Forces

Kinematics is the branch of motion studies that creates a description of an object's motion. It tells us things like: how fast, how far, the direction, the time involved. Dynamics is the branch of motion studies that provides explanations of the object's motion. It offers answers to questions like: What made the object move? What made it slow down (or speed up)? What made it change direction?

Forces A force is just a push or pull. Sometimes contact is involved, e.g., when you reach out to grab hold of a glass of pop and pull it toward you. Other times there is no contact, e.g., we feel the tug of gravity even though there is nothing touching us. Forces can cause objects to accelerate or decelerate, change their direction, flex or alter their shape.

The Four Fundamental Forces Although forces are acting in the world all around us and inside us all the time, they can be classified into one of four basic types. The gravitational force (the force Isaac Newton investigated) exists between all matter, trying to hold things together, trying to move particles closer together. It's range is enormous, reaching out across the universe, but it is the weakest of the forces. The electromagnetic force occurs between charged particles. It gives matter its strength and determines if a substance will deform or shatter when it is stressed. Note: when charged particles are in motion, they generate magnetic forces that push or pull on each other. Both magnetic and electric forces are thought to be different aspects or versions of this one force. It is much stronger than gravity but operates over a much tinier area. The strong nuclear force binds together the matter that forms the nucleus. This is by far the strongest force but, like electromagnetic forces, operates in just a tiny area. The final force, the weak nuclear force (a type of electromagnetic force), is involved in the process of radioactive decay.

Laws of Motion If forces are acting in all places all of the time, then motion must be everywhere, even if we don't notice it. Isaac Newton grouped all instances of motion into just three laws, a summary of movement.

His first law states that objects with no force acting on them move at a constant velocity, i.e., at a constant speed in a constant direction. So, an ice chunk zipping through the inky blackness of deep space, far any matter whose gravity would be reaching out to tug at it, would stay on its original straight path. The law also states that if an object was resting to begin with, it continues to do that.

The second law states that an object's rate of acceleration depends on the size of the force causing its speed to change (increase or decrease). So, a powerful engine bolted to a light weight chassis can create quite a ride, able to cause a large acceleration as the vehicle moves quickly from rest to a high speed. Why? Look at the famous equation that describes this idea: F = ma. The "F" is the applied force trying to move the object, in Newtons (N), "m" is the object's mass, in kg, and "a" is the acceleration value that indicates how rapidly the object's speed is changing (increasing if your foot is on the gas or decreasing if it's on the brake). It's unit is m/s². See that if the F is a large value (from a powerful engine), the acceleration value must also be large.

Newton's third law tells us why it takes more muscle power to move or lift heavy objects than it does to lift lightweight ones. The law states that any time a force is applied to an object, there will be a resisting force that is equal in magnitude but in the opposite direction to the applied force. The applied force and the resisting force are called action-reaction forces. An example is you trying to lift a box of books: it exerts a downward pull on your arm as you try to lift up on it. If you keep tugging up on the box, at some point, your muscles exert more of an upward force than the box can resist with its downward force. Now, there is a net upward force that creates an upward acceleration of the box and so, up it goes. Here's another application of this law. You are on a mattress in a swimming pool and drift over to the side. You push against the lip of the pool and it pushes back at you. Since there is no friction to hold you in place, you move out away from the pool edge. A similar application of this law would be in space. Imagine an astronaut trying to manoeuver a satellite out of the space shuttle's cargo bay. Although both the person and satellite experience weightlessness because of the path in which the shuttle travels around the Earth, their mass is unchanged. So, if the astronaut tugs on the satellite, it tugs back and unless she is anchored to the shuttle, she and the satellite will move toward each other herself toward it as she bends her arm. And, since her mass is so much less than the satellite's, she will move the most.

Mass and Weight These two terms are often used in an interchangeable way but do have specific meanings. Mass is a measure of the amount of matter in some substance while weight is a measure of how strongly gravity pull on the substance. We have a certain mass and when we step on a scale, we see how strongly gravity pulls us, pulls our mass, toward the Earth's core. The more mass we have, the larger will be the weight reading on the scale's dial. The equation that relates weight and mass is:

W = mg. W is the weight, in Newtons (N); "m" is the mass, in kg; "g" is the gravity acceleration created by the matter in the Earth pulling at us, it is 9.8 m/s². (On the moon, it is just 1.6 m/s².) Do you see that F = ma and W = mg are just different versions of each other?

We often do not notice the constant downward tug of gravity on us (creating our weight) because we are used to the sensation of our muscles supporting ourselves. But, what about after a long day on our tired feet? Or slipping and falling? And then limping around on a sprained ankle? In these circumstances, we become very aware of the tug of gravity and of the sensation of weight. We can "see" our weight when we walk across wet sand or snow. The depth of our footprints is a rough visual indicator of our weight, of the force of gravity pulling down on us.

Does a skydiver's mass change once she steps out of the plane? No, because the amount of matter she contains is unchanged. Does a skydiver feel her weight? No, because there is no supporting surface against gravity can pull her. Does the constant force of gravity accelerate her faster and faster? No, because as she picks up speed, the drag force due to air friction becomes larger and larger until it prevents a further increase in speed. The terminal velocity of an object depends on its mass and how smoothly its aerodynamic shape allows it to slip through the air.

Inertia is a property of matter that can be described as "resistance to change". An object's inertia tends to keep it doing what it's doing: keeping it moving in the same path and at the same speed if its in motion, or keeping it motionless if it's at rest. An object's inertial mass is the "a" value in the equation

F = ma. The same object's gravitational mass can be found by putting it on one platform of a pan balance (a type of balance that works like a teeter-totter) and adding masses to the other platform until the arm of the balance is level. Some call this process "weighing" the object, others call it "massing" the object. The two types of masses are the same.