Newton's Three Laws of Motion.First Law:Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it. Likewise, every object at rest tends to remain in that stationary position unless an external force is acting on it.Second Law:Force is proportional to mass times acceleration (when proper units are chosen, F = ma).Third Law:For every action there is an equal and opposite reaction.
OK, now that we got the theory out of the way, let's put this into a way the we can remember it. The main reason why students don't understand Newton's Laws of Motion is due to their method of learning. Emphasis should not be made on learning the laws word for word as written in the text but on the actual understanding of the laws themselves. Let's do an example to put this all into perspective for you.
Take for instance a book on a table, abiding by the first law of motion, the book will remain at rest on the flat and even surface. This is common sense. Now, let's look at the other part of the first law, the part which states that an object in uniform motion will continue in that state unless an external force acts upon it. You can think about this as a car in nuetral that is rolling. Obviously, we know it is going to stop at some point but as the las states, NO OTHER FORCES are applied to it. In other words, the car is being acted upon by other forces (ie. friction from the road surface and air resistance). Hence in order to see objects actually adhere to this rule, the object must be placed in a vacuum in which nothing else is present to act upon it. Takes for instance a spaceship, once out of earth's atmosphere and gravitational pull (in space/ vacuum), the spaceship will remain in motion without any thrust added.
For the Second Law, let's use an encyclopedia and a notepad for the illustration. Force is by definition in physics, a push or pull that can cause an object with mass to change its velocity. Place a banana on the ground and from a known distance, release the notepad directly above the banana, it can be noted that the the banana shows no sign of impact. From the same distance directly above the banana, release the encyclopedia. It can be seen that the banana is definitely no longer appetizing , its crushed to the point where the fruit has emerged from its 'skin'. Obviously in order for such very different results there must be some plausible explanation. We know one thing, in order for the banana to have been squashed , more pressure must have been applied which indicates that more force was also applied (pressure = force/area {taking area of impact to be the same}). Hence, since more force was applied it can be assumed that the book had a greater mass. All this is actually trying to explain is that the force exert on the banana was greater as a greater mass was used.
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