Galileo's ideas that an object must be propelled by a steady force was completely turned around by Galileo, who stated that in the absence of a force, a moving object will continue moving. This idea was refined by Newton and made into his first law.


Law 1: Every object continues in it's state of rest or uniform motion in a straight line unless it is acted on by an outside, unbalanced force.

The key word here is continues: an object continues to do whatever it happens to be doing unless a force is exerted upon it. If it is at rest, it continues to be at rest. If it is moving, it will continue moving in a straight line. (This should already be fairly familiar to you from our discussion of inertia in lesson 1.)


Every object in the world possesses inertia, depending on how much mass the object has. The more mass, the more inertia. We often think of mass as weight. But this is incorrect. Mass is the quantity of matter in an object. It is also the amount of inertia that object possesses. Weight is the force of gravity upon each other. Mass and weight are directly proportional, but mass is the more fundamental quantity. It does not change, whereas weight changes in different gravitational fields.

In physics, we use different units than you may be used to. We measure mass in kilograms. There are one thousand grams in a kilogram. We measure force in Newtons. A Newton is equal to a little less than a quarter of a pound. A one kilogram brick weighs 9.8 N at the surface of the earth. (Notice that this has the same numerical value as the acceleration of falling bodies.) 9.8 is the conversion factor for mass to weight (which, remember, is the force of gravity on an object.) Take the number of kilograms in an object and multiply it by 9.8 and you will get it's weight in Newtons at the surface of the earth. Remember: Mass and Weight are different from each other. In space, an object may have no weight but it would still have mass!


From Newton's first law, we see that everything that changes it's motion has to have some sort of force behind it. We know from lesson one that a change in motion is an acceleration. So force must cause an acceleration. How does force relate to acceleration? Newton covered this in his second law.


Law 2: The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.

This means that that Force is equal to mass times acceleration, or F=ma. An object is accelerated in the direction of the force acting on it. When a force is applied in the direction of an objects motion, it will increase the motion of the object. When a force is applied in the opposite direction of an objects motion, it will decrease the objects motion.


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What is a force, exactly? In the simplest terms, a force is a push or a pull. It's source may be gravitational, electrical, or muscular. Newton's second law gives a more precise definition. A force is something that produces acceleration. Since force is proportional to acceleration, twice the net force produces twice the acceleration, three times the net force produces three times the acceleration, etc. We say net force because often more than a single force acts on an object at the same time. If you and a friend pull in the same direction with equal forces on an object, the forces will add up to produce a net force of twice your single force. This produces twice the acceleration you would have produced if you were pulling alone. If you and your friend were pulling with equal forces, but in opposite directions, the net force would be zero, and the object would not move.



When the acceleration of an object is zero, we say the object is in mechanical equilibrium. The net force of an object in equilibrium is always zero. A ball at rest on a table is in mechanical equilibrium, because it is not accelerating. A ball moving at constant velocity is also in mechanical equilibrium. Consider the first case. The book lying at rest on the table is not accelerating. Therefore, the net force must be zero. Weight acts as a force, pulling the book down. There must be a second force holding the book up. This force is the support force of the table. The table exerts an upward force on the book that is equal in magnitude to the downward force of gravity on the book. It is like pushing down on a spring: The spring pushes up on your hand with the same force you push down on it.


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Now consider the second case. The ball slides without acceleration because the net force is equal. When you push a box across the floor at constant velocity, the force of your push is balanced exactly by the force of friction. The net force is zero, so the box does not accelerate. Remember, no acceleration does not mean no velocity. It means that the velocity will not change; The object will neither speed up, slow down, or change direction.



Remember from lesson 1 that Galileo discovered that all objects fall at the same rate, no matter what their masses. Galileo never understood this fact. However, Newton is able to explain this fact with his laws of motion. We know that a falling object accelerates toward the earth because of the gravitational force of attraction between the object and the earth. A heavy object is attracted to earth more than a lighter one. Why, then, doesn't the heavier object fall faster than the lighter object? The answer is in the equation F=ma. The gravitational force is larger on the heavier object, but the mass is larger too. These two masses always cancel out and the ratio of F/M is always 9.8.


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What about an object not in free fall, but one falling with air resistance? The acceleration is different between an object falling under the influence of air resistance and an object falling in free fall. The important concept to keep in mine is the idea of net force. In a vacuum or in cases where air resistance can be neglected, the net force is just the weight of the object. In cases where air resistance is present, the net force is the weight of the object and the force of the air pushing against it. The force of the air increases with the speed of the falling object. When the force of the air equals the weight of the object, we say that the object has reached terminal velocity. The object has stopped accelerating. Consider the case of a light woman and a heavy man sky diving. When their parachutes open, who will reach terminal velocity first? The answer is the light woman. Since her weight is less, it takes less time for the force of the air to equal her weight.


We said that in it's simplest sense a force is a push or a pull. Looking more closely, however, we see that a force is really an interaction between two things. Consider the example of a hammer and a stake. When you hit the stake with the hammer, the hammer exerts a force on the stake that makes it accelerate and move into the ground. But what makes the hammer stop? According to Newton's first law, there has to be a force to make the hammer stop. What exerts this force? The answer is the stake! Newton reasoned that while the hammer exerts a force on the stake, the stake also exerts a force on the hammer. So in the interaction between the hammer and the stake, there is a pair of forces-one acting on the stake and one acting on the hammer. This is Newton’s third law.


Law 3: Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

One force is called the action force, and the other the reaction force. It doesn’t matter which force we call which, only that we recognize that both forces are co-parts of a single interaction. For every interaction , the forces always occur in pairs. The hammer pushes down on the stake, the stake pushes up on the hammer. The tires push against the road, the road pushes against the tires. The rocket pushes on gas, the gas pushes on the rocket. Even the force of gravity has it's reaction force. The earth pulls on the ball, the ball pulls on the earth!



We know that forces cancel out when they are equal and act in opposite directions on the same object. Why then, don't action and reaction forces cancel each other out? Even though action and reaction forces are equal and act in opposite directions, they do not cancel out because they are on different objects. Consider a force pair of an apple pulling on an orange. Let's ignore the apple and just think about the orange. We can draw an imaginary circle around the orange and call what is inside that orange a system. The apple's pull provides a force on the system, and hence the system accelerates. The fact that the orange simultaneously exerts a force on the apple, which is outside the system, affects only the apple and not the orange. If we consider the system to be both the orange and the apple, the force pair is internal to the system. In this case, where the action and reaction pairs are considered to be inside the same system, the forces do cancel each other. The apple and the orange would move closer, but the system's "center of mass"(we'll learn about that later) stays in the same place. So the internal parts of the system accelerate, but the system itself does not. This is why our bodies don't blow apart due to atoms moving around inside of us. (This may be confusing, so don't worry if you don't understand it just yet.)


We said that when a ball is pulled on by the force of gravity, it pulls on the earth with an force of equal magnitude to the force of gravity on it. Why, then, don't we see the earth jumping up every time something falls? The answer in this is in Newton's second law. Remember that F=ma. Although the earth has a force acting on it every time a object is dropped, it's mass is so large that there is no perceptible movement! A large mass will always cancel out a small force .