# An Explanation of Newtons three Laws of Motion

Newton’s three laws of motion are: the law of inertia, the relationship of acceleration and force and inertia, and the law of action and reaction.

The law of inertia 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. (from Newton’s Principia) This is usually translated as an object at rest remains at rest and an object in motion remains in motion at a constant speed on a straight line path. Inertia is the tendency of objects to resist movement. The inertia of a penny is demonstrated by placing a penny on an index card over the mouth of a drinking glass. When you flick the card away, the penny drops into the drinking glass. Another example is if you are standing on a bus and the bus driver slams on the brakes, your body continues to move forward until you fall down in the aisle because of your body’s inertia.

Newton’s second law of motion says the acceleration of a body is directly proportional to the net forces acting on the body and inversely proportional to the mass of the body and is in the direction of the net force. (from Newton’s Principia) Newton’s second law of motion is generally explained through the equation: acceleration = force divided by mass or an object will accelerate when a force is impressed upon it. If we put the same engine into a Hummer and a Hyundai, we would expect the smaller and lighter vehicle, the Hyundai to accelerate faster than the larger and heavier vehicle, the Hummer because the same amount of force is applied to each. Anyone who has ever pushed a stalled car knows that it is hard work to get the car moving, but once the car is moving, it rolls at constant speed. It takes more fuel or energy to get the car started than it does to keep it moving. That is why we get better mileage on the highway than when driving in the city.

Newton’s law of action and reaction, the third law of motion says whenever one body exerts a force on a second body, the second body exerts an equal and opposite force on the first. (from Newton’s Principia) In a game of tug-of-war, the action is team members pulling on the rope and the reaction is the rope pulling on the team members. The team that wins tug-of-war is not the team that pulls the hardest on the rope, but the team that pushes hardest against the ground. A fish moves water in a pond backwards with its fins, and the water then pushes the fish forward. The gases escaping from the end of the rocket launch the rocket into space. Newton’s Laws of motion form the basis of NASA’s space program.