Forces and Motions

Chapter 13 – The Nature of Forces

Forces13.1 What is a Force? Anything that changes the state of rest or

motion of an object

It’s what causes ACCELERATION

Has magnitude and direction Therefore force is a vector

13.1 Force

A force can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction.

13.1 Combining Forces1) Balanced Forces:

5 N 5 N =

Equal forces in opposite directions cancel each other to produce no net force and therefore no movement.

0 N

Unbalanced ForcesResult when the net force acting on an object is NOT equal to zero

When an unbalanced force acts on an object, the object accelerates and moves in the direction of the net force

13.1 Combining Forces - Unbalanced

13.1 Combining Forces2) Adding Forces:

5 N 5 N 10 N =

Two forces, working in the same direction will add together to produce a net force larger than either original force.

3) Subtracting Forces:

13.1 Combining Forces

10 N 5 N 5 N =

Two forces working in opposite directions will subtract to produce a net force in the direction of the larger force.

There are two main types of friction: Static - keeps things “static” (i.e. stationary) Force resisting motion of an object at rest.

Ex- the force that is keeping this block from sliding downhill

Kinetic - friction that occurs with objects in motion.

Three types of kinetic friction: Sliding, Rolling and Fluid

Friction is a force that opposes motion between two surfaces that are in contact.

13.2 Friction

13.2 FrictionStatic Friction: works in the opposite direction to the intended motion. Object is at rest.

Rolling Friction: happens when an object rolls over a surface. Works opposite to the wheels’ motion.

Static friction Direction of motion

Rolling friction

Direction of motion

Seating Plan

Bassel

BeaAmaniIngrid

Alain

Mikel

Jenny

Chrissana

Demy

Anne Marie

Zeina

Patricia

Tara

Charles Elias

Mia

Sarah

ArthurRyan

Michel R

Richard

Paul

Michel B

Jad

Test

Physics (25 mins, April 4th) Acceleration Chapter 13.1 Chapter 13.2

Chemistry (25 mins, April 7th) Acids and Bases

(yes, that’s really it, so be really prepared)

13.2 Friction

Fluid Friction: happens when an object pushes fluid aside. The surfboard overcomes the fluid friction of the water.

Sliding Friction: acts between the sled and the snow, in the opposite direction to the sled.

FRICTION

STATIC SLIDING ROLLING FLUID

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

-rocks laying on the side of a mountain

-a hockey puck on ice

-a child going down a slide

-a roller blade on a sidewalk

-a bowling ball going down a bowling alley

-a plane flying through the air

-a swimmer swimming in a pool

is defined as is defined as is defined as is defined as

& an example is & an example is & an example is & an example is

Reducing Friction

A smooth surface does not have as much friction as a rough surface.

A lubricant, ball bearings or even air can be used to reduce friction.

13.2 Friction

It is an attractive force that pulls objects together.

13.3 GravityGravity is a natural phenomenon in which objects that have mass are attracted to one another.

13.3 GravityEarth’s gravity acts downward toward the

center of the Earth. There is an upward force that balances gravity.

Falling Objects

Terminal velocity: the constant velocity of a falling object when the force of air resistance is equal in magnitude & opposite in direction to the force of gravity.

Question: What other force is not present in a vacuum that would affect acceleration?

Answer = air resistance

In a vacuum, two objects would accelerate at the same rate,

regardless of their shape, because both are in free fall.

Free Fall: the motion of a body when only the force of gravity is acting on

the body.

Newton’s 1st Law of Motion According to Newton’s 1st Law, the state of motion of an object does not change as long as the net force is zero.

a) An object at rest will stay at rest unless a force acts on it.

b) An object in motion tends to stay in motion unless a force acts on it.

Ex: A soccer ball will remain at rest on the grass unless a force is acted on it. It will then stay in motion until another force acts on it.

This law is also called the “Law of Inertia”

Inertia: the tendency of an object to resist being moved or, if the object is moving, to resist a change in speed or direction until an outside force acts on the object.

Ex. In a car crash: You continue forward

because of inertia

Newton’s 2nd Law of Motion

Force equals mass times acceleration.

F = ma

Newton’s 2nd Law of Motion

Force = mass x acceleration Units: Newtons (kg·m/s2)

Acceleration of gravity on Earth = 9.8 m/s2

Acceleration of gravity on the Moon =1.6 m/s2

Weight = mass x acceleration due to gravity On Earth, weight ≈ mass x 10m/s2

On the Moon, weight ≈ 1/6 weight on Earth

Question 1: If you weigh 540N on Earth, how much would you

weigh on the Moon?

Question 2: A) If your mass is 42.5 kg on Earth, what is your

mass on the Moon? B) What would your weight on Earth be? C) What would your weight on the Moon be?

Newton’s 2nd Law of Motion

90 N540N x 1/6 =

42.5 kg!!!425 N

70.8 N

This will help you think about everything you have learned

Homework

Worksheet Newton’s 2nd LawQuestions 1-8 (see, it could be worse!)

Newton’s 3rd Law of MotionFor every action there is an equal and opposite

reaction.

Action: Earth pulls on you

Reaction: you pull on Earth

Action and Reaction on Different MassesConsider you and the Earth

Action: tire pushes on roadReaction: road pushes on tire

Action: rocket pushes on gases

Reaction: gases push on rocket