4.2 Newton’s Laws of Motion

Sir Isaac Newton (1642-1727) Discovered laws of motion and gravity

Newton's Laws of Motion First Law: An object at rest stays at rest and an object in motion, stays in motion (same direction & same speed) unless acted upon by an unbalanced force Also called the Law of inertia

The amount of inertia an object has depends on its Mass—A Measure of Inertia Inertia is a property of matter that causes it to resist any change in its motion The amount of inertia an object has depends on its more mass = more the inertia more speed = more the inertia

Mass—A Measure of Inertia You can tell how much matter is in a can when you kick it. Kick an empty can and it moves. Kick a can filled with sand and it doesn’t move as much.

Mass Is Not Volume! Mass—A Measure of Inertia Do not confuse mass and volume. Volume is a measure of space and is measured in units such as cubic centimeters, cubic meters, and liters. Mass is measured in the fundamental unit of kilograms.

Mass Is Not Weight! Mass—A Measure of Inertia Mass is often confused with weight. Mass is a measure of the amount of material in an object. Weight, on the other hand, is a measure of the gravitational force acting on the object.

Mass—A Measure of Inertia It’s just as difficult to shake a stone in its weightless state in space as it is in its weighted state on Earth.

Weight = (mass) x (acceleration due to gravity) Formula: W = mg Mass—A Measure of Inertia Mass and weight are proportional to each other in a given place: In the same location, twice the mass weighs twice as much. Mass and weight are proportional to each other, but they are not equal to each other. Weight = (mass) x (acceleration due to gravity) Formula: W = mg

What is a Force? FORCE = Any push or pull acting on an object

The SI unit of force is the newton, the symbol for the newton is N. One newton is equal to slightly less than a quarter pound. If you know the mass of something in kilograms and want its weight in newtons at Earth’s surface, multiply the number of kilograms by 10.

One kilogram of nails weighs 10 newtons, which is equal to 2.2 pounds. Away from Earth’s surface, where the force of gravity is less, the bag of nails weighs less.

Newton's Laws of Motion Second Law: The acceleration of an object is proportional to the net outside force acting on the object Force = mass object  acceleration of object Formula: F = ma

The acceleration produced depends on the mass that is pushed.

Forces can be BALANCED or UNBALANCED Balanced forces are equal in size and opposite in direction Unbalanced forces are not equal in size and/or opposite in direction. If the forces on an object are unbalanced, we say there is a NET force.

The greater the force, the greater the acceleration The greater the mass, the greater the force needed for the same acceleration

Examples of Newton’s 2nd Law Bunting the baseball, versus a grand slam The positioning of football players - massive players on the line, lighter (faster to accelerate) players in the backfield Pushing a loaded versus an unloaded truck

Newton's Laws of Motion Third Law: For every force, there is always an equal and opposite reaction force.

Example of Newton’s 3rd Law

Identifying Action and Reaction Forces When action is A exerts force on B, the reaction is simply B exerts force on A.

Action and Reaction think! We know that Earth pulls on the moon. Does the moon also pull on Earth? If so, which pull is stronger?

Action and Reaction think! We know that Earth pulls on the moon. Does the moon also pull on Earth? If so, which pull is stronger? Answer: Asking which pull is stronger is like asking which distance is greater—between New York and San Francisco, or between San Francisco and New York. The distances either way are the same. It is the same with force pairs. Both Earth and moon pull on each other with equal and opposite forces.

Weightlessness – free from the effects of gravity?

There is gravity in space The faster you run from the tower the farther you go before falling to earth Using a rocket you could get enough sideways speed to keep missing the earth as you fall. There is gravity in space Weightlessness is due to a constant state of free-fall