ForcesandNewton’s Laws of Motion ForcesandNewton’s

Causes of Motion AristotleAristotle ( BC) believed that all objects had a “natural place” and that the tendency of an object was to reside in its “natural place.” Aristotle All objects were classified into categories of earth, water, air, or fire. “Natural motion” occurred when an object sought to return to its “natural place” after being moved from it by some type of “violent motion.” The natural state of an object was to be “at rest” in its “natural place.” To keep an object moving would require a force.

These views remained widely supported until the 1500s when Galileo Galilei ( ) Galileo Galilei Galileo Galilei popularized experimentation. Isaac Newton Isaac Newton (1642–1727) Isaac Newton proposed that the tendency of an object was to maintain its current state of motion.

INERTIA the tendency of an object to resist any change in its motion Inertia is a property of matter and does not depend on the position or location of the object. MASS a quantitative measure of inertia FORCE “a push or pull” “a push or pull”

“Law of Inertia” An object at rest remains at rest, and an object in motion continues in motion at a constant speed in a straight line, unless acted on by a nonzero net force. * OR * The velocity of an object remains constant unless acted on by an unbalanced force.

The net force acting on an object is the vector sum of all the forces acting on it. Examples: 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. ? 8 lb 12 lb 3 lb 8 lb 7 lb 4 lb 6 lb 5 lb 9 lb 4 lb 7 lb 8 lb

EQUILIBRIUM the condition of an object such that it is not accelerating The first condition for equilibrium is that there must be no net force acting on the object. All forces must be balanced. F 0  F = 0 F x 0F y 0  F x = 0 and  F y = 0 F left F right F up F down  F left =  F right  and  F up =  F down

“Law of Acceleration” A net force causes an object to accelerate in the direction of the net force. The acceleration is directly proportional to the net force and inversely proportional to the object’s mass.

a = F net m =ma The SI unit of force is the Newton, named in honor of Isaac Newton. One Newton of force is the amount of force needed to cause a one kilogram mass to accelerate at a rate of 1 m/s 2.

WEIGHT a measure of the gravitational force that a massive object, such as a star or planet, puts on another mass F = ma F = ma weight = mass x acceleration of gravity W = mg An object’s weight on planet Earth inNewtons An object’s weight on planet Earth in Newtons is equal to itsmass in kilogramstimes9.8 m/s 2 is equal to its mass in kilograms times 9.8 m/s 2.

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

FRICTION the force that opposes the motion between two surfaces that are in contact Characteristics 1. Friction acts parallel to the surfaces in contact and in the direction opposite to the motion of and in the direction opposite to the motion of the object or to the force tending to produce the object or to the force tending to produce such motion. such motion. 2. Friction depends on the nature of the materials in contact and the smoothness of their surfaces. in contact and the smoothness of their surfaces. 3. Sliding friction is less than or equal to starting friction. starting friction.

4. Friction is virtually independent of surface area. 5. The friction force is directly proportional to the force pressing the two surfaces to the force pressing the two surfaces together (the normal force). together (the normal force). The ratio of the friction force, F f, to the normal force, F n, is called the coefficient of friction, .   = FfFfFfFf FnFnFnFn  has no units.

Explore forces and friction using this free simulation. simulation

“Law of Interaction” Short Version “For every action there is an equal and opposite reaction.” Longer Version When one object exerts a force on a second object, the second exerts a force on the first that is equal in magnitude, but opposite in direction.

Newton’s Third Law deals with acting on objects. two forces two different Newton’s Third Law pairs of forces always, sometimes, never cancel each other out. The only way for two forces to cancel each other out is for them to be equal and opposite and act on the same object!

Learn more about Newton’s Laws of Motion at these links: link1link1, link2, link3, link4 link2link3link4 link1link2link3link4

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