Chapter 3 Forces & Motion

Newton’s 1 st Law of Motion An object at rest will remain at rest unless acted upon by an unbalanced force. An object in motion will continue with constant speed and direction, unless acted upon by an unbalanced force. Unless you apply a force, things tend to keep doing what they were already doing.

Newton’s 2 nd Law of Motion The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. Forces cause an object to accelerate, while the object’s mass resists acceleration.

Newton’s 2 nd Law Equation Unbalanced forces cause a change in motion. They will speed things up, slow them down, or change their direction. Forces that don’t add up to be equal in size AND opposite in direction are unbalanced. a= FmFm

Newton’s 2 nd Law Equation a= FmFm F = m * a

Newton’s 3 rd Law of Motion Whenever one object exerts a force on another, the second object exerts an equal and opposite force on the first. For every action, there is an equal and opposite reaction.

Inertia The property of an object to resist changing its state of motion. Inertia is measured by the mass of an object. More mass means more inertia.

Force A push or a pull Any action that has the ability to change motion. Units –Newtons in the metric system –Pounds in the English standard –One pound equals 4.48 Newtons

Mass vs. Weight Mass: the amount of matter in an object. Weight: the response of an object (mass) to gravity. Mass is constant; weight varies as gravity changes. –The mass of your text book will be the same on the moon as on earth. It will weigh more on earth than the moon because earth’s gravitational pull is more than the moon’s.

Mass, Weight & Volume Why do farmer’s sell milk by the pound and not in gallons? Why do aircraft measure their fuel in pounds instead of gallons?

Gravity An attractive force that pulls every mass toward every other mass. Applying Newton’s 2 nd Law to weight: F w = m*g F w = weight of an object m = mass of the object g = acceleration of gravity (9.8 m/s/s on earth)

Law of Universal Gravitation The force of attraction between two objects is directly related to the masses of the objects and inversely related to the distance between them. You will not have to do any calculations using the formula given in the book.

Friction The resistance to motion. Always work against the motion of an object. Two types of friction –Static friction: the friction involved before something begins to move –Kinetic friction: the friction involved when an object is in motion (air, sliding, viscous, rolling)

Examples of Friction Air Friction (air resistance): the air is slow to move out of the way of an object moving through it. Sliding Friction: two surfaces rub against each other caused by irregularities in the surfaces in contact. (i.e. skiing) Viscous Friction: objects moving through a fluid. (i.e. oil, grease, water) Rolling Friction: one object rolls over another. (i.e. ball bearings, wheel)

Net Force & Equilibrium Net force: the sum of all the forces acting on an object. Equilibrium: When the net force is zero. There is no change in motion. These forces are said to be balanced.

Friction & Motion Friction will cause an object to come to rest if no other force balances it out. All machines have friction. A continual force must be applied to keep an object in motion.

Action & Reaction Forces always come in pairs. –Action & Reaction Action and reaction forces act on opposing objects. –If you push on the ground, the ground pushes back on you. –The reaction force (in this case) makes you move because it is an unbalanced force on you. –These forces don’t balance out because they are on two different objects.

Momentum Mass of an object multiplied by its velocity. Law of Conservation of Momentum: As long as a system is not influenced by outside forces [like friction], the total amount of momentum of the system cannot change.