These basic physics concepts will be critical to your understanding of the physics of a car collision.

# Motion

When discussing the motion of an object, two quantities must be specified, the speed and the direction of the object. Speed is the rate of change in an object’s position. An object’s change in position, otherwise known as displacement is the object’s distance from its starting point. The speed and direction of an object is the velocity. An object with the same speeds but different velocities may not end up in the same location. So the most basic concepts when dealing with motion are position, speed, and velocity. An object with constant velocity or speed has uniform motion. This means that the object isn’t changing its speed or direction. However, if the speed and/or direction of the object changes in any way, then that object is accelerating. Acceleration is the rate of change of velocity. Keep in mind that even if the speed remains constant, if the direction of object changes, then the object has acceleration.

# How and Why things move

Newton’s First Law of Motion states that every body continues in its state of rest, or of uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it. Basically, this means that anything that is moving will keep moving unless something else makes it stop moving.

Newton’s Second Law of Motion states that the change in motion is proportional to the force impressed; and is made in the direction of the straight line in which the force is impressed. Basically, Force = mass * acceleration

So, what is a force? A force can be described as any push or pull. If I were to hit you with a newspaper, then I will have exerted a force on you

Newton’s Third Law of Motion states that to every action there is an opposite and equal reaction.

Newton actually defined motion as momentum, which is equal to the velocity of an object multiplied by its mass. An object’s mass is the amount of substance or stuff in an object. Two important concepts when dealing with momentum are Impulse and Inertia. Inertia, is the force that resists acceleration and opposes momentum. Impulse, is the change in momentum which also equal the Force multiped by the change in time.

# Work and Energy

In physics, When a force acts on object over some distance, work is being done. The ability to do work is defined as Energy. There are two main types of energy which are important when dealing with car crashes. The first type is Kinetic Energy. Kinetic Energy is the energy that is related to an objects motion. Since moving objects are doing work, they have energy, or more specifically, Kinetic Energy. Kinetic energy and work are directly related because work is equal to the change in kinetic energy. The other type of energy is potential energy. Potential Energy is defined as the potential ability to do work. An object with a large potential energy will be capable of doing a large amount of work.

# Conservation of Momentum and Energy

The Law of Conservation of Energy states that energy is conserved. This means that amount of energy there is will never change. The amount of energy in the world will always be the same. The same goes for momentum. Momentum will be conserved. For example, if one car collides with another car, the amount of initial momentum will be the same as the amount of final momentum.

# Elastic and Inelastic Collisions

Inelastic Collisions occur when two objects collide and do not bounce away from each other. In an Inelastic Collision, Kinetic Energy is not conserved. Instead, the energy is converted into sound and heat waves. Inelastic objects have very little give and will become deformed in a collision. On the other hand, in an elastic collision, kinetic energy is conserved. Very little energy is converted to heat and sound. In an elastic collision, the objects will bounce off each other. Elastic objects have a lot of give and will usually "bounce back" to their original shapes.

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The Complete Idiot's Guide to Physics