FORCES AND MOTIONAND STUFF (AND WHAT-

NOT)

EVERYTHING MOVES!

• I mean e-v-e-r-y-t-h-i-n-g

• From sub atomic particles to the entire galaxy itself is in motion

• mountains, stars, clouds and every single living organism on earth is in some kind of motion...ALL THE TIME.

Aristotle

• This famous philosopher classified motion into two groups natural and unnatural

• natural motion would be the motion of a boulder falling, smoke rising or the movement of planets and such through the sky

• He believed that natural objects seeked out natural resting places– because of this, he felt that natural motion DID NOT

require a force

Unnatural motion...

• ...was the motion of a cart or some other man-made object and DID require a force because we would have to push or pull it to make it move

SORRY ARISTOTLE...

• ...but you were wrong!!!

• Well, half wrong.

• Why only half wrong?

I TRIED MY

BEST.

IT’S OKAY BUDDY. NOT EVERYONE CAN BE AS

AWESOME AS ME. stupid

son of a...

Galileo

• Aristotle’s theories were taken as fact for about 2000 years

• Until Galileo blew them away

• Galileo’s experiment at the Leaning Tower of Pisa destroyed Aristotle’s belief that larger objects fall faster

• he also showed that Aristotle was wrong about forces too

MAN...I CAN’T WIN!

BOOYAH!!

Galileo’s simple experimentswere quite powerful

• He simply rolled balls down flat planes at different angles and observed

• He noticed that a ball rolling down an angle increased speed and then decreased speed as it rolled up an incline

• He also noticed that a ball rolling on a flat surface neither increased nor decreased speed

His conclusion...

• A force is required to start an object moving, but once moving, no force is required to keep it moving-except for the force needed to overcome friction

• He also believed that if there was no friction the ball moving on the flat surface would roll forever (he was right)

Thanks for the history lesson but...

• ...what is a force?

• Forces are pushes or pulls on objects

• There are four forces in the universe– gravity– electromagnetic force– strong nuclear force– weak nuclear force

• most forces that we think of are electromagnetic

Let’s Talk About Motion and Speed and Velocity

• MOTION IS RELATIVE

• What...what does that mean?

• I’ll show you

What happened?

• What changed?

• How do you know?

• If you didn’t see it happen, how do you know?

“Motion is relative” means...

• ...motion is an object’s change in position in relation to other objects

• so, you know Spongebob moved because he is in a different position in relation to Patrick

• So, is the school moving?

Speed

• What is speed?

• Speed is the distance moved per unit time

• We can thank Galileo for this too

• Before him people thought of speed as fast or slow

• Galileo was the first to define and measure speed

• speed = distance/time (the first equation you’re responsible for)

Average vs. Instantaneous

• Imagine that you are doing this

Are you travelling at the same speed the entire journey?

• NO!!!

• Why not?

• Correct.. So as you go the speed varies from one instant to the next

• This means that there is an average speed which is equal to :– the total distance travelled/the total time it took

There is also a bunch of instantaneous speeds

• These are the speeds you are travelling at any given second during the trip

• instantaneous speed is what a speedometer measures

• The SI unit for speed is meters/second or m/s

• Ummmmm......

Back to physics...

• We know speed now. What is velocity?

• The unit for velocity is m/s

• Does that help?

• No, the unit fo speed is m/s too.

• What is the difference?

Velocity tells a direction also

• Velocity is a vector.

• A vector describes a direction as well as a magnitude

• If we say “a car travels 60 km/h”, that is speed

• The car’s velocity is 60 km/h to the north

• get it?

• Oh yeah, force is a vector too.

Newton’s First Lawof Motion

• Called the Law of Inertia

• But first, what’s inertia?

• Inertia is a property of matter. All matter has inertia. It is the object’s resistance to change in motion.

Now back to Newton’s 1st Law

• “Every object continues in a state of rest or of uniform speed in a straight line unless acted upon by a nonzero net force.”

• What?• All this means is that one needs to apply a force to

an object to change it’s motion, or put another way: Object’s at rest tend to stay at rest unless acted upon by an unbalanced force. Object’s in motion tend to stay in motion unless acted upon by an unbalanced force.

Net force

• When we say “force” we mean the net force or the total of all forces acting on an object

Look at this guy go. He is pushing the box across the floor. What is the net force? To find out we need to know ALL of the forces acting on the box.

The push

friction

gravity

Natural force (normal force)Air

resistance

Air pressure

The push

friction

gravity

Natural force (normal force)

Air pressure

If this guy is applying a force of 10 N (ten newtons) to the left, but all the other forces add up to 5 N in the opposite direction then the net force is 5 N to the left

The Newton

• The Newton is the SI unit for force

• It is named after Sir Isaac Newton

• If force is equal to the mass of an object times its rate of acceleration or: F = ma or force = kg x m/s2 then the unit for force is the kg m/s2

• nobody wants to say “kilogram meter per second squared” so we call it a Newton and we just write a “N”

Balanced vs. Unbalanced forces

• Change in motion occurs when forces are unbalanced

• the little boy can move the box to the left because he is applying more force in that direction than friction is applying in the opposite direction. However...

• If the forces are balanced there is no motion

5 N 5 N

F = 0

• 5 N to the left opposing 5 N to the right is just like saying 5 - 5, which we know equals 0

• so 5N - 5N = 0N• No force equals no motion• When the net force is zero the object is in

equilibrium

• it can be represented mathematically like this: F = 0. this Greek symbol stands for “the vector sum of” and F is force

The Support Force (the normal force or the natural force)

• This will be quick

• Put simply the normal force is the force that the ground or the floor pushes up onto the bottom of your feet

• It acts directly against gravity, it is equal to gravity but in the opposite direction

• Without it, we would be pulled into the center of the planet where we would die a horribly painful death

Equilibrium for moving objects

• We discussed equilibrium already. What was it again?

• It is when the net force acting on an object is zero or ΣF = 0

• So what if the object is moving?

• If it is in motion with a CONSTANT VELOCITY then it is also in equilibrium. This just means that the velocity is neither increasing nor decreasing

Newton's 2nd Law of Motion

• “The acceleration produced by a net force on an object is directly proportional to the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object.”

• What?

• Let's define some terms first

Acceleration

• It is simply the rate of change of velocity

• We can thank Galileo for this too

• So a = v/t

• What the heck is v?

• It means “the change in” velocity

• so...acceleration is equal to the change in velocity divided by the time it took to change

The 2nd Law of Motion re-defined

@All this law is saying is that an object accelerates in the direction that the force is applied

@so...the big equation that relats to this is

@F = ma or force = mass x acceleration

@keep in mnd that you will now be responsible for not only F = ma but also m=F/a and a = F/m

Friction

¥ Friction is the force that arise between two objects that are sliding across each other

¥ It can occur between solids, liquids or gases

¥ PLEASE KEEP IN MIND friction ALWAYS opposes the direction of motion

¥ this is what cause movement when you walk, ride a bike, drive a car or boat

Force applies by tire to the rock

Friction

Why?

œ MICROWELDS!

œ The surfaces of solid objects, no matter how smooth, are not smooth.

œ Microwelds are microscopic bumps and valleys

œ When surfaces interact, these microwelds interact this cause friction

œ the sum of the forces on the molecular level is equal to the total force of friction

Freefall

We know that acceleration rate DOES NOT depend on mass

Thanks Galileo!

DON’T MENTION IT.YOU EVER WONDER WHAT I’M LOOKING

AT?