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And now we are dealing with non-uniform movements, I mean movements which are accelerated.

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Acceleration is the variation of velocity with times.

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So this is the formula, the final velocity minus the initial velocity divided by the

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time it took to change from Vf to Vo.

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For example, if we're driving a car at, let's say, 20 kilometers per hour, and then we stop,

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and it took us to stop two seconds, the acceleration would be 0 minus 20 divided by 2.

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That results minus 10 kilometers per hour per second.

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In fact, we may have positive or negative acceleration, depending on whether the final

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velocity is bigger or smaller than the initial velocity.

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By the way, in the international system of units, acceleration is measured in meters

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per second squared.

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In other words, the variation of speed in meters per second, every second.

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The fundamental equation of dynamics was written by Newton about 400 years ago.

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This equation establishes a direct relationship between acceleration and force.

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But what role plays the constant m in this formula?

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see that if m is very big then the acceleration will be small for a given

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force. Newton called this constant m the mass, the mass of each object on which

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the force is applied. So Newton concluded that the mass of the different

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object is an inertial constant, as Galileo explained. You can use this web page

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to get acquainted with this concept, force, mass and acceleration. By the way, you know that

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acceleration is measured in meters per square second. Mass, you know, is measured in kilograms

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and force is measured in Newton. We write this unit of force with a capital N. We can define

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one Newton as the force required to make a mass of one kilogram accelerate one

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meter per second squared. Then we can calculate the force required to stop

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something in movement or conversely to speed something up. You will have to

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perform this kind of exercise next. And so I'm also proposing you some tests

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about these critical concepts. Mass, force and acceleration. We are dealing with the

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classical dynamic approach. We shall learn later that Einstein was not very

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convinced about the explanation of mass given by Newton and finally he found that mass is a kind

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of energy or in fact mass can be converted into energy and energy can be converted into mass

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but we cannot study this in second ESO so for the moment we shall just say that mass is a constant

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for every object and we call it mass. It's just a constant of proportionality between the force

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and the acceleration. That's enough for the moment. We are using Newton's equation, force

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equals mass times the acceleration, just to solve some very simple problems in mechanics. That's all.