Forces What is a Force? -A pushor pull on an object -UNITS: NEWTONS (N)
What is a Newton? • 1 Newton will cause a 1 kg object to experience 1 m/s2 of acceleration.
Force causes changes in ___________ VELOCITY! ACCELERATION! • Therefore, it causes __________________ • Because force deals with VELOCITY- it has magnitude and direction • Therefore force is a vector
Force cont. A force can cause… • a resting object to move, or … • A moving object to accelerate • How? • By changing the object’s speed or direction.
Types of Forces- • Applied – contact force in the direction the object is moving (Fa) • Tension- caused by a rope, cable, ect., directed away from the object (Ft) • Normal- caused by a surface (Fn) • Friction- opposes the motion of objects, must have contact (Ff) • Force of gravity- force at a distance, caused by attraction between two objects (Fg)
Friction is a force:Friction is a force that opposes motion between two surfaces that are in contact There are two main types of friction: Static- between stationary objects (“static”= “stationary”) Ex- the force that is keeping this block from sliding downhill Kinetic- friction of movement NOTICE: three types (on chart): Sliding,Rolling,Fluid Complete the concept map for the 4 types of friction! Use your textbook (pages 332-334)
FRICTION is defined as is defined as is defined as is defined as & an example is & an example is & an example is & an example is STATIC SLIDING ROLLING FLUID Thre friction between surfaces that are stationary force that exists when objects slide past each other force that exists when a round object rolls over a flat surface (usually less than sliding friction) force that exists when an object moves through a fluid (air, water) a book sitting on a table -hockey puck on ice -child going down a slide -a sled down hill -a roller blade on a sidewalk -bowling ball on bowling alley -a car driving down the road -swimmer swimming through pool
How can we decrease friction? Watch this demo and see… http://www.scottso.net/examples_encfri.htm Can you think of a situation in which you would want to increase friction?
The force of Gravity:objects that have mass are attractedto one another • Gravity is an attractive force pulls objects together • Earth’s gravity acts toward the center of the Earth.
WEIGHT: equal to Force of GRAVITY (Fg) • Not the same as MASS!!! • DO NOT CONFUSE THE TWO “GRAVITIES!” • FORCE of Gravity= FORCE in NEWTONS • (an object’s mass x the acceleration due to gravity) • ACCELERATION due to Gravity= 9.8 m/s2
Concept REWIND • So why don’t we get sucked into the center of the earth?
NORMAL FORCE • That upward force that balances gravity when you are standing on a surface • Can you think of other forces that can oppose gravity?
Tension and Friction • Tension force can also oppose gravity • Air resistance is a type of frictional force: • (also called “Drag”)
AIR RESISTANCE: • acts in the direction oppositetothemotion & reduces acceleration.
Forces can be COMBINED: Net Force: Fnet: the combination of all of the forces acting on the object • Use Free Body Diagrams to show the net force: • Remember the Units? • NEWTONS
Free body diagrams: • Book on desk
Balanced Forces: • Fnet = 0 (the net force is zero) • and there is nochange in the object’s motion. • Can be at rest OR moving at a constant velocity Example: Tug of war, Pushing piano
Unbalanced Forces • Fnet is NOTequaltozero • The object accelerates in the direction of the net force (can cause it to speed up OR slow down) • Fig 14, p.332 http://www.darvill.clara.net/enforcemot/friction.htm
Decide if the following objects have balanced or unbalanced net forces. • An airplane is flying at 150 km per hour for 30 minutes. • Balanced= no acceleration! • A book sitting on a table. • Balanced= no acceleration! • A man sprinting to the finish line, accelerating at 2 m/s each second. • Unbalanced= acceleration!
A) Historical Development 1) Aristotle (384 BC- 322 BC): Incorrectly proposed that force is required to keep an object moving
Newton’s Laws of Motion Newton(1643 – 1727): Defined mass and force; Introduced 3 Laws of Motion
Newton’s 1st Law of Motion The motion of an object does not change if its net force is zero. • Basically -an object at rest will stay at rest unless a force acts on it • Example: Soccer ball will remain (atrest) on the grass unless a forceacts on it
Sometimes called the “LawofInertia” a) Inertia: the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction. 1) Car crash: You continue forward because of inertia “Science and the Consumer” p.348
How is inertia related to mass?P 347 • Mass is a measure of inertia. • Who would you rather be tackled by…a toddler or a defensive lineman? • What is easier to move? An empty garbage can or a garbage can full of lead? Why? • The empty garbage can has less mass= less inertia= less resistance to being accelerated.
Newton’s 2nd Law of Motion “the acceleration of an object is equal to the net force acting on it divided by the object’s mass.” Units: F= N M= g A= m/s2 = N/kg • See Math Skills p.350 Acceleration = Net Force a = FF=ma Mass m
W=mg a=F/m Weight is a result of F=ma Mass: the amount of matter in an object Weight: measure of the gravitational force • Units for weight = N • Basically Newton’s 2nd Law W is Force g is Acceleration Weight = Mass x Acceleration due to gravity W = mg • g = 9.8 m/s2
Rewind • Do you remember Inertia? How is this related to mass? • How is that different than weight?
Your weight on Earth is affected by Earth’s gravity You would weigh less on the Moon b/c gravity on Moon is less You would weigh almost nothing in interstellar space Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object.
Newton’s Third Law of Motion & Momentum Newton’s Third Law “when one object exerts a force on a 2nd object, the 2nd object exerts an equal and opposite force on the first object.” Two forces are called action and reactionforces
Equal & opposite Action and Reaction Forces: Example: Pushing your hand against a wall 1) Hand pressing on wall = ACTION 2) Wall exerting a force on hand = REACTION
Action-Reaction forces do not cancel each other out Example: Swimming action-reaction forces do notact on the same object. • Action= pushing backward onthewater, • Reaction = water pushing forward ontheswimmer • causes the swimmer to move because that force is overcoming the friction ontheswimmer.
Action-reaction forces can produce motion, but not all do- • it depends on what other forces are acting on the objects. Pushingawall = no motion (because the wall has other forces acting on it…gravity, etc) Swimming = motion (because force of water is greater than friction on the swimmer)
Rewind • State in simple terms Newton’s Three Laws. (in a way you can remember which is which) • 1st Law of Inertia (An object at rest will stay at rest…) • 2nd- F=MA • 3rd Action-Reaction (explains conservation of momentum)
Newton’s Laws explain Projectile Motion: • Projectile motion= the curved path that an object follows when thrown, launched, or otherwise projected near the surface of the Earth [Fig 13, p.358]
What FORCES are affecting the acceleration? (A=F/M) • Force of the “throw”, • Air resistance and • gravity Water Balloon Toss- • http://library.thinkquest.org/2779/
How does this affect the acceleration? • The combination of an initialforwardvelocity and the downwardforce of gravity causes the ball to follow a curvedpath. • Animation: http://www.physicsclassroom.com/mmedia/vectors/hlp.cfm
Another Law • Law of conservation of Momentum
Momentum: the product of the mass & velocity of an object An object has a large momentum if the product of its mass and velocity is large. SI Units: kg x m/s Momentum=mass x velocityp=mv
Law of Conservation of Momentum: • momentum cannot be created or destroyed, so the total momentum is conserved [Fig 20, p.364] • In a closed system, the loss of momentum of one object equals the gain in momentum of another object—momentum is conserved.
Usually that’s not the whole story • There’s one more concept to explain the acceleration of an object
Impulse Impulse is related to momentum and force Definition: Change in Momentum over time SI Units: N(s) Impulse=Force x Time I=Ft
The fact that impulse depends on both force and time means that there is more than one way to apply a large impulse to an object