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UNIT 6: Newton’s Three Laws of Motion Objective: Students will investigate and describe applications of Sir Isaac Newton’s law of inertia, law of force and acceleration, and law of action-­‐reaction such as in vehicle restraints, sports activities, amusement park rides, Earth’s tectonic activities, and rocket launches. For an idea to become a law, it must hold true in all cases. Any object in motion can be described using all 3 of Newton’s laws of motion (all 3 laws are always in play), however usually one or 2 of them are more NOTICEABLE than the others in any situation with motion. LABEL, DEFINE AND DRAW EXAMPLES OF NEWTON’S 3 LAWS NEWTON’S 1ST LAW OF MOTION: “THE LAW OF INERTIA” -­‐ Law of Inertia: an object in motion stays in motion or an object at rest stays at rest until acted upon by an outside force. Inertia is the resistance to a change in an object’s motion. (INERTIA = RESISTANCE TO CHANGE) The more mass an object has, the greater its inertia! If the net force on an object is zero, the motion of the object does not change. The object’s motion changes only if an unbalanced force acts upon it. AKA: an object keeps doing whatever it was doing before. Examples: A book that is lying still on a table resists movement because of its inertia. Inertia explains many common events. For example, if you are in a car that stops suddenly, inertia causes you to continue to move forward. Crash-­‐test dummies don’t stop when a car does. Therefore, force is exerted (applied) by the safety belt. If the safety belt is not worn, the force may be exerted by the windshield instead. The amount of inertia depends on an object’s mass. Mass then, can also be defined as a measure of the inertia of an object. Other examples: Shaking a bottle of ketchup. When bringing the bottom down, the suddenly stopping it, inertia is what causes the ketchup to come out of the bottle. A hockey puck will continue to slide across ice until acted upon by an outside force. Astronauts in space find it more difficult to stop moving because of a lack of gravity (a force) acting against them. LABEL, DEFINE AND DRAW EXAMPLES EACH OF NEWTON’S 3 LAWS NEWTON’S 2ND LAW OF MOTION: “LAW OF FORCE & ACCELERATION” The Law of Force and Acceleration explains how force, mass and F=m acceleration are related to each other, and creates a mathematical Same Force relationship where force is equal to mass multiplied by acceleration A
(Force (Newtons)=mass (kilograms) x acceleration (typically meters/seconds2)). If the mass of an object remains constant (mass F= a stays the same), then acceleration increases as the force increases. When more mass is added to an object, its speed increases (aka acceleration). As mass increases, force increases. One Newton (N) equals the force required to acceleration one kilogram of mass at 1 meter per second per second. (This is depending on the mass of the object.) Examples: It is easier for a strong adult to push a full shopping cart than it is for a baby to push the same cart. (This is depending on the net force acting on the object.) Also, it is easier for a person to push an empty shopping cart than a full one. A baseball player hits a baseball, they are accelerating it to a high velocity. If the ball had a greater mass, the same amount of force would not result in as high of a velocity. LABEL, DEFINE AND DRAW EXAMPLES EACH OF NEWTON’S 3 LAWS NEWTON’S 3RD LAW OF MOTION: “THE LAW OF ACTION-­‐REACTION” The Law of Action/Reaction states that for every action there is an equal but opposite reaction. For every action force, there is a corresponding reaction force. All forces act in pairs. Whenever one object exerts a force on a second object, the second object opposes the action force, and exerts a force back on the first object. If one force is greater, a change in the motion of the object occurs. However, this is NOT a cause-­‐effect relationship, as the forces act simultaneously! Newton called the 1st force “action” and the 2nd force “reaction.” There are always 2 different objects. Examples: A hammer exerts a force on a nail, causing it to push into a piece of wood. The nail exerts a force back on the hammer, causing it to come to a sudden stop. A book lying still on a table has a downward force of gravity (action force) and is balanced by the upward force (reaction force) from the table. The pull (action force) is opposed by friction (reaction force). When you walk, you push the ground with your feet, and the ground pushes back equally. A bird flies by exerting a force on the air with it’s wings. The air pushes back with air resistance, causing the bird to propel forward/up. The exhaust gas flows through a rocket nozzle and is accelerated to the rear of the rocket. In re-­‐action, a thrusting force is produced on the rocket. The thrust then accelerates the rocket as described by Newton's 2nd law of motion. M