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| Tutorial ID | Title | Tutorial | Video Tutorial | Revision Notes | Revision Questions | |
|---|---|---|---|---|---|---|
| 4.2 | Types of Forces I. Gravitational Force and Weight |
In these revision notes for Types of Forces I. Gravitational Force and Weight, we cover the following key points:
Gravitational Force is the attracting force between any two objects due to the interaction between their respective gravitational fields.
The phenomenon of objects' attraction due to the presence of the gravitational field (and therefore, of gravitational force) is known as "gravity". For example, Earth attracts all objects near its surface because these objects are inside the range of its gravitational field. Hence, we can say "Gravitational Field is the space around an object in which its gravitational attraction effect is observed." This means that Gravitational Force is a distant (field) force.
Gravitational force depends on the mass of the objects involved and the distance between their centres. The equation of gravitational force is:
where m1 and m2 are the masses of the objects involved, and R is the distance between the objects (from centre to centre, not from surface to surface).
The quantity G is a constant. It is known as the gravitational constant. Its numerical value is
The value of constant G has been calculated experimentally.
Gravitational field strength (gravitational acceleration) g⃗ is the physical quantity, which characterizes mathematically the gravitational field. Its value is g⃗ = 9.81 m/s2 = 9.81 N/kg near the Earth surface but it becomes smaller when moving away from the Earth. It can be calculated by the equation
where m is the mass of the object, (not the Earth).
The long formula of gravitational field strength is
Gravitational field strength g⃗ generated by the Earth does not depend on the properties of the other object, which is inside this gravitational field, but only in the distance R from the centre of the Earth (the other quantities such as mass of the Earth M and the gravitational constant G are constants).
By definition, Weight, W⃗ is the pushing force exerted by an object on the place it lies (or the pulling force when hanged on a string), caused by the gravity.
Many people often confuse Weight and Gravitational Force with each other. Despite the common things (and the same result obtained in most cases when calculating these two forces), there are some very important differences between them. They are:
People very often confuse weight and mass. This is because when an object rests on a balance, it shows the mass in kilogram. However, this is a trick used from the balance constructors to avoid problems since people are more familiar to the mass.
In fact, the balance measures the weight in Newtons but when stamping units in the balance, the constructors convert it in kilograms by dividing weight with the gravitational field strength.
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