Secondary Mechanics/Page7: Newton's Laws

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Laws of Newton and their applications

In the first chapter of our physics course we described movements in the most different kind: times in words, times in diagrams, times with formulas and computations. We call this kind of kinematics "kinetics". Now we will ask for the causes of movements, and to examine, like forces movement’s change can ("dynamics"). Isaak Newton was first, who did this systematically, and its famous three laws formulated, which determine still today school physics.

We want to now examine, which influence of forces on movements to have.

From experience we can say that a movement, which will not artificially maintain by a driving strength comes finally to succumbing:

Reflection: Call examples of it!

However: the experience deceives! The statement is wrong! Why?

We ask first for the cause for the fact that the movement comes to the peace:



Puck on the ice, v = speed

In addition, a puck on the ice sleds far, comes with smooth ice finally to a halt. Why?
Clearly: Even with fresh and smooth ice there is always a small friction between puck and ice. It breaks it.

Without the friction movements would not come to succumbing. The puck would continue to slip with same speed straightforward.

The earth turns already since billions of years around the sun, without it needs a drive. Because the area is practically empty around the sun, there is "in space" practically no friction.

Isaak Newton recognized as the first that in truth the state of affairs is exactly different around: We do not only need driving strength, but the absence of a braking strength for the receipt of a movement. Unfortunately this is difficult on earth to realize, since usually any kind of friction is in the play.

Even a thread pendulum in the vacuum comes soon to the peace, since the thread possesses internal friction.

It applies the inertia set in such a way specified (1st Newton's law):

If no strength affects a body, then it moves on 

straight-lined and with constant speed.

Briefly: F=0? v = constantly

A special case of this sentence is that the body is at first in peace. Question: how does the sentence read then?

A resting body, which no strength affects, remains in peace.

Turned around arises thus: so that a body from the peace begins to move, it must experience a mutual force action.

Example: A football is on the point of eleven-meter. It flies a short time later on the gate too. What happened? Clearly: he got a footstep of a player. Explanation: the shoe of the soccer player exerts a substantial strength within a fraction of one second on the ball, so that this is accelerated, and for example with about 100 km/h of it flies. This happens now straight-lined and with constant speed.

Example: A motorcycle starts from the conditions.

Question: Here nevertheless does no strength influence from the outside, how it can accelerate nevertheless?

Clearly: the strength comes here from the inside, i.e. from the engine. The engine strength is transferred over different translations to the rear wheel. This exerts to the rear strength on the road. One can recognize that for example by the fact that sand under the tire squirts to the rear away. The road again reacts with a counter acting force on the tires, which will transfer to the whole motorcycle. We draw a representative in of this strength in the emphasis.

Forces at the motorcycle: S = emphasis of the motorcycle F1 = strength of the rear tire on the road F2 = reaction force of the road on the emphasis

Strength does not have to necessarily change the amount of a speed. If it works perpendicularly to the direction of motion, then it pulls neither forward nor to the rear, but laterally. It will change thus only the direction of the movement.

Example: An article is fastened to a cord and swung over the head. With something fate a constant rotation speed can be reached. The cord pulls the body with constant strength F inward. (Note: with this strength the hand must hold the cord.)

Question: doesn't contradict the inertia set? The article strength affects, and nevertheless the amount remains the same the speed! No, Newton did not also maintain this! It only stated that it cannot be that then a straight-line movement with constant speed develops. And the movement is no longer straight-line in this case!

For the case of the earth circling around the sun the situation is accordingly. It maintains its speed of 30 km/s constantly.

We hold that a strength can also cause on a body that the direction of its movement changes, without the movement becomes faster or slower. This is the case whenever the strength works perpendicularly to the direction of motion. Clearly, then the body is pulled neither forward nor to the rear, accelerated nor retarded thus neither.

Example: A car drives with constant speed by a curve. How, at the author is nevertheless fastened can no rope be, which pulls it into the curve? Correctly, the cause for it is the steering element, which causes that the car at the road pushes itself away. The road exerts thus a lateral strength on the cars.

Reflection: What happens, if this is made suddenly impossible by roll loose chippings or ice? Answer: the car continues to move straight-lined, since no more strength works (inertia set). And the cars get off the road! Accident!

Proceed to the next side