Newton’s Laws of Motion

A Little History… Isaac Newton was one of the greatest scientists and mathematicians that ever lived. While Newton attended college he wrote his ideas in a journal. Newton had new ideas about motion, which he called his three laws of motion. He also had ideas about gravity, the diffraction of light, and forces. His accomplishments laid the foundations for modern science and revolutionized the world.

Newton’s 1 st Law (Law of Inertia) Law #1 Objects in motion tend to stay in motion, and objects at rest tend to stay at rest unless acted on by an unbalanced force.  On Earth, gravity and friction are unbalanced forces that often change an object’s motion.  An object will resist any change to its motion whether it is at rest or in motion. This resistance is called inertia. (Inertia- the tendency of an object to resist a change in motion.)  Examples  Clothes laying on the floor of your bedroom will remain there until you pick them up.  A tennis ball flies through the air once you hit it with a racket. If your friend doesn’t hit the ball back, the force of gravity and friction are unbalanced forces that often change an object’s motion.

Newton’s 2 nd Law Law #2 Acceleration depends on the object’s mass and on the net force acting on the object. Acceleration = Net Force/Mass or Force = Mass * Acceleration  This means that when the same amount of force is applied to an object with more mass and an object with less mass, the object with less mass will accelerate more quickly.  Everyone unconsciously knows the Second Law- We already know that heavier objects require more force to move the same distance as lighter objects.  Example: How can you increase the acceleration of the wagon?  One way is to increase the force. If the mass is constant, acceleration and force change in the same way. So to increase the acceleration of the wagon, you increase the force used to pull it.  Another way to increase acceleration is to change the mass. According to the equation, acceleration and mass change in opposite ways. If the force is constant, an increase in mass causes a decrease in acceleration.

Newton’s 3rd Law Law #3 For every action there is an equal and opposite reaction.  This means that if one object applies a force to a second object, the second object applies an equal force to the first object, but in the opposite direction.  The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force.

Common Misconceptions Students may believe that force is a property that an object has. For example, students may believe that when an object stops moving, it has run out of force. In fact, a force is an interaction between two objects. Students may believe that a force must be constantly applied to an object for that object to stay in motion. In fact, according to the law of inertia an object in motion will stay in motion unless acted on by an unbalanced force. This means that, barring other outside forces, an object that is moving can continue moving without the constant application of a force in the direction of motion. Many students believe that if you apply a constant force to an object, the object will move forward at constant speed. Applying a constant force to a mass will cause that mass to accelerate.

The Laws in Motion Newton’s Laws The Science of NHL