Forces

On a post-it, write down anything you know about the word force or

the different ways you use the word force.

Scientific Definition

• A force is a push or a pull of an object.• When there is a force, one object is applying a force and on

object is receiving the force.

Characteristics Used to Describe a Force

1) Magnitude (size)- the amount of force being exerted

2) Direction- the direction in which the force is moving the object

Effects of a Force on An Object

MOTION!

-start moving-stop moving

-speed up-slow down

-change directions

Identifying Forces

• Some forces are obvious, because you see where the force is coming from since they act based on touch.

• Some forces are not obvious because they can act with out touch. – GRAVITY– MAGENTS– -STATIC

Measuring a Force

• The unit used to measure how much force an object is exerting is the Newton.– The abbreviation for Newton is N. – One Newton can lift one small apple.

• The tool used to measure force is a spring scale.

• The more mass an object has, the more force it will require to move the object.

• As mass increases, force increases.

Combining Forces• Sometimes there is more than one force being

exerted on an object• The forces being exerted may be working in the same

direction. • Example: a plane traveling in the same direction as the

wind

• Sometimes there are multiple forces acting on an object in different directions. – Example: a plane flying against the wind

Think, Pair, Share:Name all the forces acting on the kite.

Are all the forces working in the same direction?

Net Force• Net force is the combination of all the measure of all

the forces being exerted on an object.

• In order to determine the net force when the forces are acting in the same direction, you add up the measure of all the forces.

• Example: My sister and I are moving a piano together in the same direction. I exerted 15 Newtons while my sister exerted 10 Newtons. What is the net force applied to the piano?

• ______ N to the ___________ (direction)

Net Force: Different Directions

• To determine the net force on an object with forces acting in different directions, you subtract the smaller force from the bigger force.

• When there are forces working in different directions, it will lessen the force on the object.

Net Force

What does net force tell us?

• if the object will move• in which direction the object will move• how strong the force is on the object

Balanced Forces

• A balanced force is produced when two forces are acting in different directions and are equal, which means the net force is 0.

• When the forces applied to an object produce a net force of zero, the forces are balanced.

• Balanced forces don’t cause movement in an object.

• Balanced forces will not change the motion of a moving object.

Unbalanced Force• When the force of an object is not zero, then it is an

unbalanced force.• An unbalanced force is when one force is stronger than

another force.• An unbalanced force causes a change in the movement

of an object.1. start/stop movement2. change direction of movement3. speed up/ slow down movement

Picture WalkChoose a picture with your group. Then discuss and answer the following questions on an index card. Be ready to present the information to the class.1. What are the forces being exerted on the

object?2. Are the forces balanced or unbalanced?3. What are the effects of the force?4. How would you determine the net force being

exerted on the object?

Friction

• Question: When you kick a soccer ball, eventually it will stop moving. Why?

Description of Friction• Friction is a force that appears whenever two things rub against

each other.

• Friction always works in the opposite direction of the force being exerted on the object. Therefore, it affects it slows down its movement and eventually causes the object to stop moving.

– Example: force of kick causes ball to move in one direction, while friction is slowing down the movement of the ball and eventually slowing it down.

• Even though a surface may seem smooth, it actually has tiny bumps that can be seen with a microscope. Therefore all surfaces cause friction.

Friction Depends On:1. mass: objects that have more mass push

harder and closer against the surface and therefore cause more friction.

2. surface: Objects with smooth surfaces cause less friction. Objects with rough, bumpy surfaces cause more friction.

Exit Ticket

• What do you think would happen if you kick a ball in space? Why?

• How does friction affect the force exerted on an object?

Four Types of Friction

1) rolling friction- when one object ROLLS over another object, usually on wheels

Examples: bike, car, stroller, ball , train, rollerblading, skateboarding

2) sliding friction- friction used when one surface slides past another surface

Examples: writing, walking, itching your skin, pushing a book, touching something

Which of he two produce more friction? (think of a suitcase)

3) fluid friction- any friction that occurs between a liquid or a gas and an object

Examples: objects in water, objects in air

4) Static friction- the friction that occurs between two objects when object applies a force on another object but the object does not move. Static friction disappears once the object begins to move.

Example: when you push something heavy but it does not move, static friction occurs. Once it moves, static friction disappears and sliding friction occurs.

Is friction a good thing or a bad thing?

positive effects• friction between floor and feet helps us walk• car brakes, tires pushing against the floor to move• holding anything in your hand would be difficult without

friction- writing• sports- cleats for softball, gloves for better grip, sneakersnegative effects• car parts rub together causing engine parts to heat up and

be worn down• holes in socks, clothing, shoes worn out from friction• friction by water and wind causes erosion/rust –bike in rain

Increasing/Decreasing Friction

decreasing friction:• adding wheels• adding liquids• lubricant-oil, wax, grease, air• making the surface smoother (sandpaper)increasing friction: • make surfaces more rough (brillo, salt, gloves)• add mass

Exit Ticket

• Based on what we learned today, explain why a ballerina wears ballet slippers.

• Why is friction good for a car? Bad for a car?

What do you already know about gravity?

Record what you know about gravity on a postit. Then post it onto the board.

What is gravity?

• Gravity is a force of attraction between two objects.

• All objects with mass have a gravitational force. The gravitational force pulls objects towards each other.

• The force of gravity can affect the motion of an object by changing its speed, direction or both.

Gravity depends on:1) mass- the more mass an object has, the more

gravitational force it will have. (Example: you and the desk vs. you and Earth.)

2) Distance- the closer two objects are to one another, the greater the gravitational force.

Isaac Newton• British scientist named Isaac Newton realized while

watching an apple fall from the tree that there has to be some unbalanced force that is causing the apple to move.

• He came to the conclusion that the force was gravity which is pulling everything on Earth towards Earth.

• This became known as the law of universal gravitation- it is a law because it applies to all things in the universe.

Effects

• What are some effects of gravity on you?• What would happen without gravity?

Review: What is the difference between weight and mass?

• Mass- the measure of how much matter is in an object• weight-the measure of gravitational force on an object • Mass never changes, but weight changes depending on

how much gravitational force there is on the object. • You weigh less on the moon than you do on Earth-

why?• Why is there less gravity on the moon?

Directions• You will get into groups of 3 and choose roles.

– pens/pencils keeper– teacher getter/clarifier – coordinator /point keeper

• You will discuss the answers to each of the questions based on what you learned about gravity. While you discuss, all pens/pencils must be placed in the bin.

• After you discuss the answers, each student will write down the answer to the question on a post-it and place it underneath the question they answered.

• If one person has a question, then all students must put their pens/pencils down to discuss and answer the question.

• Reminders: – Do now write any of the answers in your notes– When there is talking there is no writing. When there is writing there is no

talking.

Use the picture to help answer the questions.

How does gravity affect the tides?•Tides are the rise and fall in sea level on the ocean shore. •The side of the Earth that is facing the moon experiences a high tide, and the opposite side has a high tide because gravity is pulling the water towards the moon. •The sides that are not facing the moon are experiencing low tide.•When the sun is aligned with the moon it enhances the pull of gravity on the ocean.

Objective

How do you describe motion?-frame of reference

-speed (type and amount)-velocity

-acceleration

How do you detect motion?

• In order to see that something is moving, you need to compare it to something else that is not moving.

• The background used to determine if something is moving is called a frame of reference.

• Your frame of reference changes depending on how you are looking at something or experiencing something. – ARE YOU MOVING?

airplane, rollercoaster, Earth

Types of Speed1. circular- motion that takes place around a central point with a circular

path -Examples: Ferris wheel, tire, planets

2. linear motion-motion that happens in a straight lineExample: walking

3. vibrational- rapid back and forth movement examples: guitar strings, vocal cords

Speed • Speed is the measure of how fast an object is moving.• In order to determine the speed of an object you need

to determine the distance the object travels and the time it takes to move that distance.

• In order to calculate speed you can divide the distance by the time to get the speed.

SPEED= DISTANCE TIME

• The unit for distance in the metric system is the meter or kilometer.

• The unit for time is hours, minutes, or seconds.• Therefore the unit for speed is km/h, m/s (depending

on the units being used).

Exit Ticket

A boy rides his bike for ten minutes and travels 30 meters. Give of a description of his motion.

Include:-frame of reference -type of speed-speed

Use this formula to solve the problem:

A train travels 200 meters in 10 seconds. What is the speed of the train? ________

A train is traveling at an average speed of 20 meters per second for 10 seconds. How far did it travel? _______

A train travels 200 meters at 20 meters per second. How long did it take the train to travel? _______________

Velocity• Velocity is speed in a given direction:– 20 mph south– 37 kilometers per second north

• A change in velocity is a change in either speed or direction or both, which is caused by an unbalanced force acting on the object.

Turn and Talk: • How would you find your velocity if you were walking

forward on a moving bus, in the same direction of the bus?• What if you were walking in the opposite direction of the

bus’s motion?

Acceleration

• When there is a change in velocity, acceleration occurs. • Acceleration is the rate at which velocity changes. • Acceleration occurs when the movement of an object:

1. slows down2. speeds up3. changes direction

• To calculate the acceleration of an object:1. find the initial velocity(speed& direction when started

2.Find the final velocity and subtract the initial velocity from the final velocity.

3. Divide that number by the amount of time it took to go from the initial velocity to the final velocity.

final velocity- initial velocity= m/s2 timehttps://www.youtube.com/watch?v=rZo8-ihCA9E -3:36

Practice

1) A roller coaster’s velocity at the top of a hill is 10 meters/second. Two seconds later, it reaches the bottom of the hill with a velocity of 26 meters/second. What is the acceleration of the roller coaster? _______________________ 2) A skater goes from a standstill to a speed of 6.7 m/s in 12 seconds. What is the acceleration of the skater?________________________________

Exit Ticket

• What is the difference between speed, velocity, and acceleration?