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Basic Newtonian Physics
Newtonian Physics can be described as the math of how everything moves here on Earth and throughout the universe. The principles that Isaac Newton discovered can show us how objects react to different forces, from gravity to torque supplied to a vehicle's wheel. These principles can show us the forces at work during a vehicle's acceleration, stopping and cornering. The information on this article is intended to give a general idea of these principles, and don't take into account any number of variables that exist in the real world.
Physics Measurements
Speed, Velocity and Acceleration
Speed is a rate of distance per time. Miles per hour and meters per second are examples of units of measure for speed. Velocity is a speed with a direction. A compass is divided into 360 degrees (360o). 10MPH at 0o (due North) is an example of a velocity. Any change in speed or direction would be a change of velocity, and therefore an acceleration. Acceleration is a change of speed or direction, and are measured at rates of speed per time. Acceleration can be either negative or positive, with negative acceleration usually referred to as deceleration outside of physics. An object accelerating at 10MPH per second would take 10 seconds to increase its speed by 100MPH. Changing direction is also an example of acceleration. As a vehicle turns a corner, it undergoes what is known as centripetal (center seeking) acceleration. This acceleration is directed toward the center of the circle in which the vehicle is turning.
Mass, Force, and Momentum
Mass and weight are often confused. Mass is a constant value for an object, whether it is on the surface of the earth or floating in space, usually measured in kilograms. Weight is the force that gravity applies to an object, which is actually not the same as the weight we measure on a scale. An astronaut in orbit is actually not weightless, but feels weightless because the spacecraft and the astronaut are both undergoing the same centripetal acceleration around the Earth. Similarly, a person's true weight is higher than their measured weight because the earth is rotating, andtherefore the surface of the earth is essentially "falling" away from the person. Weight is just one example of force, with force commonly measured in Newtons or pounds. An object's momentum is equal to its mass multiplied by its velocity. Since an object with a certain mass essetially weighs the same anywhere on earth's surface, a car which weighs 3,000lbs moving at 10MPH would have the same momentum as a car weighing 6,000lbs moving at 5MPH, and twice the momentum as a 3,000lb. car moving at 5MPH or a 1,500lb. car moving at 10MPH, provided they were moving in the same direction.
Isaac Newton's Laws of Motion
Newton's three laws of motion are the basis of Newtonian Physics, and quite simply and thoroughly explain questions about the motion of objects that had plagued the wisest philosophers for thousands of years. Some of our most basic understanding of the world around us are thanks to these laws,
First Law
Newton's first law of motion, the law of inertia, can be simply explained as follows: an object at rest will stay at rest until a force acts to accelerate it, and an object in motion will stay in motion at the same speed and in the same direction until a force acts to either accelerate or decelerate it. Imagine a car sitting in a perfectly level parking lot, in neutral with the parking brake released. There is nothing holding the vehicle from moving, but since no force is acting upon it to cause it to move, it remains motionless. The other side of this example is a bit more complicated; placing the vehicle in gear and accelerating to 5MPH and returning to neutral while still in the perfectly level parking lot, according to this law, should cause the vehicle to continue to roll at 5mph until it hits a curb or comes to an uphill, right? Wrong, because forces of friction, such as friction between the tires and the road, between drivetrain components, and between the car's body and the air are acting upon the vehicle. Taking this into account, it is important to note that in order for the vehicle to accelerate in the first place, the force applied to the vehicle, whether it is pushed or driven by its own wheels, must be greater than the force of friction or other forces that act on the object. Imagine a tug-o-war. If both teams pull on the rope with the exact same force, regardless of how much force this might be, the rope doesn't move. It is only when one team pulls with more force than the other team that the rope moves in their direction. The difference in opposing forces that causes the movement is known as net force. Net force causes acceleration, and no net force means no acceleration.
Second Law Newton's second law of motion, the law of resultant force, basically tells us that an object's change in momentum is related to the amount of force applied to it. The acceleration caused by a certain amount of force is equal to that force divided by the object's mass. If a car weighing 3,000lbs. accelerates at a certain rate with a certain net force applied to it (remember a certain amount of force is required just to overcome friction), twice the net force applied to it will cause the car to accelerate at double the rate. A car weighing 6,000lbs with twice the original net force applied to it would accelerate at the same rate as the 3,000lb. car. It is also important that the car will accelerate in a straight line in the same direction that the force was applied to it, assuming no other forces are applied to cause the vehicle to turn, such as the force caused by turning the wheels when steering the vehicle. In this case, the friction between the tires and the road provide a centripetal force, which causes a centripetal acceleration.
Third Law Newton's third law of motion, the law of reciprocal actions, explains that there is always an equal and opposite reaction to every action. If a car weighs 3,000lbs., it is pressing on the earth with a force of 3,000lbs. The earth, in turn, is pressing back on the car with a force of 3,000lbs. A better example might be a boxer throwing a punch. As the boxer lands a punch on his opponent's face, his fist applies a force to the face. In turn, his opponent's face applies an equal and opposite force to the boxer's fist. Anyone who has ever thrown a punch can attest to the presence of this opposing force, which can result in pain or even broken bones in the hand!
Preservation of Momentum
If a car is moving at 10MPH and collides head-on with another 3,000lb. car traveling at 5MPH, the two cars will exert an equal force upon each other. Assuming all factors aside from the velocity of the vehicles are identical, the two vehicles would suffer identical damage and the passengers in both vehicles would suffer identical injuries. Assuming the two vehicles were completely rigid and stuck together on impact, and ignoring all other forces such as friction, the difference in momentum between the two vehicles at the time of the collision would cause the now-joined vehicles to move at 2.5MPH in the same direction as the vehicle originally moving at 10MPH. This is because the 10MPH vehicle had twice the momentum of the 5MPH vehicle. The total amount of momentum remains constant throughout the collision, but remember velocity consists of both speed and direction. If the 10MPH vehicle was traveling in a 0o direction and the 5MPH vehicle at a 180o direction, the net velocity would be 5MPH in a 0o direction. Since the net weight of the cars combines to 6,000lbs, double the weight of each individual vehicle, maintaining the 5MPH at 0o velocity would require doubling the net momentum of the now-joined vehicles. Since no outside force was available to cause an increase in net momentum in the now-joined vehicles, the 6,000lbs. would have a velocity of just 2.5MPH in a 0o direction, preserving net momentum.
Summary
As you can see, physics can explain how vehicles move and interact with the world around them. Physics is an extremely complex science that really attempts to explain how everything in the universe moves and interacts. If you have difficulty understanding the principles in this article, don't worry. Even Albert Einstein had his limits of understanding of physics.
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