance without requiring physical contact, it is classified as a non-contact force.

Problem 3:

A pointed nail is easier to push into a wooden plank than a blunt one. Explain using the concept of pressure.

Solution:

For the same applied force FF, a pointed nail has a much smaller area of contact AA. Since P=FAP = \frac{F}{A}, a smaller AA results in a larger Pressure PP.

Explanation:

By reducing the area of contact at the tip, the force applied by the hammer creates enough pressure to pierce the wood easily.