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.

Forces can cause 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, 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)

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: this friction depends on what two objects are in contact! (rough vs slippery surfaces) 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?

More on the force of Gravity: Gravity is an attractive force  pulls objects together (objects that have mass are attracted to one another)

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

WEIGHT is a FORCE that can be calculated… More on that later in the unit…

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 Free-body diagrams are used to show the relative magnitude and direction of all forces acting on an object.

This diagram shows four forces acting upon an object This diagram shows four forces acting upon an object. There aren’t always four forces, For example, there could be one, two, or three forces.

Problem 1 A book is at rest on a table top. Diagram the forces acting on the book.

Problem 1 In this diagram, there are normal and gravitational forces on the book.

Problem 2 A baseball is free-falling from a spot where it was stuck in a tree. Neglect air resistance. Draw a free-body diagram showing the forces involved.

Gravity is the only force acting on the ball as it falls.

Problem 3 Add to the drawing of the baseball falling and consider air resistance. A free body diagram for this situation looks like…

Gravity pulls down on the ball while air resistance opposes the ball’s acceleration.

Balanced Forces: Ex’s? Write one down! When the forces on an object are balanced, the net force is zero (Fnet = 0) 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! (Any object at rest or constant Velocity)

Unbalanced Forces Results when net force acting on an object is NOT equal to zero ( Fnet is not =0 ) 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!

Calculating Net Force and predicting motion… Remember: When forces are not balanced ___________________ occurs! (don’t confuse with velocity!!! It’s a CHANGE in velocity!!!) ACCELERATION!!!

Example 1 Notice the upward force of 1200 Neutons (N) is more than gravity (800 N). The net force is 400 N up.

Example 2 Notice that while the normal force and gravitation forces are balanced (each are 50 N) the force of friction results in unbalanced force on the horizontal axis. The net force is 20 N left.

Problem 1 A rightward force is applied to a book in order to start it moving across a desk. Consider frictional forces. Neglect air resistance. Construct a free-body diagram. Then describe the net force.

Note the applied force arrow pointing to the right Note the applied force arrow pointing to the right. Notice how friction force points in the opposite direction. Finally, there is still gravity and normal forces involved. Net force is NOT zero because the movement is CHANGING!! Net force is to the right!

Problem 2 A skydiver is descending with a constant velocity. Consider air resistance. Draw a free-body diagram. Then describe the net force.

Gravity pulls down on the skydiver, while air resistance pushes up as she falls. Net force is ZERO because there is NO ACCELERATION! (Velocity is constant)

Problem 3 A man drags a sled across loosely packed snow with a rightward acceleration. Draw a free-body diagram. Then describe the net force.

The rightward force arrow points to the right The rightward force arrow points to the right. Friction slows his progress and pulls in the opposite direction. Normal forces still apply as does gravitational force since we are on planet Earth. Net force is NOT ZERO because it is ACCELERATING! (V is not constant)

Another way to look at balanced and unbalanced forces

Balanced or unbalanced?

Balanced or Unbalanced?

EVALUATION Tell your partner the answers to these questions: What is force? What is net force? How is a free-body diagram used to understand forces acting on an object?

Evaluation Complete questions on the Free-body Diagram Worksheet.

NEWTON’S LAWS

Historical Development 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 and an object in motion will not change its motion 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? 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= kg A= m/s2 = N/kg Acceleration = Net Force a = F F=ma 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 (due to gravity) Weight = Mass x Acceleration due to gravity W = mg g = 9.8 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

PRACTICE After lab… See Math Skills p.350 Read p 350, do practice problems #1-3 on 351 If done with lab and above… Complete handout (scenarios A-D)

Net force= 0

Net force= 5N to the left

Net force = 0

Net force = 15 N upward

Newton’s Third Law of Motion “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 (Also called “interacting 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)

Extension- do not teach in 2014

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 initial forward velocity and the downward force of gravity causes the ball to follow a curved path. 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 velocity p=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