We will talk a lot about forces. By this we’re not referring to any special powers from a time long ago or a galaxy far, far away. Instead, we are referring to the ways that physical objects interact with each other.
Even if you are quietly seated in your armchair, you are influenced by a number of forces. In fact, two in particular are fighting against one another right now and are locked in a perfect stalemate.
First of all, there is a force pulling you down toward the floor. If you opt for a second cup of coffee, it is this force that you will resist when you stand up to return to the kitchen. This is the force of gravity.
The second force, as you may have guessed, must be pushing upward. Otherwise, you’d be falling straight through the chair onto the floor. Actually, you’d even fall straight through the floor and continue to hurtle downward toward Earth’s center. This second force, essentially your chair supporting your backside, is due to something called electromagnetism. We’ll go into more detail about this later, but suffice it to say that you and the chair are composed of small, electrically charged particles that repel one another.
As gravity pulls your particles down toward those of the chair, the chair’s particles push upward with just enough force to keep you comfortably seated.
Whenever two objects are near each other, they interact by means of forces. When the forces cancel out, then a body at rest (e.g., you on your chair) will stay at rest. Moreover, a body moving at a constant speed (e.g., you gliding on a pair of ice skates) will continue moving at that speed. This is what physicists call Newton’s first law of motion: when the sum total of forces acting on an object is zero, then the change in its motion (or lack of motion) is zero.
DEFINITION
Newton’s first law of motion states that an object at rest will remain at rest, and a body in motion will continue moving in a straight line, unless acted upon by a force
Newton’s second law of motion states that the acceleration of a body is directly proportional to the force acting on that body and inversely proportional to its mass
Newton’s third law of motion states that for every action there is an equal and opposite reaction
In many cases, however, the sum total of forces acting on an object does not exactly cancel out. In this case, something’s got to give and the object’s motion will be changed.
The game of billiards presents a great example. When you begin the game with an opening break shot, you line the cue stick up behind the cue ball, then give it a bump. When you do, the stick imparts a force on the ball. Because there is nothing on the other side of the cue ball to push back, it begins to move, or accelerate, in the direction of the bump.
If you know how hard you hit the cue ball (the force, F), and you know its mass (m), you can predict how fast and in what direction it will begin to move (its acceleration, a). This relationship is known as Newton’s second law of motion, which says that the acceleration of the object is directly proportional to the force and inversely proportional to its mass: F = ma.
ATOM TRAP
The terms mass and weight are often used interchangeably. Strictly speaking, though, these are different things. Mass is a measure of the amount of matter in a body, and it is often measured in terms of kilograms. Weight is the force by which that body is attracted to Earth, and it is often given in terms of pounds. At sea level, 1 kilogram of stuff “weighs” about 2.2 pounds. High above Earth’s surface, where Earth’s gravitational pull is much less, it “weighs” appreciably less. When we refer to a “massive body” We simply mean a body that has mass, and not necessarily a heavy object.
Once you’ve hit the cue ball, it rolls along until it encounters the other balls, nicely racked up into a triangle. When it crashes into the rest of the balls, it imparts a force of its own on them. What happens next will depend on how fast and in what direction the cue ball was moving, where exactly it hit the triangular arrangement, and how perfectly the balls were racked.
If you know enough about the pool balls’ initial locations, you can use Newton’s laws to determine their movements and locations on the table at all times following your opening break shot. The inherent ability to predict this, along with the ability to control the speed and direction of the cue ball in the first place, is what separates the pool sharks from the mere guppies.
Newton’s first and second laws of motion were certainly enough to earn him a place in history. For good measure, though, he came up with a third. It says that whenever one object imparts a force on a second object (e.g., you pushing down onto your chair), then the second object imparts an equal and opposite force on the first (e.g., your chair holding you up). In plain speak, for every action there is an equal and opposite reaction.
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