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Science · 13 chapters
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Chapter 5: Exploring ForcesClass 8 Science — summary, notes, extra questions & NCERT solutions

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What is the SI unit of force?

Summary

This chapter explores force as a push or pull that arises from the interaction between at least two objects. A force can make a resting object move, change the speed or direction of a moving object, or change an objects shape. The SI unit of force is the newton (N). Forces are classified as contact forces, which act only when objects touch, and non-contact forces, which act from a distance. Muscular force and friction are contact forces; friction opposes motion between surfaces in contact, arises from surface irregularities, and is greater on rough surfaces. Magnetic force, electrostatic force, and gravitational force are non-contact forces. Rubbing certain materials produces static charges, and like charges repel while unlike charges attract. Gravitational force is the attraction of the Earth on objects; it is always attractive and gives objects their weight. The chapter distinguishes weight, the gravitational pull measured in newtons that can vary from place to place, from mass, the amount of matter measured in grams or kilograms that stays constant. Weight is measured with a spring balance, and students learn to read its range and smallest division. Finally, it introduces buoyant force (upthrust): liquids push objects upward, and whether an object floats or sinks depends on the balance between gravity and this buoyant force, linked to density.

Force as push or pullContact forces: muscular and frictionNon-contact forces: magnetic, electrostatic, gravitationalMass, weight, and spring balanceBuoyant force, floating and sinking

Key terms

Force
A push or pull on an object resulting from its interaction with another object; its SI unit is the newton (N).
Contact force
A force that acts only when two objects are in physical contact, such as muscular force and friction.
Friction
The contact force that opposes the motion of one surface over another, arising from surface irregularities.
Non-contact force
A force that acts even without physical contact, such as magnetic, electrostatic, and gravitational forces.
Gravitational force
The always-attractive force with which the Earth pulls objects towards itself, giving them weight.
Buoyant force
The upward force, also called upthrust, that a liquid exerts on an object placed in it.

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