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02. Newton’s Law of Motion

Before attempting to make anything move in CG, it is essential to understand the laws that govern movement: Newton’s three Laws of Motion.

Introduction

Much of what makes up Newton’s three Laws of Motion is already a part of our consciousness; we move and observe movement all day. We understand, for instance, that heavy objects move more slowly than light ones. However, by actually studying Newton’s laws, this implicit understanding is made much more active and can be born in mind when animating. For while we may not be able to recite the three laws spontaneously, when they are absent from an animation, they are very conspicuous by their absence and the bubble of illusion is burst. Let’s look at the three laws in turn.

Newton’s First Law of Motion

An object in motion stays in motion at the same speed and in the same direction unless acted on by an unbalanced force.

An object will either keep on doing what it’s doing, that is either resting or moving, until a force acts on it to change that. This law is also known as the Law of Inertia, and it is the reason why wearing a seatbelt while in a car is a very good idea. If you are driving and the car stops very suddenly, wearing a seatbelt will ensure that you and the car share the same motion, and you will also come to a stop. Without a seatbelt, however, your motion will continue and off you fly through the windscreen and beyond.

The principle of animation called Follow Through which we looked at previously is based on this law. Let’s take the example of the driver above who was wise enough to wear a seatbelt. It is easy to visualise what would happen to his body on impact. His torso, which has the seat belt, shares most closely the motion of the car and is brought to a stop equally fast. The head and neck, however, will continue to move for a few moments, and the hair, if it is long, will be the last part of the body to become stationary.

Newton’s Second Law of Motion

When a force acts on a mass, acceleration is produced. The greater the mass, the greater the force required to achieve acceleration. Therefore, the same force acting on two different objects will produce two different accelerations. This is why it is far easier to push your broken down scooter than your broken down SUV.

The law provides an equation: force = mass x acceleration

Let’s relate this to Anticipation, one of the principles of animation described above. Imagine that you are going to animate two characters, one is going to lift a heavy weight, the other is going to lift something very light. Clearly, the first character is going to need to prepare to lift the heavy object, so the Anticipation will be bold moves: he may pull up his sleeves and brace himself to handle the weight. Anticipation for the second character will need to be far more subtle: the character will turn his eyes to look at the object and his hand will reach for it, with the fingers adopting an appropriate pose.

Newton’s Third Law of Motion

For every action, there is an equal and opposite reaction. Acting forces meet other forces acting in the opposite direction. As a perfect example of this natural symmetry, when you sit down, you exert a downward force on the chair. At the same time, though we are not conscious of it, the chair is exerting a similar force upwards on you. Were the chair unable to exert this equal upward force, it would collapse.

Let’s relate this to a gun being fired. The gunpowder explosion inside the gun pushes the bullet forwards at great speed. The same force is exerted backwards causing the gun to “kick”. Naturally the bullet travels much faster and further because its mass is far smaller than that of the gun itself. If this equal and opposite reaction, in this case the “ kick”, was not depicted in an animation, the audience would register its absence.

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