Actions

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What is a mechanical action?

Every time you push, pull, hit, or press on an object, you exert a mechanical action.
It is simply the effect that an object (or a person) exerts on another. This action can:

  • modify its motion (make it speed up, slow down, change direction),
  • or deform it (bend, crush, stretch it...).

For example, hitting a ball makes it move: it is an action that modifies its motion. Crushing a metal can is an action that deforms it.

Two types of mechanical actions

We distinguish two main families of mechanical actions:

  • Contact actions: the objects must be in contact.
    Example: pushing a door, pulling a drawer, walking (your feet press on the ground).
  • At-a-distance actions: the objects do not touch each other.
    Example: the Earth attracts objects towards itself without touching them (gravity), a magnet attracts a paperclip at a distance.

Representing a mechanical action

To better understand actions, scientists use two tools:

  1. Forces: they represent an action with an arrow called a vector.
  2. Interaction diagrams (DOI): they show all the interactions of an object with its environment.

What is gravity?

Gravity, or weight, is an action at a distance exerted by a celestial body (like Earth) on objects close to its surface. It is a special case of gravitation.

When you drop an object, it falls to the ground: this is because Earth attracts it. It is this force that is called weight.

How do we measure weight?

Weight (notated P) is measured using a dynamometer and is expressed in Newtons (N).
It depends on two things:

  • the mass of the object (the heavier it is, the more it is attracted),
  • the field strength or acceleration of gravity (g) which depends on the planet.

On Earth, we take g ≈ 10 N/kg (more precisely, 9.81 N/kg). This quantity can also be expressed in m/s², an equivalent unit, because g is both a gravitational field strength and an acceleration.

Formula to know

$P= m \times g$

With:

  • P: weight (in N),
  • m: mass (in kg),
  • g: gravitational field strength (in N/kg, or equivalently in m/s²).

  Example:
A 2 kg object on Earth.

The gravitational field strength is g = 10 N/kg
The formula: P = m x g
  ➝ P = 2 × 10 = 20 N

The same object on the Moon (g ≈ 1.6 N/kg) 
  ➝ P = 2 × 1.6 = 3.2 N

 

 

A force present everywhere in the universe

All objects with mass exert an attractive force on each other. This is universal gravitation.
Even if we do not feel it between small objects, it is responsible for:

  • the weight we feel on Earth,
  • the motion of planets around the Sun,
  • objects falling to the ground.

Gravitation ≠ Gravity

  • Gravitation: attractive force between two masses, regardless of their size or position.
  • Gravity: effect of a celestial body's gravitation on objects near its surface.

   Gravity is therefore a special case of gravitation.

What influences gravitation

The strength of gravitation depends on:

  • the mass of objects (the more massive they are, the stronger the attraction),
  • the distance separating them (the further apart they are, the less they attract each other).

It is the universal law of gravitation (developed by Isaac Newton) that allows us to calculate this gravitational force:

$$F=G\frac{m_1 \times m_2}{d^2}$$

m₁ m₂ d
Masses
Distance
← Drag the spheres →
Calculation of F
F = G × m₁ × m₂ / d²
m₁ = — 
m₂ = — 
d = — 
— N