CBSE Class 8 Annual Assessment

Chapter 5: Exploring Forces

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.

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