Chapter 10-Gravitation

We are aware that the moon orbits the earth. When something is thrown upward, it rises to a given height before falling.

UNIVERSAL LAW OF GRAVITATION

• Every thing in the universe is attracted to every other object with a force that is inversely proportional to the square of the distance between them and directly proportional to the product of their masses.

• The force is along the line joining the centres of two objects.

• F is the gravitational force

• M is the object with greater mass

• m is the object with smaller mass

• d is the distance between the objects

• G is gravitational constant whose value is

IMPORTANCE OF THE UNIVERSAL LAW OF GRAVITATION

• It is the force that binds us to the earth.

• The motion of the moon around the earth is due to gravitation

• The motion of planets around the Sun is due to gravitation

• The tides due to the moon and the Sun is due to gravitation

Free Fall

• We now understand that objects are drawn to the earth. The gravitational pull of earth is  responsible for this. We refer to an object as being in free fall once it is falling towards the ground solely due to this force of gravity.

• The objects' motion is still in the same direction as they fall. However, the magnitude of the velocity will change because of the earth's attraction. Acceleration is a necessary part of every velocity change. There is an acceleration involved whenever something is falling towards the earth. The gravitational pull of the earth is what is causing this acceleration. As a result, this acceleration is also known as the acceleration due to gravity. It is represented by g. The SI unit is the same, which is m/s^2.Value of g is 9.8m/s^2 or 10m/s^2.

FORMULAS

• v = u + gt

• H = ut + 1/2 att

• v^2 = u^2 + 2gH

These equations are valid in case of gravitation where ‘H’ is the height of the body and ‘g’ is the accleration due to gravity which is 9.8m/s^2 or 10m/s^2.

Mass

• Mass of an object is the measure of its inertia

• Greater the mass, the greater is the inertia.

• It remains the same whether the object is on the earth, the moon or even in outer space.

• Thus, the mass of an object is constant and does not change from place to place.

Weight

• The weight of an object is the force with which it is attracted towards the earth.It is denoted with W. W=mg

• As the weight of an object is the force with which it is attracted towards the earth, the SI unit of weight is the same as that of force, that is, newton (N).

• Weight of a body is not constant,it varies from place to place.

WEIGHT OF AN OBJECT ON THE MOON

• The weight of an object on the moon is the force with which the moon attracts that object.

• The mass of the moon is less than that of the earth. Due to this the moon exerts lesser force of attraction on objects.

Thrust and Pressure

The force acting on an object perpendicular to the surface is called thrust.

• When you stand on loose sand, the force that is your body weight acts on an area that is equal to the area of your feet. When you lie down, the same force is applied to a surface greater than the area of your feet and equivalent to the contact area of your entire body. As a result, different areas are affected differently by forces of the same magnitude. The thrust is the same in the examples above. But the results vary. As a result, the thrust's impact is determined by the area where it is applied.

• The effect of thrust on sand is larger while standing than while lying. The thrust on unit area is called pressure

• Pressure=thrust/area Its SI unit is Pascal (Pa) or N/m^2.A larger pressure is applied by the same force to a smaller area,but if the area is large,the same force applied will result in less pressure. This explains why blades have sharp edges, buildings have wide foundations, and nails have pointed tips.

PRESSURE IN FLUIDS

All liquids and gases are fluids. A solid exerts pressure on a surface due to its weight.Similarly, fluids have weight, and they also exert pressure on the base and walls of the container in which they are enclosed.Any confined mass of fluid under pressure transmits that pressure in all directions without loss.

BUOYANCY

The upward force applied to an object that is fully or partially submerged in a fluid is known as the buoyant force. Upthrust is another name for this upward thrust.

• All objects experience a force of buoyancy when they are immersed in a fluid.

• The magnitude of this buoyant force depends on the density of the fluid.

• Objects having density less than that of the liquid in which they are immersed, float on the surface of the liquid. If the density of the object is more than the density of the liquid in which it is immersed then it sinks in the liquid.