Changes

'''Value of “g” at different places'''

'''Value of “g” at different places'''

[[Image:Gravitation%20for%20wiki_html_m7724e27c.gif]] <br>

[[Image:Gravitation%20for%20wiki_html_m7724e27c.gif]] <br>

 

=== How do we perceive this weight? ===

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

How do we perceive this weight?

When you stand on a surface, the force of the Earth's gravity is acting upon you downwards and there is a normal force exerted by the surface on which you stand. Since you stand on a firm surface and there is no acceleration, the normal force is equal to the gravitational force and this is equal to mg. If an object is suspended from a spring, the gravitational force will be balanced by the tension force in the string.

When you stand on a surface, the force of the Earth's gravity is acting upon you downwards and there is a normal force exerted by the surface on which you stand. Since you stand on a firm surface and there is no acceleration, the normal force is equal to the gravitational force and this is equal to mg. If an object is suspended from a spring, the gravitational force will be balanced by the tension force in the string.

When an object is in equilibrium, it only experiences the gravitational and restoring force/ Weight is mass multiplied by the acceleration due t gravity

When an object is in equilibrium, it only experiences the gravitational and restoring force/ Weight is mass multiplied by the acceleration due t gravity

Measured weight can change:

'''Measured weight can change:'''

    when acceleration due to gravity changes

*when acceleration due to gravity changes

    when the object is accelerating (non-inertial frame)

*when the object is accelerating (non-inertial frame)

When used to mean force, its magnitude (a scalar quantity), often denoted by an italic letter W, is the product of the mass, m, of the object and the magnitude of the local gravitational acceleration g;. thus: W = mg. When considered a vector, weight is often denoted by a bold letter W. The unit of measurement for weight is that of force, which in the International System of Units (SI) is the newton.

When used to mean force, magnitude of weight (a scalar quantity), often denoted by an italic letter W, is the product of the mass, m, of the object and the magnitude of the local gravitational acceleration g;. thus: W = mg. When considered a vector, weight is often denoted by a bold letter W. The unit of measurement for weight is that of force, which in the International System of Units (SI) is the newton.

For example, an object with a mass of one kilogram has a weight of about 9.8 newtons on the surface of the Earth, about one-sixth as much on the Moon, and very nearly zero when in deep space far away from all bodies imparting gravitational influence.

For example, an object with a mass of one kilogram has a weight of about 9.8 newtons on the surface of the Earth, about one-sixth as much on the Moon, and very nearly zero when in deep space far away from all bodies imparting gravitational influence.

Earlier concepts of weight

'''Earlier concepts of weight'''

Concepts of heaviness (weight) and lightness (levity) date back to the ancient Greek philosophers. These were typically viewed as inherent properties of objects. Plato described weight as the natural tendency of objects to seek their kin. To Aristotle weight and levity represented the tendency to restore the natural order of the basic elements: air, earth, fire and water. He ascribed absolute weight to earth and absolute levity to fire. Archimedes saw weight as a quality opposed to buoyancy, with the conflict between the two determining if an object sinks or floats. The first operational definition of weight was given by Euclid, who defined weight as: "weight is the heaviness or lightness of one thing, compared to another, as measured by a balance.".

Concepts of heaviness (weight) and lightness (levity) date back to the ancient Greek philosophers. These were typically viewed as inherent properties of objects. Plato described weight as the natural tendency of objects to seek their kin. To Aristotle weight and levity represented the tendency to restore the natural order of the basic elements: air, earth, fire and water. He ascribed absolute weight to earth and absolute levity to fire. Archimedes saw weight as a quality opposed to buoyancy, with the conflict between the two determining if an object sinks or floats. The first operational definition of weight was given by Euclid, who defined weight as: "weight is the heaviness or lightness of one thing, compared to another, as measured by a balance.".

It is a very common misconception that when astronauts are in orbit they are weightless because they are somehow far enough from the earth that the force of earth's gravity does not pull on them. This is totally incorrect. If they were that far away, earth's gravity would not pull on the shuttle either and it would be impossible for it to be in orbit around the earth.

It is a very common misconception that when astronauts are in orbit they are weightless because they are somehow far enough from the earth that the force of earth's gravity does not pull on them. This is totally incorrect. If they were that far away, earth's gravity would not pull on the shuttle either and it would be impossible for it to be in orbit around the earth.

Gravity (a force we call weight) is actually responsible for keeping the space craft and the astronaut in orbit around the earth. Gravity is still pulling on the astronaut. The feeling of weight;ess is no differenet than when in ree fall. What they are not experiencing is the normal force, which is the opposing force. When that force is gone, we feel say we feel "weightless." In fact, whenever a person is in freefall they feel weightless even though gravity is still causing them to have weight. While in orbit, the space shuttle does not have to push on the astronaut (or anything else in the cabin) to keep him up. The space shuttle and the astronaut are in a constant state of freefall around the earth.

Gravity (a force we call weight) is actually responsible for keeping the space craft and the astronaut in orbit around the earth. Gravity is still pulling on the astronaut. The feeling of weight;ess is no differenet than when in ree fall. What they are not experiencing is the normal force, which is the opposing force. When that force is gone, we feel say we feel "weightless." In fact, whenever a person is in freefall they feel weightless even though gravity is still causing them to have weight. While in orbit, the space shuttle does not have to push on the astronaut (or anything else in the cabin) to keep him up. The space shuttle and the astronaut are in a constant state of freefall around the earth.<br><br>

Discovery of planets

 

== Discovery of planets ==

Accurate measurements on the orbits of the plantes indicated that they did not precisely follow Kepler's laws. Slight deviations from perfectly elliptical orbits were observed. Newton was aware that this was to be expected from the Law of Universal Gravitation. The derivation of perfectly elliptical ignores the forces due to the other planets. These deviations called perturbations are observed and led to the discovery of Neptune and Pluto. Planets around distant stars were also inferred from the regular wobble of each star due to the gravitational attraction of the revolving plant.

Accurate measurements on the orbits of the plantes indicated that they did not precisely follow Kepler's laws. Slight deviations from perfectly elliptical orbits were observed. Newton was aware that this was to be expected from the Law of Universal Gravitation. The derivation of perfectly elliptical ignores the forces due to the other planets. These deviations called perturbations are observed and led to the discovery of Neptune and Pluto. Planets around distant stars were also inferred from the regular wobble of each star due to the gravitational attraction of the revolving plant.

Ocean tides

== Ocean tides ==

Ocean tides are caused by differences in the gravitational pull between the Moon and the Earth on the opposite sides of the Earth. Gravitational force is stronger on the side of the Earth nearer to the Moon and is weaker on the side of the Earth farther from the Moon. The bulge that is caused in the Earth's oceans due to this gravitational pull results in two sets of tides on the Earth.

Ocean tides are caused by differences in the gravitational pull between the Moon and the Earth on the opposite sides of the Earth. Gravitational force is stronger on the side of the Earth nearer to the Moon and is weaker on the side of the Earth farther from the Moon. The bulge that is caused in the Earth's oceans due to this gravitational pull results in two sets of tides on the Earth.

Activity 1 – Observe a freely falling body

=== Activity 1 – Observe a freely falling body ===

Objective: To observe the behaviour of a freely falling object

'''Objective: To observe the behaviour of a freely falling object'''

 

'''Procedure:'''

Procedure:

Ask a child to drop a piece of chalk from terrace, Start the stop clock as soon as the child drops it. Put off the clock as soon as the chalk touches the ground, note down the time taken , Repeat the same expt with a stone,& calculate the time, Time taken will be same in both the cases. Inference : All bodies accelerate equally irrespective of their mass.

Ask a child to drop a piece of chalk from terrace, Start the stop clock as soon as the child drops it. Put off the clock as soon as the chalk touches the ground, note down the time taken , Repeat the same expt with a stone,& calculate the time, Time taken will be same in both the cases. Inference : All bodies accelerate equally irrespective of their mass.

Activity 2 : Thought Experiment

=== Activity 2 : Thought Experiment ===

Objective: To understand the nature of gravitational force

'''Objective: To understand the nature of gravitational force'''

Ask the children to think about what will happen if we had a hollow tunnel running through the centre of the Earth and we dropped a ball into it. What will happen to the ball?

*A projectile motion of a body thrown is due to the gravitational force.

    A projectile motion of a body thrown is due to the gravitational force.

*Satellites are projectiles that are continuously falling in the orbit around planets

In the first case, the ball is just dropped from the cliff and it falls down in a straight line, subject to the force of gravity. In the second and third instances, the ball is thrown upwards, reaches a certain height and still falls down. In the third case, the ball covers a horizontal range as well.

In the first case, the ball is just dropped from the cliff and it falls down in a straight line, subject to the force of gravity. In the second and third instances, the ball is thrown upwards, reaches a certain height and still falls down. In the third case, the ball covers a horizontal range as well.