Unit 3 Forces. Forces A force is a push or a pull A force is a push or a pull A force affects how an object moves A force affects how an object moves

Unit 3 Forces Unit 3 Forces

ForcesForces

A force is a push or a pullA force is a push or a pullA force affects how an object movesA force affects how an object moves

Can make objects start or stop Can make objects start or stop movingmoving

Can make objects move faster or Can make objects move faster or slowerslower

Can make objects change directionCan make objects change direction

Push/Pull ActivityPush/Pull Activity

Move around the classroom and find Move around the classroom and find at least 10 things that you push, pull, at least 10 things that you push, pull, or push and pull (3 minutes)or push and pull (3 minutes)

Complete the Venn diagram by Complete the Venn diagram by placing the items in the appropriate placing the items in the appropriate categories (2 minutes)categories (2 minutes)

ForcesForcesRemember acceleration? Remember acceleration?

change in speed or directionchange in speed or directionForces cause a change in the speed or Forces cause a change in the speed or

direction of an object direction of an object F=ma F=ma Forces are measured in Newtons (N)Forces are measured in Newtons (N)

1N=1kg·m/s1N=1kg·m/s22

Combining ForcesCombining Forces

Forces usually act in pairs Forces usually act in pairs Forces act in a particular direction Forces act in a particular direction Forces usually cannot be seen, but Forces usually cannot be seen, but

their effects cantheir effects can

ForcesForces

More than one force can act on an More than one force can act on an object at a timeobject at a time

Forces can push or pull in any direction Forces can push or pull in any direction What happens when more than one What happens when more than one

force acts on an object depends on:force acts on an object depends on:How strong the forces areHow strong the forces areThe direction of the forcesThe direction of the forces

Combining ForcesCombining Forces

When more than one force acts on an When more than one force acts on an object, the forces combine to form a object, the forces combine to form a net forcenet force

The net force is the combination of all The net force is the combination of all forces acting on an object forces acting on an object

Forces may work together or they may Forces may work together or they may be opposite forcesbe opposite forces

Balanced ForcesBalanced Forces

The total or net force is the difference The total or net force is the difference between the individual forcesbetween the individual forces

Total/Net force= 5N - 5N=0NTotal/Net force= 5N - 5N=0NWhen the forces are balanced, the When the forces are balanced, the

object is at equilibrium and does not object is at equilibrium and does not movemove

Unbalanced ForcesUnbalanced Forces

When 2 or more forces act in the When 2 or more forces act in the opposite direction and areopposite direction and are not not equal in equal in magnitude (number), the forces will be magnitude (number), the forces will be combined by subtracting to produce a combined by subtracting to produce a net forcenet force


The object will move in the direction of The object will move in the direction of the greater forcethe greater force


Total/Net force= 10N - 5N=5N rightTotal/Net force= 10N - 5N=5N right


PracticePractice

You are pulling a wagon with your You are pulling a wagon with your niece in it up a hill with a force of 18N. niece in it up a hill with a force of 18N. She is holding your dog’s leash, and She is holding your dog’s leash, and the dog is pulling in the opposite the dog is pulling in the opposite direction with a force of 12N. What is direction with a force of 12N. What is the force on the wagon?the force on the wagon?

Total/net Force = 18N – 12N = 6NTotal/net Force = 18N – 12N = 6N


When 2 or more forces act in the same When 2 or more forces act in the same direction, the forces are added to direction, the forces are added to produce the total forceproduce the total force

Unbalanced forces result in motionUnbalanced forces result in motion


The total force is the sum of the The total force is the sum of the individual forcesindividual forces

Total/Net force= 25N + 30N = 55NTotal/Net force= 25N + 30N = 55NWill the box move?Will the box move?

PracticePractice

Example:Example:

You are pushing your twin brothers up You are pushing your twin brothers up a hill in a stroller with a force of 18N. a hill in a stroller with a force of 18N. Your best friend sees you and comes Your best friend sees you and comes to help. He or she pushes with a force to help. He or she pushes with a force of 12N. What is the force on the of 12N. What is the force on the stroller?stroller?Total/Net Force = 18N + 12N = 30NTotal/Net Force = 18N + 12N = 30N

FrictionFriction

Dragon Racers ActivityDragon Racers Activity

FrictionFriction

Friction exists when two surfaces are Friction exists when two surfaces are touching and comes from the fact that touching and comes from the fact that no two surfaces are perfectly smoothno two surfaces are perfectly smooth

Friction acts in the opposite direction of Friction acts in the opposite direction of motion and will cause a moving object motion and will cause a moving object to slow down and eventually stopto slow down and eventually stop

FrictionFriction

To make an object move, the force you To make an object move, the force you exert on an object must be greater than exert on an object must be greater than the force opposing itthe force opposing it

What did you observe about the friction What did you observe about the friction of the surfaces that were rougher or of the surfaces that were rougher or softer in the Dragon Racer Activity?softer in the Dragon Racer Activity?

FrictionFrictionFour types Four types

Static friction – occurs between Static friction – occurs between stationary objects stationary objects Books on table, box on floorBooks on table, box on floor

Sliding friction – occurs when solid Sliding friction – occurs when solid objects slide over each otherobjects slide over each otherBooks or box being pushedBooks or box being pushed

FrictionFrictionFour types Four types

Rolling friction – occurs when a solid Rolling friction – occurs when a solid object rolls over another solid objectobject rolls over another solid objectTires on a roadTires on a road

Fluid friction – occurs when an object Fluid friction – occurs when an object moves through a fluid (liquid or gas)moves through a fluid (liquid or gas)Fish swimming, airplane flyingFish swimming, airplane flying

FrictionFrictionIdentify the type(s) of friction involved Identify the type(s) of friction involved

in walking or running.in walking or running.

FrictionFrictionIdentify the type(s) Identify the type(s)

of friction involved of friction involved in stopping a car in stopping a car with brakes.with brakes.

What would be an What would be an example of fluid example of fluid friction?friction?

Rolling friction?Rolling friction?

FrictionFriction

How can we reduce friction?How can we reduce friction?Use oil in an engine between moving Use oil in an engine between moving

partspartsGrease the bearings in a car wheelGrease the bearings in a car wheelHave you ever used candle wax on a Have you ever used candle wax on a

sled?sled?Use a lubricant (oil, grease, etc.)Use a lubricant (oil, grease, etc.)

Friction PracticeFriction Practice

Consider a book on a tableConsider a book on a table

If I push the book with a force of If I push the book with a force of 32N and the table applies 4N of 32N and the table applies 4N of friction, what is the net force on the friction, what is the net force on the book?book?


Consider a book on a tableConsider a book on a table

32N32N 4N4N

The net force on the book is The net force on the book is

F= 32N - 4N = 28NF= 32N - 4N = 28N

Force CalculationsForce Calculations

Force = mass x accelerationForce = mass x accelerationF=maF=maThe triangle:The triangle:

Force

Mass Acceleration


Your mass is 70kg. Your friend Your mass is 70kg. Your friend pushes you down a hill on a sled pushes you down a hill on a sled with a force of 66N. The snow has with a force of 66N. The snow has a friction of 2N. What is your a friction of 2N. What is your acceleration?acceleration?


66N66N

2N2N


Calculate net force:Calculate net force:

F = 66N – 2N = 64NF = 66N – 2N = 64N

Then find acceleration:Then find acceleration:

a = F/ma = F/m

a = 64N/70kg = 0.91m/sa = 64N/70kg = 0.91m/s22

PracticePractice

A 50kg skater pushed by a friend A 50kg skater pushed by a friend accelerates at 5m/saccelerates at 5m/s22. How much force . How much force did the friend apply? did the friend apply?

How much force is required to push a How much force is required to push a 16kg child in a 22kg stroller with an 16kg child in a 22kg stroller with an acceleration of 2.5m/sacceleration of 2.5m/s22??

PracticePractice

If a 90kg water skier is being pulled If a 90kg water skier is being pulled behind a boat with a force of 360N, behind a boat with a force of 360N, what is the skier’s acceleration?what is the skier’s acceleration?

If a 60kg person on a 15kg sled is If a 60kg person on a 15kg sled is pushed with a force of 300N, what will pushed with a force of 300N, what will be the person’s acceleration?be the person’s acceleration?

PracticePractice

A bowling ball rolled with a force of 15N A bowling ball rolled with a force of 15N accelerates at a rate of 3m/saccelerates at a rate of 3m/s22. A . A second ball rolled with the same force second ball rolled with the same force accelerates at 4m/saccelerates at 4m/s22. What are the . What are the masses of the two bowling balls?masses of the two bowling balls?

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Unit 3 Forces. Forces A force is a push or a pull A force is a push or a pull A force affects how an object moves A force affects how an object moves