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Friction - Types of Friction: Static, Sliding, and Rolling

Grade 8CBSE

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

🔑Concepts

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Friction is a force that opposes the relative motion between two surfaces in contact. It acts in the direction opposite to the applied force.

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The magnitude of friction depends on the nature of the surfaces (roughness or smoothness) and the force pressing the two surfaces together, known as the Normal Force (NN).

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Static Friction (fsf_s) is the force that resists the initiation of motion. It acts when the object is at rest. Its maximum value is called Limiting Friction.

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Sliding Friction (fkf_k) comes into play when an object is sliding over another surface. It is slightly less than the limiting static friction because once motion starts, the irregularities on the surfaces do not get enough time to lock into each other strongly.

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Rolling Friction (frf_r) occurs when an object like a wheel or a sphere rolls over a surface. It is significantly smaller than sliding friction, which is why wheels and ball bearings are used to reduce friction.

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The order of magnitude for the three types of friction for the same pair of surfaces is: fstatic>fsliding>frollingf_{static} > f_{sliding} > f_{rolling}.

📐Formulae

f=μNf = \mu N

N=m×gN = m \times g

fstatic(max)>fsliding>frollingf_{static(max)} > f_{sliding} > f_{rolling}

💡Examples

Problem 1:

A heavy wooden crate of mass 50 kg50 \text{ kg} is placed on a floor. If the acceleration due to gravity g=9.8 m/s2g = 9.8 \text{ m/s}^2, calculate the Normal Force NN exerted by the floor on the crate.

Solution:

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

Explanation:

On a horizontal surface, the Normal Force (NN) is equal to the weight of the object, which is the product of its mass and the acceleration due to gravity.

Problem 2:

Explain why it is easier to move a luggage bag fitted with wheels than one without wheels.

Solution:

Rolling friction is much smaller than sliding friction (frolling<fslidingf_{rolling} < f_{sliding}).

Explanation:

When wheels are used, the contact area between the surface and the object is minimized, and the motion involves rolling instead of sliding. Since rolling friction is significantly less than sliding friction, less force is required to move the luggage.

Problem 3:

If the limiting static friction of a block is 20 N20 \text{ N}, what happens if an external force of 15 N15 \text{ N} is applied?

Solution:

The block will remain at rest, and the static friction will be 15 N15 \text{ N}.

Explanation:

Static friction is a self-adjusting force. It matches the applied force up to its maximum limit (limiting friction). Since 15 N<20 N15 \text{ N} < 20 \text{ N}, the friction produced is exactly 15 N15 \text{ N} to keep the object in equilibrium.