Forces and Motion

Forces What is a Force? -A push or 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 __________________ Is it a vector? 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, etc., 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)

Applied Force Examples

Tension Force Examples

Normal Force Examples

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 STATIC SLIDING ROLLING FLUID is defined as & an example is The 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 attracted to one another Gravity is an attractive force  pulls objects together

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= 10.0 m/s2

Gravity is a force: natural phenomenon in which objects that have mass are attracted to one another Gravity is an attractive force  pulls objects together Earth’s gravity acts downward toward the center of the Earth. There is an upward force that balances gravity AIR RESISTANCE

Falling Objects have two forces acting on them: Gravity causes objects to accelerate downward air resistance acts in the direction opposite to the motion & reduces acceleration.

Free Fall: the motion of a body when only the force of gravity is acting on the body Free fall acceleration of an object is directed toward the center of Earth Because free fall acceleration results from gravity, its symbol is g Acceleration due to gravity on Earth = 9.8 m/s2 Formula for objects in free fall: d = ½ gt2

In a vacuum, two objects would accelerate at the same rate because both are in free fall (Fig 8, p.354) Question: What other force is not present in vacuum that would affect acceleration? Answer = air resistance

So why don’t we get sucked into the center of the earth? Concept REWIND So why don’t we get sucked into the center of the earth?

NORMAL FORCE Can you think of other forces that can oppose gravity?

Tension Force and Friction Forces Air resistance can also be called “drag”

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

Free fall with air resistance: (no friction)

Balanced Forces: Ex’s? Write one down! Fnet = 0 When the forces on an object are balanced, (the net force is zero) and there is no change in the object’s motion. Does this mean the object has NO VELOCITY? NO!!! Can be at rest OR moving at a constant velocity Ex’s? Write one down!

Results when net force acting on an object is NOT equal to zero Unbalanced Forces Fnet is not =0 Results when net force acting on an object is NOT equal to zero When an unbalanced force acts on an object, 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. A man sprinting to the finish line, accelerating at 2 m/s each second. Unbalanced= acceleration!

Drawing Free Body Diagrams Identify the types of forces acting on the object Use a dot or a box to represent the object Use arrows to represent the direction and size of the force -change the size of the arrow if more force is being applied in one direction versus the other Label the arrows with the type of force

Free Body Diagrams How will the object move in the diagram to the right? Answer: Object may be at rest and not move or it could be moving at a constant velocity since all the forces are balanced and the net force is zero

Free body diagram practice

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 (at rest) on the grass unless a force acts on it

What about an object in motion? An object in motion will also stay in motion unless a force acts on it. Why does a baseball stop after you’ve thrown it? Forces are acting on it: Friction force (air resistance) Force of gravity

“Science and the Consumer” Sometimes called the “Law of Inertia” 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 = F F= maa Mass 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 = 10 m/s2 W=mg a=F/m

Rewind Do you remember Inertia? How is this related to mass? How is that different than weight?

Mass is a measure of the inertia of an object; weight is a measure of the force of gravity acting on an object. 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

Newton’s Third Law of Motion & Momentum “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 reaction forces

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 Equal & opposite

Action-Reaction forces do not cancel each other out Example: Swimming  action-reaction forces do not act on the same object. Action= pushing backward on the water, Reaction = water pushing forward on the swimmer causes the swimmer to move because that force is overcoming the friction on the swimmer.

Action-reaction forces can produce motion, but not all do- it depends on what other forces are acting on the objects. Pushing a wall = 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)