What is missing in Newtonian physics

Newton's laws

1. Newton's law (principle of inertia)

Every body remains in a state of rest or uniform, straight-line movement as long as no force acts on it.

2. Newton's law (principle of action)

If a force acts on a body, it is accelerated in the direction of the force.

The acceleration is directly proportional to the force and inversely proportional to the mass of the body. ( F. = ma )

3. Newton's law (reaction principle)

If there is a force between two bodies A and B, the force that A exerts on B is the opposite of the force that B exerts on A.

("Actio = Reactio")

Newton's axioms make a statement about the state of motion of bodies depending on an external size - the force- and a property of the body - the Dimensions.


Unit of mass: kilogram (kg)
(The kilogram is defined by an arbitrary mass standard - a cylinder made of a platinum-iridium alloy)

Examples of some dimensions:

Body / particle

Mass / kg



Milky Way













One distinguishes between inert mass and heavy mass:

  • If a gravitational force acts on a mass (e.g. weight force in the area of ​​attraction of the earth), then this weight force is due to the property "heavy mass".
  • The acceleration that a mass experiences through any force is defined by the amount of the inertial mass. Different masses are accelerated to different degrees by the same forces.


Lecture hall experiments for observing inertia:

·       Pulling a cloth away from under a glass filled with a liquid:

If you pull the tablecloth away quickly, only sliding friction acts instead of static friction. This is not sufficient to accelerate the body appreciably during the short interaction time between the cloth and the glass.

·       Breaking a thread


If the handle is pulled in such a way that the load is slowly increased, the upper thread breaks, as it is additionally loaded by the weight of the mass.


If you pull jerkily, the lower thread will break. The mass "resists" its acceleration by exerting an opposing force. The short acceleration phase is not sufficient to transfer the force to the upper thread. The lower thread will break beforehand.




The unit of force is determined with the help of the equation of motion of mechanics



Are defined:

1 kg m / s2 = 1 N (Newtons)

The power is a vector


Measurement of masses

a) Comparison of accelerations of different masses by the same forces: (inertial mass)

Different sizes m1 and m2 are accelerated with the same force F:

b) Beam balance: comparison of the weight forces of two masses (heavy mass)


In both cases a comparison with a calibrated mass standard is necessary.



Measurement of forces

Forces can be measured

·       by measuring the acceleration of a body

·       by observing the deformation of a body


Lecture hall experiment: Stretching a spring - Hooke's law

Observation of the deformation of a body.
force F. stretches a spring
For small changes in length x one observes the proportionality between the change in length of a spring and the amount of the applied force:


The constant of proportionality D is called the spring constant. The spring force is opposite to the acting force. Hooke's law describes the spring force as a function of the deflection, so you get:

Force gauges that make use of this connection are called spring force gauges