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# 10.1 The Nature of Force and 10.2 Friction and Gravity

pg 334
by

## Cassandra Schmidt

on 4 February 2014

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#### Transcript of 10.1 The Nature of Force and 10.2 Friction and Gravity

10.1 The Nature of Force
10.2 Friction and Gravity

Rotate
axis
and drag
frames

to change balance
10.2 Friction and Gravity
10.1 The Nature of Force
(W) FORCE: A push or a pull of an object on another object.

List 5 forces that you encountered today.

(W) Similar to velocity and acceleration, a force is described by its strength and by the direction it goes.
What is a force?
(W) The strength of a force is measured in Newtons.
NEWTON: The SI unit for force is called the Newton (N).

Forces are represented with arrows. They point in the direction of a force, the longer the arrow the greater the force.
Newton
Before we start, on your white board, list at least two forces you have encountered today.
Discover Activity:
1) Push down on your desk with your hand. What do you feel?
2) Hold on end of the string while your partner holds the other end. GENTLY pull. What do you feel?
3) Rub your hands together. Now, what do you feel?

Over the course of chapter 10, we will come back to these three activities and identify exactly what is going on with each of these activities....
Many forces can be applied on an object at one time.

(W) NET FORCE: the combination of all forces acting on an object.

Of the examples of forces we shared with the class or performed in the activity, pick three and draw a picture with arrows that describes the force.
Combining Forces
UNBALANCED FORCE: results in a net force that can cause an object to move.

BALANCED FORCE: results in a stationary object, all forces are canceled out.
Unbalanced/Balanced
Describe all of the friction you see in the picture.
GRAVITY: a force that pulls objects toward one another.

(W) The law of universal gravitation states that the force of gravity acts between all objects in the universe.

(W) Two factors affect the gravitational attraction between objects: mass and distance.
Static Friction: acts on objects that are not moving.

Sliding Friction: two solid surfaces slide over each other.

Rolling Friction: an object rolling across a surface.

Fluid Friction: a solid object moving through liquid.
(W) Friction is the force that two surfaces exert on each other when they rub together.

(W) Depends on two things:
how hard the surfaces push together
the types of surfaces involved.
As you watch the clip, think about what type of friction was shown in the phone book? What type of friction would have been shown if they could have separated the phone book?
Write down each type of friction and name an example:
You have 10 minutes to work with your group and demonstrate the 4 types of friction. For each one, draw a quick sketch on your whiteboard to show what you did to demonstrate that type of friction and label the the sketch's motion using arrows. Take turns presenting the whiteboards after the 10 minutes are up:

Whiteboard for Friction Activity:
Static (draw, and label with motion arrows)
Sliding (draw, and label with motion arrows)
Rolling ( draw, and label with motion arrows)
Fluid (draw, and label with motion arrows)
Weight: the force of gravity on a person
weight = mass x gravity
gravity = 9.8 m/s
Mass: amount of matter a person has
(W) FREE FALL: when the only force acting on a falling object is gravity.
An object in free fall is accelerating.
All objects accelerate at the same rate: 9.8 m/s (often called the acceleration of gravity), regardless of their masses.
Record this diagram into your notes:
You can determine the weight of an object if you measure its mass:
measure the mass in kilograms.
multiply each mass by 9.8 m/s squared to find the weight in newtons.
Micro-Lab:
Pick an object.
Find its mass on the scale.
Record the mass in kilograms.
Find its weight in newtons.
Gravity and Motion
On Earth, gravity is a downward force.......
(W) AIR RESISTANCE: an upward force of fluid friction on an object falling through the air.
objects with a greater surface area experience more air resistance.
Take two pieces of paper that are the same size.
Crumple the first one into a ball and leave the second one flat.
Stand on your chair and drop both pieces of paper at the same time.
(W) TERMINAL VELOCITY: is reached when the force of air resistance equals the force of the weight falling.
this means that there is no acceleration, the speed of the object remains constant as it is falling.
If we have time....
Felix Baumgartner's supersonic free fall...
Projectile Motion
(W) An object thrown is called a projectile.

(W) If you throw a projectile at an angle, it will fall at the same rate as any dropped object.
Which piece of paper fell faster?
Why do you think it fell faster?
If you were to drop both pieces in a vacuum, which one would reach the ground first? Why do you think that?