Sir Isaac Newton IPC: Newton’s Laws Forces Newton’s Laws

Sir Isaac Newton Newton discovered the relationship between an object’s mass, acceleration, and the forces acting upon it. Before the age of 30, he formulated the laws of mechanics discovered the law of universal gravitation invented calculus. Newton lived from 1642-1727; the same time that the first colonists were trying to settle the American Colonies. He was a professor at Cambridge University, and was a bit of a recluse. He locked himself in his office for nearly a year while writing his book describing the laws of mechanics, letting himself out only to get the meals being placed outside his office door. Before digging into Newton’s Laws of Motion, let's talk about some of the concepts that you will need to better understand Newton’s discoveries.

Force A FORCE is a push or a pull, or any action that has the ability to change motion. A force has the ability to cause motion but doesn’t always cause motion--if you don’t push hard enough on a car sitting still, it won’t move Metric unit of force is the Newton. One Newton, abbreviated N, is 1 kg•m/s2 .

Other important units Metric unit of mass is the kilogram. English unit of force is the pound. We tend to use mass and weight interchangeably in our measurement system, but mass and weight are NOT the same thing.

Mass vs Weight Your mass is the amount of matter in an object. An object’s mass is the same no matter where the object is in the universe.   Mass resists the action of forces by making objects harder to accelerate. The weight of an object is the result of the gravitational force pulling on an object’s mass. In different gravitational fields, such as the moon or the Earth, an object’s weight will be different.

Gravity Gravity is a force that pulls objects towards each other There is a gravitational attraction between every two objects in the universe You are most familiar with Earth’s gravity The acceleration due to gravity (on Earth) is 9.8 m/s2 Mars and the moon both have gravitational fields smaller than Earth g=9.8 m/s2 Jupiter has more gravity than Earth

Calculating Weight Weight is a force measurement Gravity is different on earth and on mars so the same mass has a different weight on each planet Weight is a force measurement Weight is the force exerted by gravity on an object’s mass Formula for calculating weight (on Earth) is: mass Acceleration due to gravity weight

Inertia INERTIA is defined as the property of an object to resist changing its state of motion. Mass is a measure of the inertia of an object, because the greater the mass of an object, the greater the inertia of the object.

Newton’s First Law of Motion “An object at rest will remain at rest unless acted on by an unbalanced force. An object in motion will continue with constant speed and direction, unless acted on by an unbalanced force.” Referred to as the Law of Inertia, because it describes the property of inertia. Your personal experience of this: you are in a car and you hit the brakes--your body tries to keep moving forward.   The greater the mass of an object, the greater it’s inertia.

Newton’s Second Law The acceleration of an object is directly proportional to the force acting on it and inversely proportional to its mass. When F, a (if mass remains constant); when F remains constant, if a, then m Mathematically, we write this law as F = ma Force causes an object to accelerate, while the object’s mass resists acceleration. Acceleration is equal to the ratio of force over mass. Changes in speed or direction all involve acceleration. Force causes acceleration, this is how we create changes in motion

Why do all objects with negligible air resistance accelerate toward the Earth at the same rate? How can Newton’s 2nd Law be used to explain how air resistance affects the acceleration of an object in free fall? How does Newton’s 2nd Law relate to what a weight scale would read as you move up and down on an elevator?

Newton’s Third Law The force & reaction force are a “force pair”, and they act on a single object

You Try It!!! How do Newton’s Laws of motion apply to these situations? an object rests in your hand a ball is tossed upward a car windshield hits a bug a person sits on a table a person jumps up from the floor a baseball bat hits a baseball a truck and car hit head-on Others?

Net Force (F) The motion of an object depends upon the total of all forces acting on the object. + - The total of all forces is the NET FORCE.   Forces that push or pull in opposite directions have opposite signs. Imagine that the object that has forces acting upon it is at the 0 place in the center of a four quadrant grid. Forces that push or pull the object to the right are positive Forces that push or pull the object to the left are negative. Forces that push or pull the object below the x axis are negative Forces that push or pull the object above the x axis are positive.  

The net force acting on an object is the vector sum of all the forces acting on it. Examples: 6 lb 5 lb 9 lb 4 lb ? 3 lb 8 lb 4 lb 7 lb 8 lb 12 lb 7 lb 4 lb 8 lb If an object is remaining at rest, it is incorrect to assume that there are no forces acting on the object. We can only conclude that the net force on the object is zero.

Equilibrium When the net forces on an object cancel each other out (they are balanced), the object is in equilibrium. When two teams playing tug of war are not moving the rope one direction or the other, they are pulling equally, nobody moves at all--equilibrium When an object is in equilibrium, the net force on the object is equal to zero. 5N 5N 5N 5N

Friction Friction is a force that occurs between two objects moving across each other The surface of any object is not perfectly smooth; the microscopic hills and valleys cause objects to resist sliding across each other Friction is always opposite the direction of motion (opposite the applied force) Friction in engines and other machines causes wear, damage due to friction interfering with part movement

Friction and Motion Friction opposes motion, so when friction is present in a system, we get speed and other motion measurements that are less than we predict Friction is subtracted from the force causing motion

Types of Friction Static friction Kinetic friction Air resistance between two objects that prevents them from sliding across each other Kinetic friction two objects sliding across each other Air resistance opposing force caused by air moving around an object Viscosity the resistance of liquids to flow