BellringerBellringer

EXPLAIN IN COMPLETE SENTENCES

WHAT IS ACCELERATION

ObjectivesObjectives Describe examples of force and identify appropriate

SI units used to measure force Explain how the motion of an object is affected

when balanced and unbalanced forces act on it Compare and contrast the four kinds of friction Describe how Earth’s gravity and air resistance

affect falling objects

ForceForce Definition

- push or pull that acts on an object

- can accelerate or decelerate on object

ex. kicking a soccer ball at rest

wind pushing against you as you walk Measured

- units: Newtons (N) = 1kg m/s2

Represented

- force vectors: arrow = direction

length = strength or magnitude

Forces Cont.Forces Cont. Combining Forces

- forces acting in the same directions: add 3N + 5N = 8 N

- force in opposite directions: subtract 6N – 2N = 4 N Net Force

- the overall force acting on an object after all the forces are combined

Two Types- balanced- unbalanced

Balance vs. Unbalanced ForcesBalance vs. Unbalanced Forces Balanced forces

- forces that combine to produce no net force

ex. tug of war with no winner

2 N - 2 N = 0

- no net force, no movement Unbalanced forces

- force that results when the net force acting on an object is not equal to 0

ex. 2 N - 4 N = 2 N

- object will move in direction of net force

Practice ProblemsPractice Problems You push a car with a force of 50 N, your friends

pulls with a force of 25 N. Draw force diagrams, and calculate the net force acting upon the car. Will the car move?

50N + 25N = 75 N You push a box towards your friends with a force of

80N while one friend pushes the box against you with a 55 N, the other 25N. Draw force diagrams and calculate the the net force acting upon the car. Will the box move?

80 N - 55 N + 25 = 0

Common ForcesCommon Forces

Two common forces

- gravity: force that acts between any two masses

- attractive force that can act over large distance

- Earth’s gravity acts downward toward the center of Earth = 9.8 m/s/s = 10m/s/s

ex: Earth’s gravity holds you on the ground

Common Forces Cont.Common Forces Cont.

Friction- force that opposes the motion of objects that touch as they move past one another

- four types: static, sliding, rolling, fluid - static: acts on objects that are not moving

ex. - sliding: force that opposes the direction of motion of an object as it slides over

a surfaceex.

Common Forces Cont.Common Forces Cont.

- rolling friction: force that acts on rolling objects

ex. soccer ball rolling across the floor

- fluid friction: force that opposes the

motion of an object through fluid

* any mixture of gases is considered a fluid

ex. airplane flying through the air

Force DiagramsForce Diagrams

A diagram that identifies all force acting upon on object

1. Identify the situation of the object

ex. Static

2. Identify the force acting upon the object

ex. Gravity, static friction

3. Draw the forces acting upon the object

ex. Fn forces cancel out

Fg net force = 0

Practice ProblemsPractice Problems

An object is decelerating due to friction

An object is static

An object is hanging from the ceiling

ObjectivesObjectives

Compare and contrast Aristotle Galileo’s and Newton’s ideas

Define inertia Explain Newton’s 1st law of motion, and apply

them to physical situations

HomeworkHomework

DESCRIBE THE FORCES ACTING ON YOU

WHEN YOU SIT ON A CHAIR

CALCULATE IT!

WHAT FORCES ACTING ON YOU WHEN YOU

MOVE WITH CONSTANT SPEED?

Galileo Vs. Newton: InertiaGalileo Vs. Newton: Inertia

Galileo- introduced the idea of inertia- maintained that motion of an object requires a force

Newton- grasped the significance of inertia- motion of an object requires and initial force, not continual- law of inertia defines natural motion and tells us what kinds of motion are the result of applied forces

NewtonNewton’’s First Law of Motions First Law of Motion

- Newton redefined Galileo’s idea of inertia

Inertia: tendency of things to resist a change in it’s motion

Law 1

- Every material object continues in its state of rest or uniform motion in a straight line, unless it is compelled to change that state by forces impressed upon it

- Key Word: continues

NewtonNewton’’s 1s 1stst Law Cont. Law Cont.

ex. stamp our feet to remove snow shake a garment to rid dust or dirt

table cloth and dishes stunt

* Holds true whether your are at rest or moving at a constant velocity

ex. when we jump straight up, we land in our footsteps rather than at a location equal to the

distance the earth moves during our jump?

Inertia ProblemsInertia Problems

Q: A hockey puck sliding across the ice finally comes to rest. How would Aristotle interpret this behavior? How would Galileo and Newton interpret it?

A: Aristotle: a constant force was not applied to the puck therefore it would come to a stop.Galileo & Newton: The forces acting against the puck become greater than that of force acting on the puck and therefore it will come to a stop

ObjectivesObjectives

Be able to understand and apply Newton’s 2nd Law of motion

Using Newton’s 2nd: calculate force, mass, and acceleration

Recognize that the free fall acceleration near Earth’s surface is independent of the mass of the falling object

Explain the difference between mass and weight

NewtonNewton’’s 2s 2ndnd Law Law The acceleration of an object is directly proportional

to the net force acting on the object, in the direction of the net force, and is inversely proportional to the mass of the object

- in short: A = net force or F = ma mass

- symbols: a = acceleration Fnet = force m = mass

Practice ProblemsPractice Problems

A boy pushed forward a cart of groceries with a total mass of 40.0 kg. What is the acceleration of the cart if the net force on the cart is 60.0 N?

An automobile with a mass of 1200 kg accelerates at a rate of 3.0m/ s2 in the forward direction. What is the net force acting on the automobile?

A 25 N force accelerates a boy in a wheelchair at 0.5m/s2. What is the mass of the boy and the wheelchair?

Practice Problems Cont.Practice Problems Cont.

A = F/m

60.0 N = 1.50 m/s2

40.0 kg F = ma

1200 kg x 3.0 m/s2 = 3600 N M = F/a

25 N = 50 kg

0.50 m/s2

Inertia ProblemsInertia Problems

Q: Would it be easier to lift a huge truck on the Earth or on the Moon?

A: On the moon, when you lift an object you are dealing with weight, since weight is the gravitational pull on the most massive body, and the moon has less gravity than the earth

AccelerationAcceleration Acceleration can be:

Equal to 0: Net Forces = 0static motion

- object is not moving or accelerating- all forces are balanced

ex. a book at rest on the tabledynamic motion

- object is moving but not accelerating- all forces are balanced

ex. a book sliding across the table at a constant velocity

Acceleration Cont.Acceleration Cont. < gravity: net forces >0

- object is moving & experiencing friction

- forces are unbalanced

ex. a feather falling to the ground

*important to remember NET FORCES

ex. air resistance is neglected: net force is the objects weight

ex. presences of air resistance: net force is less than the weight

objects weight – air resistance

Acceleration Cont.Acceleration Cont.- Air resistance of an object depends on 2 factors

1. The frontal area of an object

- greater the area greater the air resistance

2. The speed of the object

- greater the speed greater the air resistance

Acceleration Cont.Acceleration Cont.

Terminal Speed

- When acceleration of an object equals zero

- If concerned with direction; use terminal velocity

Why?

- Velocity indicates a direction and speed

Acceleration Cont.Acceleration Cont.Ex. Skydiving

- as you fall you gain speed, air resistance therefore builds until finally it equals your weight. If this happens, the net force is equal to zero and you no longer accelerate, reaching terminal speed

feather few ~ centimeters per second

sky diver ~ 200 kilometers per hour

Acceleration Cont.Acceleration Cont.

= gravity (free fall)

- object is falling at 9.8m/s2

- air resistance can be neglected

ex. Any object falling in a vacuum

Why does the object with double the mass not accelerate greater?

Acceleration ProblemAcceleration Problem

Q: A jumbo jet cruises at a constant velocity of 1000 km/hr when the thrusting force of it’s engine is a constant 100,000 N.

What is the acceleration of the jet?What is the force of air friction (air resistance) on the jet?

A: The jet is not accelerating, it is at a constant velocity and therefore the net force must be 0 meaning the thrust is canceled out by the air resistance, therefore the air resistance must be 100,000N

Air Drag ProblemsAir Drag Problems

Q: Consider a man and a woman parachuting together from the same altitude. Suppose the man is twice as heavy as the woman and that their same sized chutes initially open simultaneously. Who gets to the ground first?

A: The man, he will fall a further distance before reaching his terminal velocity

Air Drag Problems Cont.Air Drag Problems Cont.

Q: A skydiver jumps from a high flying helicopter. As she falls faster and faster through the air, does her acceleration increase, decrease, or remain the same?

A: Acceleration decreases because the net force acting on her decreases. Net force is equal to her weight minus her air drag, and being that air drag, increases with speed, net force and hence acceleration decrease, Newton’s 2nd Law

ObjectivesObjectives

Explain Newton’s 3rd law of motion and relate it to everyday events

Explain how action and reaction forces are related according to Newton’s 3rd law

Be able to define a system of interactions

NewtonNewton’’s 3s 3rdrd Law Law Action Reaction Forces

- whenever on object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.

ex. produces motion:car tires push backwards against the road the road pushes forward against the car tires

ex. no motion:hand pushes against the wall wall pushes against your hand

NewtonNewton’’s 3s 3rdrd Law Cont. Law Cont.- if we drop a sheet of tissue paper in front on the

heavy weight champion of the world and challenge him to hit it in midair with 50 lbs of force, his best punch couldn’t even come close

Why?- a force is not a thing in itself but makes up an interaction between itself and another (system)

- the action force and the reaction force - net force only equals 0 when force are

equal and oppositeex.

System of InteractionsSystem of Interactions

Define the system

- Jack and Jill: Nf =0, hence no acceleration, force is internal to the system

ex.

- Jack: Nf > 0, acceleration, force is external to the system

ex.

- Jill: = Nf > 0, acceleration, force is external to the system

ex.

Systems of Interactions ProblemsSystems of Interactions Problems

Q: One cold and rainy day, you car battery is dead and you must push the car to get it started. Why can’t you push the car by remaining comfortably inside pushing against the dash?

A: Pushing again the dash creates a internal force to the system, in order to accelerate the car you must have an external force to the system

ObjectivesObjectives

Define momentum Understand how momentum and Newton’s laws

related Explain how impulses affect momentum

MomentumMomentum Definition

- a quantity defined as the product of an object’s mass and its velocity

Formula

- M = m x v p = m x v Units

- Kg x m/s

Momentum ProblemsMomentum Problems

Which object has more momentum: a car traveling at a speed of 10 km/hr or a baseball pitched at 150 km/hr. Explain your answer

What is the momentum of an 0.30 kg blue jay flying at 17 m/s?

What is the mass of the train car moving at 14 m/s with a momentum of 140 kg-m/s? of the car moving at 10 m/s with a momentum of 100kg-m/s? What is the total momentum of the system?

Momentum AnswersMomentum Answers

The car has more momentum because its mass is so much greater than the baseball. It compensates for the difference in velocity

p = m x v 0.30 kg x 17 m/s = 5.1 kg-m/s

a.) 10 kg b.) 10 kg c.) total = 240 kg/m/s

Momentum Cont.Momentum Cont. Why is a heavy truck harder to stop than a small

moving car at the same speed?

- truck has a greater mass, therefore greater momentum

Can you change an objects momentum?

- Yes, using forces, but most importantly “how long” that force is applied

ex. force applied briefly to a stalled car, small change in it’s momentum

ex. force applied over an extended time interval, greater change in momentum

ImpulsesImpulses Definition

- product of force and time interval General

- the relationship between impulse and momentum can be seen by rearranging Newton’s 2nd law (a = F/m)

- time interval part of impulse is “buried” in the term for acceleration (change in v/t interval)

- F x t interval = change in (mass x velocity)

-shorthand: Ft = mv

Impulses Cont.Impulses Cont.- rearrangement of Newton’s 2 law explains, why

“follow through” is important in increasing the momentum of things

Q: Would there be a difference in the momentum of a long barrel cannon or a short barrel cannon, and if so which would be greater?

A: Long Barrel: time interval is increased

Impulses Cont.Impulses Cont.

- decrease momentum over a long time, a smaller force results

ex. A truck out of control is better off hitting a haystack than a brick wall. When the truck hits the haystack, the time of impact may be

extended 100 times, force is reduced by a 100 times

*If you wish the force of impact to be smaller, extend to time of impact

Boxer:

Momentum ProblemsMomentum Problems

Q: Explain how a karate expert can sever a stack of bricks with the blow of his bare hands.

A: He imparts a large impulse to the bricks in a short time, hence producing considerable force.

***Remember small “t” large “F”