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Introduction to Forces in Physics

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    Introduction to Forces in Physics

    The objective of this video is to define the term "force" in physics in general and in mechanics in particular namely, to demonstrate, by a few simple experiments, that a force is any influence on an object that changes the object's shape or motion.

    1st Experiment:

    Bending a strip of flexible steel or metal, in this case a saw blade, by pressing the strip's end with a finger or by placing a small weight (change of shape).

    2nd Experiment:

    Flipping a coin into the air by placing the coin on the end of the flexible saw blade and then bending and releasing it (change of motion).

    3rd Experiment:

    Stretching a coil spring by a hanging mass or by pulling with hand (change of shape).

    4th Experiment:

    Earth's gravitational attraction is demonstrated by letting a tennis ball fall freely downwards on the floor. Here the ball is attracted by Earth's gravity from distance, without any direct contact, (change of motion since the ball is falling from rest).

    5th Experiment:

    Earth's gravity is also demonstrated by a rolling object racing down an inclined plane. Since the object can't fall vertically downwards it races down the plane with increased speed to the lower point possible (change of motion since the object is rolling down from rest).

    6th Experiment:

    A magnet attracts, without direct contact, a resting iron cylinder even if an object is placed between them (change of motion).

    7th Experiment:

    When we rub a plastic comb or an ebony rod with a piece of wool or silk they attract pieces of paper (change of motion).

    8th Experiment:

    A compass needle (or magnetic needle) is deflected when placed nearby a wire with a current flowing through it. For this, we use a simple series electrical circuit consisting of a battery, a switch and wires. When we close the switch a current flows through the wire and the needle of the nearby compass is deflected. (Change of motion since the needle is deflected from rest).


    1. A force is exerted on an object whenever its shape or motions are affected.
    2. In the first and third experiments some objects (weight, finger) changed the shape of other objects (saw blade, spring).
    3. In other experiments some objects (saw blade, the Earth, magnet, charged comb, wire with a current flowing through it) set other objects into motion.
    4. In a few cases the influence was achieved by direct contact between the objects whereas in other cases the influence was achieved from some distance.
    5. We have demonstrated a few types of forces: mechanical forces (experiments 1-5), magnetic force (experiment 6), and electrical forces (experiment 7-8).
    6. There are other types of forces in existence that were not demonstrated here. For example, other mechanical forces induced by air resistance and friction and molecular forces in the form of capillary and adhesive forces and more.

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    Last updated: October 2012
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