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Exploring Forces - Contact forces

Grade 8CBSE

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

🔑Concepts

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Contact forces are forces that act only when two objects are in physical contact with each other. Examples include muscular force and frictional force.

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Muscular Force is the force resulting from the action of muscles. It is used to perform physical tasks like lifting, pushing, or pulling. For example, when we lift a bucket of water, the force exerted by our muscles is a contact force.

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Frictional Force (FfF_{f}) is the force that opposes the relative motion between two surfaces in contact. It always acts in the direction opposite to the direction of motion.

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The magnitude of friction depends on the nature of the surfaces (roughness or smoothness) and the force with which the two surfaces are pressed together.

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Air Resistance is a type of contact force (fluid friction) exerted by air on objects moving through it, such as a parachute or a moving car.

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Normal Force (NN) is the contact force exerted by a surface on an object pressing against it, acting perpendicular to the surface.

📐Formulae

Fnet=Fapplied−FfrictionF_{net} = F_{applied} - F_{friction}

Ff=μ×NF_{f} = \mu \times N

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

💡Examples

Problem 1:

A wooden crate is pushed across a floor with a muscular force of 150 N150\text{ N}. If the frictional force opposing the motion is 45 N45\text{ N}, calculate the net force acting on the crate.

Solution:

The net force (FnetF_{net}) is calculated by subtracting the frictional force from the applied muscular force: Fnet=150 N−45 NF_{net} = 150\text{ N} - 45\text{ N} 150−45105\begin{array}{r} 150 \\ - 45 \\ \hline 105 \end{array} The net force is 105 N105\text{ N}.

Explanation:

Since friction acts in the opposite direction to the applied force, we subtract the magnitude of friction from the applied force to find the resultant or net force.

Problem 2:

Identify the type of contact force involved when a person climbs a rope.

Solution:

The primary contact forces are Muscular Force and Friction.

Explanation:

The person uses their muscles to pull themselves up (Muscular Force), and the grip between their hands and the rope involves friction, which prevents them from sliding down.