Are Newton's laws wrong

Newton's laws and their applications

In the first chapter of our physics course, we described movements in a wide variety of ways: sometimes in words, sometimes in diagrams, sometimes with formulas and calculations. We call this type of movement teaching "kinematics". Now we will ask about the causes of movements and investigate how forces can change movements ("dynamics"). Isaac Newton was the first to do this systematically, formulating his famous three laws that still govern school physics today.

We now want to investigate what influence forces have on movements.

From experience we can say that a movement that is not artificially sustained by a driving force eventually comes to a standstill:

Reflection: Give examples of this!

However: experience is deceptive! The statement is wrong! How so?

Let us first ask about the reason why the movement comes to rest:


Puck on the ice, v = speed

A puck on the ice slides a long way, but eventually comes to a standstill even on smooth ice. Why?

Sure: Even with fresh and smooth ice there is always a little friction between the puck and the ice. She slows him down.

Without the friction, movements would not stop. The puck would slide straight ahead at the same speed.


The earth has been revolving around the sun for billions of years without needing a drive. Because the space around the sun is practically empty, there is practically no friction "in space".

Isaak Newton was the first to recognize that in truth the situation is exactly the other way around: We do not need a driving force to maintain a movement, only the absence of a braking force. Unfortunately, this is difficult to do on Earth, as there is usually some kind of friction involved.


Even a thread pendulum in a vacuum soon comes to rest because the thread has internal friction.

The so-called applies Theorem of inertia (Newton's 1st law):

If there is no force acting on a body, it will continue to move in a straight line and at a constant speed.

In short: F = 0 ⇒ v = constant

A special case of this sentence is that the body is initially at rest. Question: what is the sentence then?

A body at rest, on which no force acts, remains at rest.

Conversely, the result is: in order for a body to begin to move out of rest, it must experience a force effect.


A soccer ball is on the penalty spot. A short time later it flies towards the gate. What happened? Sure: he got a kick from a player.

Explanation: The footballer's shoe exerts a considerable force on the ball within a fraction of a second, so that it is accelerated and, for example, flies away at around 100 km / h. This now happens in a straight line and at a constant speed.


A motorcycle starts from a standstill.

Question: There is no external force here, how can it still accelerate?

Sure: the power comes from within, namely from the engine. The engine power is transmitted to the rear wheel via various gear ratios. This exerts a rearward force on the road. This can be seen, for example, from the sand splashing backwards under the tire. The road, in turn, reacts to the tire with a counterforce that is transmitted to the entire motorcycle. We draw a representative of this force in the center of gravity.

Forces on the motorcycle:

S = center of gravity of the motorcycle
F.1 = Force of the rear tire on the road
F.2 = Reaction force of the road on the center of gravity

A force does not necessarily have to change the magnitude of a speed. If it acts perpendicular to the direction of movement, it pulls neither forwards nor backwards, but sideways. So it will only change the direction of movement.


An object is attached to a string and swung over the head. With a little skill, a constant rotation speed can be achieved. The cord pulls the body inwards with a constant force F. (Note: with this force the cord must be held by the hand.)

Question: Doesn't that contradict the law of inertia? A force acts on the object, and yet the amount of speed remains the same!

No, Newton didn't say so either! He only asserted that it cannot then be a straight line movement with constant speed. And the movement is no longer straight in this case!

In the case of the earth orbiting the sun, the position is corresponding. It maintains its speed of 30 km / s at a constant rate.

We note that a force on a body can also cause the direction of its movement to change without the movement becoming faster or slower. This is always the case when the force acts perpendicular to the direction of movement. Sure, then the body is neither pulled forwards nor backwards, i.e. neither accelerated nor decelerated.


A car drives through a curve at a constant speed. How can that be, isn't there a rope attached to the author to pull him into the curve?

Right, the reason for this is the steering, which causes the car to push itself off the road. So the road exerts a lateral force on the car.

Reflection: What happens if this is suddenly made impossible by gravel or black ice?

Answer: the car continues to move in a straight line because there is no longer any force acting (law of inertia). And the car comes off the road! Accident!