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Showing posts from December, 2017

### Hydrogen is in abundance around the sun

Hydrogen is in abundance around the sun but not around earth. Why?     ANSWER:     The escape velocity on the sun is very high compared to that on the earth. The gravitational pull of the sun is very large because of its larger mass compared to that of the earth. So it is very difficult for hydrogen to escape from the sun's atmosphere because r.m.s velocity of hydrogen is more than escape velocity(11.2 km/sec). But on the sun r.m.s velocity of the hydrogen is less than the escape velocity of the sun. Hence hydrogen is abundant on sun. Note:  r.m.s velocity means root mean square velocity) escape velocity of the planet/star is the minimum velocity required to a body to escape form the planet/star.

### IF THE POLAR ICE CAPS OF THE EARTH WERE TO MELT

·          If the polar ice caps of the earth were to melt, what would the effect of the length of the day be?      Answer: Earth rotates about its polar axis. When ice of polar caps of earth melts, mass concentrated near the axis of rotation spreads out. Therefore, moment of inertia I increase.    As no external torque acts, angular momentum constant. i.e L = I ω = I(2π/T)    With increase of I, T will increase i.e. length of the day will increase.

### PENDULUM CLOCK ON THE TOP OF A MOUNTAIN

Will a pendulum clock gain or lose time when taken to the top of a mountain? Answer:   On mountains the acceleration due to gravity is less as compared on the surface of earth. Since time period is inversely proportional to the square root of the acceleration due to gravity, the time period increases. The pendulum clock loses time on the top of a mountain. A girl is swinging seated in a swing. What is the effect on the frequency of oscillation if she stands? Answer : The frequency of oscillation will increase because in the standing position, the location of centre of mass of the girl shifts upwards. Due to it, the effective length of the swing decreases.   As υ α 1/   √l    therefore ,  υ increases.

### EFFECT OF MASS ON TIME PERIOD OF SIMPLE PENDULUM

·       The bob of a simple pendulum is made of wood. What will be the effect on the time period if the wooden bob is replaced by an identical bob of aluminum? Answer:         The time period of a simple pendulum does not change, if the wooden bob is replaced by an identical bob of aluminum why because the time period of a simple pendulum is independent of the material of the bob. The bob of a simple pendulum is a hollow sphere filled with water. How will the period of oscillation change, if the water begins to drain out of the hollow sphere? Answer :             When water begins to drain out of the sphere, the  center  of mass of the system will first move down and then will come up to the initial position. Due to this the equivalent length of the pendulum increases hence time period first increases, reaches a maximum value and then decreases till it becomes equal to its initial value.

### WHY ARE SPOKES PROVIDED IN A BICYCLE WHEEL?

QUESTION:  Why are spokes provided in a bicycle wheel? Answer:             The spokes of cycle wheel increases its moment of inertia. If it is a disc some mass will be very near to axis of rotation. Due to spokes maximum mass for rim leads to more moment of inertia, greater is the opposition to any change in uniform rotational motion. As a result the cycle runs smoother and steadier. if the cycle wheel had no spokes, the cycle would be driven in jerks and hence unsafe. Is it necessary that a mass should be present at the center of mass of any system? ANSWER:         No. It is not necessary to have some mass at center of mass of the system. Eg: For bangle or ring center of mass is at its center but there is no mass particle at center of mass.

### MANHOLE INDICATOR

SCIENCE PROJECT Manhole Indicator Project - Manhole Indicator Video