# Forces and Motion.

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Forces and Motion

Benchmarks Standard III - Physical Science Benchmarks
D. Explain the movements of objects by applying Newton’s three laws of motion. E. Trace the historical development of scientific theories and ideas, and describe emerging issues in the study of physical science.

Force Force - a push or a pull exerted on an object. Units are Newtons (N) Ex: gravity pulls objects to the Earth Forces can… 1. Speed up an object 2. Slow down or stop an object 3. Change the direction of an object (cause an object to travel in a circle Ex: satellites)

Force Vectors Forces have both a magnitude (amount) and a direction, this is called a force vector Forces can add together or they can cancel eachother out

Fundamental Forces 1. Gravitational Forces - the mass of an object attracts the mass of another object. Usually the weakest of all forces, but applies to long distances Ex: Weight on Earth, centripetal force 2. Electromagnetic Forces - a charged object attracts or repels another charged object. These forces are stronger. Ex: Friction, electrons, most regular forces 3. Nuclear Forces - two types: strong and weak forces. Occur in the nucleus of a atom and are the strongest forces. Ex: nuclear power and weapons

The Force of Gravity The Law of Universal Gravitation Fgravity = Gm1m2
d2 Fgravity = force of gravity (N) m1 = mass of 1st object (kg) m2 = mass of 2nd object (kg) G = gravitational constant (look up) d = distance between object (m)

Gravity Continued *The force of gravity is directly proportional to the masses of both object, so if either object has more mass the gravity goes up *The force of gravity is indirectly proportional to the distance squared between the two objects, so if the distance increases the gravity decreases *Question: Which effects gravity more the masses of the objects or the distance between them?

Friction Friction - force that resists motion, units are Newtons (N)
Friction increases with the roughness of the surface called mu (µ) It also increases with the weight of the object

Friction Friction increases with the velocity of the object when traveling through liquids and gases. Ex: skydiving, water skiing During energy transformations a great deal of the energy is lost to friction in the form of heat energy. Ex: Much of the chemical energy in the gasoline in your car is lost to friction (heat) instead of mechanical energy to make the car move

Types of Friction There are four main types of friction:
Sliding friction: ice skating Rolling friction: bowling Fluid friction (air or liquid): air or water resistance Static friction: initial friction when moving an object

Reducing Friction Friction can be a good thing (helps us walk) but it can also be a bad thing (reduces effeciency of engines) Four ways to reduce friction… 1. Lubricating between surfaces 2. Using ball bearing 3. Polishing surfaces 4. Using wheels

Motion

Motion Motion - the change in position of an object relative to another object that is assumed to be at rest. Ex: car drives past a sign Uniform motion - objects travels same speed in a straight line

Speed and Velocity Speed - distance traveled per unit of time, units are meters per second (m/s) Speed (s) = distance (d) / time (t) s = d/t Velocity - speed with a direction (same formula, same units as speed) v = d/t

Speed and Velocity If a car traveled 400km West in 4hr, what is the speed and velocity of the car? s = d/t = 400km/4hr = 100km/hr v = d/t = 400km/4hr = 100km/hr West *Velocity is more discriptive then speed

Acceleration Acceleration is the rate of change in velocity, units are m/s2 Acceleration = final velocity - intial velocity time a = vf - vI t *When an object slows down, it has a negative acceleration, this is called deccelartion.

Falling Objects Falling formulas: v = gt d = gt2 2 V = velocity (m/s)
g = acceleration of gravity (9.8m/s2) t = time (s) d = gt2 2 d = distance (m) *All objects fall at the same rate (g) unless they are slowed by air drag

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