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From Mechanics

Newton's First Law

A particle will move with a constant velocity or remain at rest if the resultant force on the particle is zero.

Equilibrium

If the resultant force on a particle is zero, then the forces acting on the particle are said to be in equilibrium.

The Universal Law of Gravitation

F=d2Gm1m2

G=66710−11 kg-1m3s-2

Gravity on Earth

The force of gravity is often called the weight.

F=mgg=98 ms-2

Data

Radius of Earth is 637106 metres

Mass of Earth is 5981024 kg

Example 2.1

Describe whether or not the forces acting on the following objects are in equilibrium:

(a) A passenger in a train that travels at a constant speed.

(b) A hot air balloon rising at a constant rate.

(c) A stone dropped into a very deep well full of water.

Solution

(a) Yes, if it is travelling in a straight line.

(b) Yes, if it is travelling in a straight line.

(c) Yes, if it reaches a terminal velocity, so that it is travelling in a straight line at a constant speed.

Example 2.2

Find the magnitude of the force of gravity (weight) acting on a lorry of mass 22 tonnes.

Solution

This is calculated using the fact that the weight is given by mg.

mg=2200098=215600 N

The diagram shows the lorry and its weight.

Note that reaction forces also act upwards on each wheel.

R1+R2+R3+R4=215600