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Important Notes : Physics

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Gravitation
• Every body attracts other bodies by force of gravitation
• Gravitational force of the earth is called Gravity
Mass
• The quantity of matter contained in
• It is a Scalar quantity
Weight
• Force with which is attracted towards the center of the Earth.
• It is a Vector quantity
Escape velocity
• Escape velocity at the earth’s surface is 11.2 km/s
• Escape velocity at the moon’s surface is 2.4 km/s. Therefore there is no atmosphere on the moon
Centre of Gravity

That point, at which the whole weight of the body appears to act.

Variation in g
• The value of g decreases with height or depth from earth’s surface
• The value is Maximum at poles and min at equator
• The value Decreases due to rotation of earth
Kepler's Law

• Every planet revolves around the sun in elliptical orbit.
• The area covered by the vector joining sun and planet covers equal areas in equal interval of time.
• The square of the time period of revolution is directly proportional to the cube of semi major axis.
Weight of a Body in a Lift
• If the lift is stationary or moving with uniform speed (either upward or downward), the apparent weight of a body is equal to its true weight.
• If the lift is going up with acceleration, the apparent weight of a body is more than the true weight. If going down with acceleration then the apparent weight will be less than the true weight.
• The cord of the lift is broken, it falls freely.
Geostationary Satellite

• Revolves around the Earth at a height of 36000 km.
• Used reflect TV signals and telecast TV programs from one part of the world to another in weather forecasting in predictions of floods and droughts.
Polar Satellite
• The height of 800 km.
• Used for weather forecasting, mapping etc.

Q. Two bodies kept at a certain distance feel a gravitational force F to each other. If the distance between them is doubled, the force will be-

(a) 2F

(b) 1/5 F

(c) 4F

(d) ¼ F

Ans - d

Q. It is easier to carry two buckets of water in one hand each, than to carry only one in one hand because.

(a) Weight of buckets are balanced

(b) Centre of gravity falls within the body

(c) Centre of gravity and center of equilibrium fall within the feet

(d) Resultant weight of buckets is zero

Ans - c

Q. In the Earth, the weight of a body is maximum in -

(a) Oslo

(b) Mexico

(c) Santiago

(d) Singapore

Ans - a

Q. Intensity of gravitational field of the Earth is maximum at -

(a) Poles

(b) Equator

(c) Centre of Earth

(d) Surface

Ans - a

Q. The spoon dropped by an astronaut in satellite will-

(a) Fall to the floor

(b) Remain stationary

(c) Continue to follow the motion of the satellite

(d) Move tangentially away

Ans - b