Forces and machines

Forces surround us constantly. We need force to play sports, to open a bottle and even to write!

Lets discuss the mechanics of the world around us in this last chapter of the year.

Forces

Discussion in class: If you are riding a bus and suddenly, the bus comes to a stop.

What do you feel? If you take the elevator, how do you feel if you go up? Or

down?

In a game of “tug-o-war” Now think about your favorite sport. What kind of forces

does it entail? There is also a push or a pull.

Force

The symbol of force is “F” and the unit of measurement is “newton” or “N” ex: 5 N ou 20 N

A force is an action that can provoque the movement of and object or change its shape.

Ex: - Throw a baseball- Skating- Push (or pull) a drawer- Compress a sponge- Etc.

Types of forces (p 410)

We will discuss a few different types.Compression: it is a force applied to compress an object.

It can provoque or modify and object’s motion. Ex: compressing a balloon! Also, the muscles in your arm must compress to lift objects.

Tension: the force applied to pull an object in one direction. (especially on a rope)

Ex: Such as in the game of tug-o-war or like the acrobats.

Friction: It is a force that slows down or stops the movement or motion) between two surfaces.

Friction acts in the opposite direction of the motion.

Ex: Your shoes have rubber, so you dont stop and fall.

Also the tires from a car.Air resistance is also a source of fricftion.

Observe:

What is the purpose of a zamboni?

To reduce friction between the skates and the ice.

Air friction

What is the difference between these vehicles?

Aerodynamic cars and… cats! (cheetahs)

http://www.youtube.com/watch?feature=player_detailpage&v=KIeXEiJuJUY

All have a slender body, which makes for less air friction

The consequences of forces

Elastic deformation: ex trampoline (takes its original shape).

Permanent deformation: ex:can of soft drinks that changes shape qui (it will take any shape, different from the initial shape)

Rupture: material breaks like the cables of the Olympic stadium.

Newton’s laws of motion

What made the apple fall?

Newton’s laws of motion

Newton, is a british physist, born the year Galileo died. He attended Trinity College at University of Cambridge in 1661, he did his own research and taught while he was still a student!

His work "Philosophiae naturalis principia mathematica", was published in 1687, has three sections and contains the universal law of gravity and the famous Newton’s laws of motion.

Do you like roller coasters?

Newton’s first law of motion

The first law, also known as inertia, states that all objects tends to remain in the same speed and direction it was moving at (even at rest) unless an outiside force acts upon it.

In a roller coaster, the train keeps moving until the breaks are applied.

The first law… another example

The golf ball remains at rest until an outside force acts upon it.

Newton’s second law of motion

All acceleration of an object is proportional with the forces applied on it. All acceleration is inversely proportional to the mass.

Example 1: The more we pull on a rope at tug-o-war, the more we accelerate.

Example 2: A person pushing a table, is not the same as three pushing a table. The three together will accelerate way more!

Examples of acceleration:

Examples with mass:

If these same three people applied the same force on a car, they won’t accelerate as much. Because it’s a lot heavier.

Newton’s third law

To every action, there is an equal and opposite reaction

The second and third laws applied on roller coasters…

The force that we feel from the gravity as we free fall from a very big height is enormous and can be calculated:Using F = mxa

m = mass of the train and passengers.

a = gravitational acceleration (g) 9.8 m/s2

2nd law 3d law

The roller coaster undergoes curves that you can feel. When the train goes in a curve we feel like the seats are pushing us.

But you are also pushing the seats with an equal and opposite force. Which helps you to hold on to your seat!