NEWTON'S UNIVERSAL LAW OF GRAVITATION

Universal law of gravitation

 Any two masses in the universe attract each other with a force which is 

i) directly proportional to the product of the masses,

ii) inversely proportional to the square of the distance between their centres.

F=GmM/r² 

where G= universal gravitational constant.

m and M are the masses of two bodies.

r = distance between them. 

* Larger the masses , greater is the force between them.

* Larger the distance , smaller is the force.

* If the distance is halved , force increases 4 times.

* If the distance is doubled,  force becomes one-fourth.

* The force is not effected with any medium between the masses. 

ANGLE OF DEVIATION BY A PARALLEL SLAB

Q: What is the angle of deviation produced by a parallel glass slab?

Ans: 0°. The incident ray is parallel to the emergent ray.

 Class 10                                                                     Marks: 20

Physics Revision

ATTEMPT ALL QUESTIONS.

Define moment of force. Write relationship between the SI and CGS unit of moment of couple. 2

Find the mass of the scale in equilibrium .  

                                      

Draw diagrams and locate the centre of gravity  of hollow cone and triangular sheet. 2

 State the  type of work done by a coolie  when i)  he lifts a load up ii) when he moves on a horizontal platform carrying load on his head.     2

A force is applied on a body of mass 20kg movong with a velocity of 40 m/s. The body attains a velocity of 50 m/s in 2  s. Calculate the work done by the force. 2

Give one example of  each  i) when chemical energy is converted into electrical energy .

                                                  Ii when electrical energy changes into sound  energy.   2

A ball of mass  200g is thrown vertically upwards with initial velocity of 20 m/s. Calculate its maximum potential energy.       2

State law of conservation of energy and prove it for a feely falling body taking any two points in the fall. 3

Give two conditions for which a force does no work. 2

Refractive index and colour of a light

 Red colour has longest wavelength so it has maximum speed in a medium compared to other colours.  As it travels with maximum speed in a medium,  the refractive index of the medium for Red colour is minimum.

MECHANICAL ADVANTAGE , VELOCITY RATIO

Which is always constant for a machine of given design: MA or VR ?

Ans: VR remains constant for a machine of given design as it depends on the distances moved by the load and effort but MA can vary due to the friction developed between the moving parts of the machine.

LENZ'S LAW

LENZ'S LAW is based on law of conservation of

a) mass

b) charge

c) momentum 

d) energy.

Ans: d)   energy 

Why is uniform circular motion an accelerated motion?

 Q: Why is uniform circular motion an accelerated motion?

A: The velocity of  Uniform Circular motion changes continuously due to its changing direction. As there is change in velocity, the motion is accelerated motion.

DOOR HANDLES

 Question: Why are door handles fitted at the edge of the door ?

Answer: Door handles are fitted at the edge to increase  the perpendicular distance of the force from the axis of rotation so that the minimum force can be applied to achieve the given torque.

Fog/Cloud appears white. Why?

 Q: Fog/ cloud appears white. Why ?

Ans: Light is scattered by the fog particles. As the size of the particles is bigger than the wavelength of light, all colours are scattered to equal extent giving back white light. 

The property of light responsible for white clouds: scattering of light.

Convex lens behaves as a concave lens! Surprising!!

 Convex lens can behave as a concave lens and vice versa when the lens is placed in a medium with refractive index more than that of the lens.

Convex lens diverges and concave converges the rays.

What is an image? How is an image formed?

Ans:  The replica of an object  formed by the intersection of reflected rays is called an image.

Image is formed when the reflected rays actually meet at a point        ( real image)or appear to meet( virtual image)

ABSOLUTE REFRACTIVE INDEX AND RELATIVE REFRACTIVE INDEX

 ABSOLUTE  REFRACTIVE INDEX 

The absolute refractive index of a medium is the ratio of speed of light in vacuum and the speed of light in the medium.

Absolute refractive index = Speed of light in vacuum or air

                                                    Speed of light in the medium 

For example,

A.r.i of glass = speed of light in vacuum/speed of light in glass 

3×10⁸/2×10⁸= 3/2=1.5

Its value is equal or more than 1. For vacuum or air, its value is 1. So its minimum value is 1.

RELATIVE REFRACTIVE INDEX 

The relative refractive index of a medium (a) with respect to another medium (b) is the ratio of speed of light in medium (b) and the speed of light in medium (a).

Relative refractive index of  (a) wrt (b)=Speed of light in medium (b)

                                                                        Speed of light in medium (a)

For example, 

r.r.i of glass with water=speed of light in water/speed of light in glass 

=2.25×10⁸/2×10⁸=9/8=1.1

r.r.i of water wrt to glass= speed of light in glass/speed of light in water

=2×10⁸/2.25×10⁸= 8/9=0.88

Its value can be less than 1, more than 1 or equal to 1