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 Can make objects start or stop moving Can make objects start or stop moving Can make objects move faster or slower Can make objects move faster or slower Can make objects change direction Can make objects change direction

Push/Pull Activity Move around the classroom and find at least 10 things that you push, pull, or push and pull (3 minutes) Move around the classroom and find at least 10 things that you push, pull, or push and pull (3 minutes) Complete the Venn diagram by placing the items in the appropriate categories (2 minutes) Complete the Venn diagram by placing the items in the appropriate categories (2 minutes)

Forces Remember acceleration? Remember acceleration? change in speed or direction change in speed or direction Forces cause a change in the speed or direction of an object Forces cause a change in the speed or direction of an object F=ma F=ma Forces are measured in Newtons (N) Forces are measured in Newtons (N) 1N=1kg·m/s 2

Combining 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 their effects can Forces usually cannot be seen, but their effects can

Forces More than one force can act on an object at a time More than one force can act on an object 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 force acts on an object depends on: What happens when more than one force acts on an object depends on: How strong the forces are How strong the forces are The direction of the forces The direction of the forces

Combining Forces When more than one force acts on an object, the forces combine to form a net force When more than one force acts on an object, the forces combine to form a net force The net force is the combination of all forces acting on an object The net force is the combination of all forces acting on an object Forces may work together or they may be opposite forces Forces may work together or they may be opposite forces

Balanced Forces The total or net force is the difference between the individual forces Total/Net force= 5N - 5N=0N Total/Net force= 5N - 5N=0N When the forces are balanced, the object is at equilibrium and does not move When the forces are balanced, the object is at equilibrium and does not move

Unbalanced Forces When 2 or more forces act in the opposite direction and are not equal in magnitude (number), the forces will be combined by subtracting to produce a net force When 2 or more forces act in the opposite direction and are not equal in magnitude (number), the forces will be combined by subtracting to produce a net force The total or net force is the difference between the individual forces The total or net force is the difference between the individual forces The object will move in the direction of the greater force The object will move in the direction of the greater force

The total or net force is the difference between the individual forces The total or net force is the difference between the individual forces Total/Net force= 10N - 5N=5N right Total/Net force= 10N - 5N=5N right Unbalanced Forces

Practice You are pulling a wagon with your niece in it up a hill with a force of 18N. She is holding your dog’s leash, and the dog is pulling in the opposite direction with a force of 12N. What is the force on the wagon? You are pulling a wagon with your niece in it up a hill with a force of 18N. She is holding your dog’s leash, and the dog is pulling in the opposite direction with a force of 12N. What is the force on the wagon? Total/net Force = 18N – 12N = 6N Total/net Force = 18N – 12N = 6N

Unbalanced Forces When 2 or more forces act in the same direction, the forces are added to produce the total force When 2 or more forces act in the same direction, the forces are added to produce the total force Unbalanced forces result in motion Unbalanced forces result in motion

Unbalanced Forces The total force is the sum of the individual forces The total force is the sum of the individual forces Total/Net force= 25N + 30N = 55N Total/Net force= 25N + 30N = 55N Will the box move? Will the box move?

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

Friction Dragon Racers Activity Dragon Racers Activity

Friction Friction exists when two surfaces are touching and comes from the fact that no two surfaces are perfectly smooth Friction exists when two surfaces are touching and comes from the fact that no two surfaces are perfectly smooth Friction acts in the opposite direction of motion and will cause a moving object to slow down and eventually stop Friction acts in the opposite direction of motion and will cause a moving object to slow down and eventually stop

Friction To make an object move, the force you exert on an object must be greater than the force opposing it To make an object move, the force you exert on an object must be greater than the force opposing it What did you observe about the friction of the surfaces that were rougher or softer in the Dragon Racer Activity? What did you observe about the friction of the surfaces that were rougher or softer in the Dragon Racer Activity?

Friction Four types Four types Static friction – occurs between stationary objects Static friction – occurs between stationary objects Books on table, box on floor Books on table, box on floor Sliding friction – occurs when solid objects slide over each other Sliding friction – occurs when solid objects slide over each other Books or box being pushed Books or box being pushed

Friction Four types Four types Rolling friction – occurs when a solid object rolls over another solid object Rolling friction – occurs when a solid object rolls over another solid object Tires on a road Tires on a road Fluid friction – occurs when an object moves through a fluid (liquid or gas) Fluid friction – occurs when an object moves through a fluid (liquid or gas) Fish swimming, airplane flying Fish swimming, airplane flying

Friction Identify the type(s) of friction involved in walking or running. Identify the type(s) of friction involved in walking or running.

Friction Identify the type(s) of friction involved in stopping a car with brakes. Identify the type(s) of friction involved in stopping a car with brakes. What would be an example of fluid friction? What would be an example of fluid friction? Rolling friction? Rolling friction?

Friction How can we reduce friction? How can we reduce friction? Use oil in an engine between moving parts Use oil in an engine between moving parts Grease the bearings in a car wheel Grease the bearings in a car wheel Have you ever used candle wax on a sled? Have you ever used candle wax on a sled? Use a lubricant (oil, grease, etc.) Use a lubricant (oil, grease, etc.)

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

Friction Practice Consider a book on a table Consider a book on a table 32N4N The net force on the book is F= 32N - 4N = 28N

Force Calculations Force = mass x acceleration Force = mass x acceleration F=ma F=ma The triangle: The triangle: Force Mass Acceleration

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

Friction Practice 66N2N

Calculate net force: Calculate net force: F = 66N – 2N = 64N Then find acceleration: a = F/m a = 64N/70kg = 0.91m/s 2

Practice A 50kg skater pushed by a friend accelerates at 5m/s 2. How much force did the friend apply? A 50kg skater pushed by a friend accelerates at 5m/s 2. How much force did the friend apply? How much force is required to push a 16kg child in a 22kg stroller with an acceleration of 2.5m/s 2 ? How much force is required to push a 16kg child in a 22kg stroller with an acceleration of 2.5m/s 2 ?

Practice If a 90kg water skier is being pulled behind a boat with a force of 360N, what is the skier’s acceleration? If a 90kg water skier is being pulled behind a boat with a force of 360N, what is the skier’s acceleration? If a 60kg person on a 15kg sled is pushed with a force of 300N, what will be the person’s acceleration? If a 60kg person on a 15kg sled is pushed with a force of 300N, what will be the person’s acceleration?

Practice A bowling ball rolled with a force of 15N accelerates at a rate of 3m/s 2. A second ball rolled with the same force accelerates at 4m/s 2. What are the masses of the two bowling balls? A bowling ball rolled with a force of 15N accelerates at a rate of 3m/s 2. A second ball rolled with the same force accelerates at 4m/s 2. What are the masses of the two bowling balls?