 In the absence of external forces, an object at rest remains at rest and an object in motion remains in motion with a constant velocity.  This law can be best observed in space, far from the gravity of a star or planet, where there is no friction or air resistance. If, in the middle of deep space, you give a rock a little push, it will continue with the direction and velocity you gave it forever. The only way to stop it is to apply a force in the opposite direction.  This law is not intuitive because we are surrounded by air and gravity - if we give a rock a little push on the surface of the earth, it won't travel far.

 The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.  This boils down to force equals mass times acceleration, F = ma. This little equation turns out to be immensely useful, again and again. If you add together all the forces acting on an object, they equal the mass of the object (in kg) times the acceleration of the object (in m/sec 2 ).  Force is measured in newtons. One newton is the force required to accelerate a 1-kg mass to 1 m/sec 2.

 The force exerted by object 1 onto object 2 is equal in magnitude and opposite in direction to the force exerted by object 2 onto object 1.  This law appears to make little sense and can be difficult to grasp. If you push on a brick wall, the wall is pushing back on you with an equal force. If the wall was not pushing back, then your hand would be moving away from you. However, the wall is firmly attached to the ground, so it can match the force you are applying to it.

 Every single action has an equal and opposite reaction. If you were standing in the middle of a perfectly slick frozen lake, and needed to get off, you could throw an object away from you. When you applied force to the object you threw, it also applied an equal force pushing you back. You would travel in the opposite direction from the throw, although slower, because you probably have mass more than the object you threw.

 Friction is a key concept when you are attempting to understand car accidents.  The force of friction is a force that resists motion when two objects are in contact.  If you look at the surfaces of all objects, there are tiny bumps and ridges. Those microscopic peaks and valleys catch on one another when two objects are moving past each other.

 The level of friction that different materials exhibit is measured by the coefficient of friction.  The formula is µ = f / N, where µ is the coefficient of friction, f is the amount of force that resists motion, and N is the normal force.

 Normal force is the force at which one surface is being pushed into another. If a rock that weighs 50 newtons is lying on the ground, then the normal force is that 50 newtons of force.  The higher µ is, the more force resists motion if two objects are sliding past each other.

 There are two forms of friction, kinetic and static.  If you try to slide two objects past each other, a small amount of force will result in no motion. The force of friction is greater than the applied force. This is static friction.  If you apply a little more force, the object "breaks free" and slides, although you still need to apply force to keep the object sliding. This is kinetic friction. You do not need to apply quite as much force to keep the object sliding as you needed to originally break free of static friction.

 In some places, especially Alaska in the winter, you must keep friction in your mind constantly as you drive, in order to avoid an accident. You have to limit your speed in order to be able to stop at a reasonable distance, and to negotiate curves.  Braking distance can be calculated using the equation d = V 2 / 2g µ  Where: d = Braking Distance g = Acceleration due to gravity (9.80 m/sec 2 ) V = Initial vehicle speed (m/sec) µ = Coefficient of friction between the tires and the roadway

 Causes you to experience a slightly different set of forces, as you must deal with the tendency for a car to want to travel straight ahead.  This is explained by Newton's 1st law: an object will not change velocity without a force acting on it.

 In this case, you are causing the car to change lateral velocity and move to the side by applying frictional force from the tires. If the tires don't have a coefficient of friction large enough to provide the force needed to move the car laterally, then you slide straight forward and off the road.