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Exploring Forces - Non-contact forces

Grade 8CBSE

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

🔑Concepts

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Non-contact forces are forces that act on an object without any physical contact between the two objects involved.

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Magnetic Force: It is the force exerted by a magnet on another magnet or on magnetic materials like iron, nickel, and cobalt. Like poles (N-N or S-S) repel each other, while unlike poles (N-S) attract each other.

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Electrostatic Force: It is the force exerted by a charged body on another charged or uncharged body. For example, a plastic comb rubbed with dry hair attracts tiny bits of paper due to electrostatic force. Like charges repel and unlike charges attract.

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Gravitational Force: It is the force of attraction between any two objects in the universe due to their masses. It is always an attractive force. Gravity is the force with which the Earth pulls objects towards itself.

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The strength of a non-contact force generally decreases as the distance between the two objects increases, often following an inverse-square relationship like F∝1d2F \propto \frac{1}{d^2}.

📐Formulae

W=m×gW = m \times g

Fgravity=Gm1m2r2F_{gravity} = G \frac{m_1 m_2}{r^2}

Felectrostatic=kq1q2r2F_{electrostatic} = k \frac{q_1 q_2}{r^2}

💡Examples

Problem 1:

An object has a mass of 50 kg50\text{ kg}. Calculate the force of gravity (weight) acting on it on Earth, taking g=9.8 m/s2g = 9.8\text{ m/s}^2.

Solution:

W=50 kg×9.8 m/s2=490 NW = 50\text{ kg} \times 9.8\text{ m/s}^2 = 490\text{ N}

Explanation:

Weight is the non-contact gravitational force exerted by the Earth. It is calculated by multiplying the mass of the object by the acceleration due to gravity (gg).

Problem 2:

Explain why a ball thrown upwards eventually falls back to the ground.

Solution:

The ball falls back due to the gravitational force of the Earth.

Explanation:

The Earth exerts a non-contact force of attraction on the ball, pulling it towards its center even though there is no physical connection between the Earth and the ball while it is in the air.

Problem 3:

A plastic straw rubbed with a piece of paper is found to attract another suspended plastic straw that has not been rubbed. Identify the force and its nature.

Solution:

The force is Electrostatic Force and it is attractive in nature.

Explanation:

Rubbing the straw creates a static charge on it. A charged body exerts an electrostatic force on an uncharged body (the second straw), pulling it closer without touching.