CBSE – 8th Standard Science

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Friction

• Friction is caused by the irregularities on the two surfaces in contact

• The force of friction always opposes the applied force.

• Spring Balance is a device used for measuring the force acting on an object.
• In spring balance stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of the force.
• Friction is caused by the irregularities on the two surfaces in contact.
• Force of friction will increase if the two surfaces are pressed harder.
• The force required to overcome friction at the instant an object starts moving from    rest is a measure of static friction.
• If an object started moving, it would never stop if there were no friction.
• The sliding friction is slightly smaller than the static, any object started moving, it would never stop if there were no friction.
• Friction can also produce heat.
• The substances which reduce friction are called lubricants.
• Friction can never be entirely eliminated. No surface is perfectly smooth. Some irregularities are always there.
• To reduce friction in order to increase efficiency. oil, grease or graphite is applied

• When one body rolls over the surface of another, the resistance to motion is called the rolling friction. It reduces friction.
• Rolling reduces friction.
• It is always easier to roll than to slide a body over another. That is the reason it is convenient to pull the luggages fitted with rollers.
• Fluids exert force of friction on objects in motion through them. The frictional force exerted by fluids is also called drag.
• Fluids exert force of friction on objects in motion through them.
• The frictional force on an object in a fluid depends on its speed with respect to the fluid.
• Friction opposes the relative motion between two surfaces in contact. It acts on both the surfaces.
• Friction depends on the nature of surfaces in contact.
• For a given pair of surfaces friction depends upon the state of smoothness of those surfaces.
• Friction depends on how hard the two surfaces press together.
• Static friction comes into play when we try to move an object at rest.
• Sliding friction comes with play when an object is sliding over another.

• Sliding friction is smaller than static friction.
• Friction can be increased by making a surface rough.
• Friction is important for many of our activities.
• The sole of the shoes and the tyres of the vehicle are treaded to increase friction.
• The friction is sometimes undesirable.
• Friction can be reduced by using lubricants.
• When one body rolls over another body, rolling friction comes into play. Rolling friction is smaller than the sliding friction.
• In many machines, friction is reduced by using ball bearings.
• Fluid friction can be minimized by giving suitable shapes to bodies moving in fluids.
• Even those surfaces which appear very smooth have a large number of minute irregularities on them Irregularities on the two surfaces lock into one another.
• You saw that if you apply the force along the left, friction acts along the right.
• Finally it stops. Have you not seen a moving ball on the ground stopping after some time? Why do we slip when we step on a banana peel.
• It is difficult to walk on a smooth and wet floor