Newton’s Laws of Motion Explained for H2 Physics Students

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If you are a student studying H2 physics, you must be familiar with Newton’s three laws of motion. After all, they are the pillars of modern physics. The motion of every object in the universe, including manmade objects like aeroplanes and cars, can be explained using these three laws.

But first a little background, Sir Isaac Newton developed the ground-breaking theory of gravitation in 1666. At that time, he was only 23 years old. About 20 years after that, he published a short treatise with the title “Principia Mathematica Philosophiae Naturalis”, in which he presented his three laws of motion. These laws changed the face of physics and helped usher the age of machines.

Newton’s First Law of Motion

Newton’s first law of motion states that (in his own words) “Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.”

In simpler words, an object at rest will remain at rest, and an object in motion will continue to be in motion unless an external force compels it to change its action. The state of rest or motion is called inertia.

What this law means is straightforward. A ball at rest remains at rest until someone kicks it or pushes it. A spacecraft flying in space keeps flying in the same direction with the same velocity unless it is brought to a stop. The reason we do not see that happening with objects on earth is that gravity and friction (external forces) are constantly acting on them. 

Newton’s Second Law of Motion

Newton’s second law of motion states that (in his own words) “Force is equal to the change in momentum per change in time. For a constant mass, force equal mass times acceleration. F = m x a.” Momentum is mass multiplied by velocity (m x v) and the change in momentum per change in time is acceleration.

What this law means is that the faster an object moves, the greater the force it will have. If two objects having the same mass (m) are coming at you with the same acceleration (a), then they will hit you with equal force. But if one has a larger mass or a higher acceleration, then it will hit you with greater force.

An object at complete rest has mass, but zero acceleration. It doesn’t have any force. This is the reason a parked car cannot kill you while a speeding car can.

Newton’s Third Law of Motion

Newton’s third law of motion states that (in his own words) “For every action, there is an equal and opposite reaction.”

If an object, X, exerts a force on another object, Y, then object Y also exerts a force of equal amount on object X. This is the reason a ball bounces back when you kick it against a wall. The harder you kick, the harder the ball will bounce back. This law has made it possible for jets and rockets to fly. The thrust produced by the engine pushes the jet or rocket forward. The greater the thrust, the higher the acceleration.

If you are having a tough time with this concept, then you should consider taking up a physics tuition to clarify your doubts and get some extra practice. Like other chapters of Physics, getting a good grasp of Newton’s Laws of Motion comes with continuous practice. With the help of your tutor, you can ensure the anchoring of this concept in your learning.