Physical Science Newton’s Laws And Forces

 GALILEO: Since he experimented to get EVIDENCE for his conclusions, he is considered to be the Since he experimented to get EVIDENCE for his conclusions, he is considered to be the father of experimental physics. father of experimental physics.

He also figured out that…

 Things in motion tend to STAY IN MOTION; they do NOT tend to SLOW DOWN OR STOP ON THEIR OWN.  In fact, Galileo figured out that it TAKES a force to make something slow down or stop. The usual force is (he discovered): FRICTION.

 So in fact, GALILEO figured out the principle behind what we call Newton’s 1 st Law

NEWTON’S 1 ST LAW  An object at rest tends to stay at rest, and an object in motion tends to stay in motion in a straight line at constant velocity unless acted on by an outside net force.

INERTIA refers to Newton’s 1 st Law  Inertia is the TENDENCY of ANY MASS to KEEP DOING what it is ALREADY DOING (resting, or going in a straight line at a constant speed).  A FORCE is a push or a pull, and INERTIA IS NOT A FORCE !!!  IT IS A TENDENCEY OR CHARACTERISTIC!

…INERTIA…  Inertia is a measure of an object’s RESISTANCE to CHANGE in state of motion. (Things—like people—don’t want to change!)  The word inertia comes from the Italian word for “laziness.”  Inertia is DIRECTLY PROPORTIONAL TO MASS ONLY. (More mass means more inertia.)  Inertia has nothing to do with size, speed, or anything else.

What is the difference between mass and weight? WEIGHT is the pull due to gravity. –Weight CHANGES due to location! –Weight is a FORCE! MASS is the amount of MATTER in an object. Mass does NOT change, regardless of location. Mass is measured in kg.

inertia…  If you are floating around in space outside the space shuttle, and go up to it and give it a kick, what will be the results? a)The space shuttle will go flying off, of course, since it has no weight! b) You will go flying off in the opposite direction with a hurt foot, of course, since the space shuttle has the same amount of mass (and inertia) in space as it does here on earth!

 WHAT IS VOLUME? –The amount of space something takes up  WHAT IS DENSITY? –The amount of mass per unit of volume D = m / V

 WHAT HAS MORE VOLUME: A kilogram of lead, or a kilogram of bread? –THE BREAD TAKES UP MORE SPACE, OF COURSE! WHAT HAS A HIGHER DENSITY: A kilogram of lead, or a kilogram of bread? –THE LEAD DOES—IT IS MORE COMPACT—it has more matter packed into a smaller space!

What is force?  A push or a pull.  A NET force is the leftover (or unbalanced) amount of force after you add up all the forces acting in a certain direction.

 What is ACCELERATION?  A CHANGE in motion! –That is ANY change in SPEED AND/OR DIRECTION!!!  Only NET forces cause accelerations!  No net force means NO CHANGE in speed or direction!

Newton’s 2 nd Law:  The BIGGER the amount of the force, the BIGGER the acceleration!  The BIGGER the mass, the LESS the acceleration!  In fancy terms  In fancy terms The acceleration of a body is directly proportional to the net force on it, and inversely proportional to the mass.

FORCE UNITS  Think of Newton’s 2 nd law: F = m a  N = kg m/s 2  Since mass is in kg, and a is in m/s/s, or m/s 2, F or Forces are measured in kg m/s 2.  That is the SAME as a Newton, or N.

F = ma 100N = 100kg X 1.00m/s 2 a = F/m 1.00m/s 2 = 100N / 100kg

…Newton’s 2 nd Law…  This makes sense…the more massive something is, the harder it is to accelerate. And the more force there is, the easier something will accelerate.  Zero net force means zero acceleration.

F = ma  A 2 kg mass accelerating at 3 m/s 2 has how much net force on it?

SOLUTION  F = ma = 2kg X 3 m/s 2 = 6 N

F = ma W = mg  If you weigh 150 pounds, your mass is about 68 kg. (1 kg = 2.2 lbs.) What is your WEIGHT in Newtons here on Earth?

SOLUTION  W = mg = 68kg X 10m/s 2 = 680 N  F = ma

F = ma a = F/m  A force of 16 N is pulling on a 2 kg wagon. If there is no friction, how fast is the wagon accelerating?

SOLUTION  a = F/m = 16N / 2kg = 8 m/s 2

F = ma a = F/m  If the wagon above is going at a CONSTANT VELOCITY with the 16N pull, how much friction is there?

SOLUTION  16 N ! If going constant velocity, acceleration is 0, & net force is 0 so all forces are BALANCED (equal) in any 1 direction. in any 1 direction.

Newton’s 3 rd Law:  Every action has an equal and opposite reaction.  There are NO single forces. They are always an action/reaction pair.”

 For example, you touch the table with 1N of Force, it touches you back with 1N of Force SIMULTANEOUSLY (not a little later!).

 Newton’s 3 rd Law says that an action reaction pair would be hands pushing on barbell, barbell pushing on hands.  Then HOW can the barbell be ACCELERATED upward?  What OTHER action reaction pairs are involved?

More on Newton’s 3 rd …  If you smash the wall with your fist with 80 N of force, the wall will hit you right back with 80 N.  But what if you hold a piece of paper in the air and try to hit it with 80 N?

If the paper …  cannot support or hit you back with 80 N, can YOU really hit IT with 80 N?  If the paper can only hit you back with 5 N, what happens to the other 75 N from your punch??! HINT:

How do they accelerate?

Force of friction  Opposes motion  Occurs between surfaces in contact

Gravitational force…  Is the WEAKEST of all the forces, but acts over the GREATEST distances! You are pulling and pulled by EVERY ATOM IN THE UNIVERSE!!!

Electromagnetic forces…  Are the forces that act due to electron configuration in atoms  These forces are responsible for the PROPERTIES or characteristics of elements, compounds, and materials

 This force makes paint stick to a wall, makes diamonds extremely hard, makes gold malleable, rubber stretchy, water a bipolar molecule, etc.