Forces and Laws of Motion

WHAT IS A FORCE? A force, when exerted on an object, may change the object’s state of rest or state of motion.

Forces can change motion But How? forces start motion forces stop motion forces change direction

Forces Often Make Contact Example:

Air Resistance

(Twisting or turning) Spring Force Tensional Force torque

Force - Pretty much any push or pull exerted on an object

Forces that don’t require contact Gravity Magnetism inside atomic nuclei

The Normal Force The "normal" force is the force applied by a surface in a direction perpendicular to the surface.   It's called "normal" because, in mathematics, "normal" means "perpendicular."   The normal force only exists when a surface is in contact with an object. -Jacob’s Physics website Physics text resting on the table

Balanced forces – Equilibrium Balanced forces are equal in size and direction. The book and table are motionless relative to the floor because all forces “cancel each other out.”

The all forces exerted on the bus are balanced so that the bus does not move in the apparatus below see in Jacob’s Physics website Mass and pully Mass and pully

Newton – A unit of force The N represents the standard SI unit of force. It is named for Sir Isaac Newton. 1 N = The amount of force needed to accelerate a 1 kg mass . 1m/s2

Free Body Diagrams A free body diagram can be used to show forces action on an object. The object below is in equilibrium because all forces acting on the object are equal. Force diagrams just show the forces acting in any situation (Usually just vector arrows)

Net Force If a force in one direction is not balanced by a force in the opposite direction, there is a net or resultant force acting on the object. A net force always changes the velocity of an object. It accelerates the object in the direction of the greater force.

Pg 120 Serwey/Faughn Physics- Holt

FIRST LAW OF MOTION A body moving with constant velocity will keep moving (in a straight line) at that velocity unless a net or resultant force acts on that body. A body at rest, will stay at rest unless a net or resultant force acts on that body.

According to Newton’s first law, a body naturally tends to continue in whatever state of motion or rest it has at any instant, and tends to resist any changes.

The first law is often called the Law of Inertia Inertia is the tendency of an object to resist any change in its motion. It is related to mass because the greater the mass the greater an object’s inertia.

Seatbelts are good things to have in cars because without them, even when the car stops suddenly, your inertia maintains your movement. Newton’s first law makes seatbelts a necessity!

Second Law of Motion The Law of Force and Acceleration (or The Law of Constant Acceleration) A net or resultant force (unbalanced force), acting on a body, causes that body to accelerate in the direction of the force.

Relationship between force and mass according the 2nd law a) The larger the force, the large the acceleration.

Relationship between force and mass according the the 2nd law b) The larger the mass, the less the body will be accelerated by the force A much greater acceleration can be imparted to the basketball than the car if using the same force. This is due to the fact that the car has a much greater amount of mass than the ball. – Connections: OpenStax College online

Calculations with the Second Law of Motion

Third Law of Motion The Law of Action & Reaction When an object exerts a force on a second object, the second object exerts a force on the first that is equal in size and in the opposite direction. or To every action, there is an equal and opposite reaction force.

If a book is resting on a table, the weight of the book pushing down on the table, while the table pushed up on the book cancel each other out. If a horse pulls a cart, there are other forces at work besides the horse and cart. The hose is also exerting a force on the ground, while the ground exerts a force on the horse & cart, causing them to accelerate forward.

Some thought that rockets would not work in space because they reasoned that the exhaust gases needed to push against the air of the atmosphere. Actually the rocket pushes against the gases, as the gases push against the rocket.