forces

DEFINITION OF FORCE

Types

Units

Definition of a Force

• Forces are any push or pull on an object– Created by contact or at a distance

• Forces are portrayed by arrows on pictures, with the arrow pointing in the direction of the force

• The size of the arrow indicates the size of the force

How do you know if you are applying a force?

How forces effect objects

• Balanced: when all forces applied to an object cancel each other out.

• Results in NO change in motion

• (Constant velocity)

Unbalanced forces

• Definition: Where more force is pushing or pulling on object in one direction than opposite direction

• Results in a change in motion (Acceleration)

TYPES OF FORCES

Forces

• Contact (applied) Forces– Electromagnetic forces (most common)

• Non-contact Forces– Gravitational (most common)– Fields

Types of forces

• Contact forces: source of force is a push or pull that results from direct contact between the surfaces of 2 or more objects. Related to the forces that repel or attract molecules and/or atoms

• Field forces: source of force applies influence over a distance, not direct contact

Examples

Contact Field

• Applied Gravitational

• Friction Electric

• Ground Magnetic• Air Resistance

GRAVITATIONAL FORCE

Gravity

• Force of attraction between 2 objects that have mass

•Fg or W

What determines the gravitational pull?

• The mass of each object

• The distance between the items

Gravitational force

• Depends on:

• The amount of mass in each object– The larger the masses, larger the force

• The amount of distance between each object– The greater the distance, the smaller the force

The effect on force of gravity by distance is exponential, mass is linear

Law Universal Gravitation

• There is a gravitational attraction between any two items that have mass.

• The amount of the force is related to the mass of each object and the distance their centers are apart

• More mass, less distance greater force

Do you think Weight of an object is…

An applied forceA gravitational force

A magnetic force

Weight = force on object due to gravity

• If there is no big object near you, you have no weight

WHAT OBJECT HAS A LOT OF GRAVITATIONAL FORCE?

Blackholes

Black hole stripping matter from nearby star

Black holes are

• Some the strongest sources of gravitational pull in the universe

• The remains of dying stars

• Stars much larger than ours (20x)

• Objects that collapse in on itself because of gravity, become impossibly dense, compact

Weight influences shape

• Tree trunks

• Jelly fish

• Humans 10 feet tall

Biological connection

• Blood pressure is related to gravity

• More in lower part of body than upper

• Take blood pressure on arm because same position as heart

Blood pressure and gravity

Third law and gravity

• Third law dictates that each object attracts the other with the same amount of force

WHAT ARE APPLIED/ CONTACT FORCES?

Applied/Contact force

• On a macroscopic level

• Any force that results from contact between two objects or the environment

• Electromagnetic in nature, the repulsion between atoms

• Some applied forces are further identified (normal force)

Examples

• Ex. The forces of a hand pushing a box The force of the rope pulling on a horse’s

bridle

Others?

What applied force is pushing the rocket upward?

MASS

What does mass mean?

Mass

• As previously defined:

• The amount of stuff, or matter that composes an object

• More matter = more massive an object

• Does not depend on location of object like weight. Mass ≠ Weight

Units of mass

• For chemistry (grams)

• For Physics (kilograms)

• 1 kg = 1000 g

Which is more massive?

A kilo of feathers or

A kilo of marbles

Difference in mass

COMPARISON OF WEIGHT AND MASS

Weight and mass

• Are related, not the same

• Mass is not a force, but a measurement of matter

• Weight changes as the objects moves around the universe, mass will not

HOW TO CALCULATE THE WEIGHT

Weight is a Force

• Force due to the pull of gravity

• Weight (W) = mass X acceleration due to gravity

• On earth, everything in free-fall accelerates at the same rate (9.8 m/s2) towards Earth

Mathematical equation for Weight of an object

• Weight =

mass x acceleration due to gravity

On earth’s surface, acceleration due to gravity is = to 10 m/s2

Demonstration that all objects are pulled at the same rate

• Paper and book

• Paper-vs-paper

• The only reason why things do not fall at sam rate is air resistance

Acceleration due to gravity

• g

• Value on surface of the earth

• Changes as object is moved away from surface of the earth

Weight is a force

• Weight is the measure of the Force of gravity

• To find your weight in Newtons:

– Take your weight and divide it by 2.2

– Take the result and multiply it by 9.8

Calculate your mass value

• Take your weight in Newtons and divide by 10

UNITS OF FORCE

Units

• Some units that are made out of 2 or more fundamental units are renamed after a Famous Scientist

• Units of Force– 1 Kg (m/s2) = 1N (Newton)

WHAT WEIGHS 1000 N?

Football player (220 lbs)

WHAT WEIGHS 100 N?

Freshman backpacks

WHAT WEIGHS 10 N?

The average textbook

WHAT WEIGHS 1 N?

A smallish apple

COMBINING FORCES

Net, Unbalanced, and Balanced forces

Equilibrium

Fundamental ideas

Balanced forces on an object will NOT cause the object to change its motion

• Change of motion is caused by an imbalance of forces in opposite directions

• Forces come in many types, including contact, gravitational, and frictional forces

Balanced forces

• What happens to object’s motionwhen the forces applied to them are balanced?

Balanced forces

• Forces on an object are balanced when there is the same amount of force applied in opposite directions

Balanced forces keep the object’s motion _____________

Forces

• Balanced forces = when all the forces acting on an object cancel each other out

• Unbalanced forces = when all the forces do not cancel each other out (more force on object in one direction than in others)

Unbalanced forces

• What happens to the object’s motion when the forces applied to them are unbalanced?

Unbalanced Forces

• Situation where there is more force applied in one direction than in the opposite

• Results in ____________

• Balanced ForcesNet force = zeroForces cancel outNo change in motion

• Unbalanced ForcesNet force does not = 0Forces do not cancel out

• Unbalanced forces and acceleration

Net force

• The difference in size of the opposing forces is equal to the Net force on the object

• If the net force is zero, forces are balanced– Constant velocity

• If the net force is not zero, forces are not balanced– Acceleration

10 3

Net force

• The combination of all forces on an object

• For balanced forces Fnet = 0

– At rest or at constant velocity

• For unbalanced forces Fnet ≠ 0

– acceleration

Resultant Forces

Equilibrium

Free body diagrams

Homework

In textbook: Page 89 problems 1-5

Page 93 problems 6-8

Homework : part 2

Research question: How far can a human fall and survive?

1: Research accounts, reports on the internet, bring in documentation

2: What factors do you think contribute to the survival of the human upon impact?

3: Write up a short summary of what you found to discuss and hand in tomorrow.

Newton’s Laws of motion

Inertia

CAUSES AND EFFECTS OF APPLYING FORCES

To change the motion of something

• Need to have a net force (unbalanced forces) on the object in order to change its motion

Force of rope on box

Friction force

RELATIONSHIP BETWEEN FORCES AND ACCELERATION

Fundamental ideas

• Motion is caused by forces

• Change of motion is caused by an imbalance of forces in opposite directions

• Forces come in many types, including contact, gravitational, and friction

What forces effect their ability to roll down a hallway?

Forces and Freefall

Free Fall

• When the only significant force acting on object is gravity

Acceleration due to gravity

• If gravity is the only force on object, then the forces are unbalanced

• Result is downward acceleration of the object

If object is in freefall, gravitational acceleration is constant

• For all objects

• Hammer, Feather

on Moon

Acceleration due to gravity

• Paper and Book test

• Which drops faster?

• Any other forces at work besides gravity (weight)?

Orbital Free-Fall

• Something in orbit around the earth has to be moving fast enough so it falls at the same rate that the earth curves.

• The moon orbits because of the same reason

• We orbit the sun the same way

• The closer to the surface the faster the object needs to travels to maintain orbit

Freefall

• A specific type of motion• Examples :

– _____________– _____________

(regardless of direction)

• To be in freefall, only the force of ______ can cause change in motion (acceleration)