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BellRinger: September 18 Fg=maFg is weight (force due to gravity)

m=mass of the object a=acceleration of gravity 9.8 m/s2

1. What is the weight of a 6.7 kg box?

2. What’s the mass of a 76 kg crate on the moon? On Jupiter?

3. What’s the mass of a 80N crate on earth?

Figure this out……….

You were kidnapped and knocked out. Some time later, you wake up in a dark warehouse. All the doors are locked and there are no windows. You have a spring scale and 5 kg mass. Weighing the mass, you get a reading of 44.2N. Are you still on earth? What’s your evidence?

What planet are you on?

Planet g (m/s2)

Mercury 3.61Venus 8.83Earth 9.8Earth's Moon 1.6Mars 3.75Jupiter 26Jupiter's Moon Io 1.81Saturn 11.2Uranus 10.5Neptune 13.3Pluto 0.61

POST LAB DISCUSSION

Look at Part 1

When you changed mass what else changed? Only force should have, but your acceleration

should have stayed about the same> WHY??

The acceleration part of our equation F=ma was the acceleration due to gravity…gravity is constant….it doesn’t change!

Post Lab Discussion

Part 1For #3 you were supposed to take the

highest mass you had from your experiment and see what the force would be like on the moon.

For example, if 0.500 kg was your highest, and you had that on the moon, your Force would be F=0.5 x 1.6 = 0.8 N (it is lower)

GRAVITY….

http://gravitymovie.warnerbros.com/#/videos/main-trailer

Time to talk about that thing that slows you down…that’s always holding you back…

(not “the man”)

Friction is a force that opposes motion, or “pushes back” when two surfaces are in contact with each other

It’s always around and it affects everything.

Without friction…

3 Types of friction:

1)Static Friction

2)Fluid Friction

3)Kinetic Friction

Types of friction:

1) Static friction: Friction that resists the initiation (start) of sliding motion between two objects that are in contact and at rest.

Remember Newton’s 1st Law of Inertia…a body at rest, stays at rest, unless an unbalanced force acts on it? Usually the force keeping it at rest is friction…. To make something move, you just overcome static friction

Types of friction:

2) Fluid friction: The friction between layers of a fluid. Described by viscosity.

Viscosity – the “thickness” of a fluid.

Low viscosity=“thin;” high viscosity = “thick”

The less viscous a fluid is, the easier it moves or you move through it.

http://www.youtube.com/watch?v=OikVWMMOTeE

http://www.youtube.com/watch?v=hvbGrfpCsjo

Which has a higher viscosity?

Air or Water???

How can you tell??

Types of friction:

3) Kinetic friction: The force that opposes the movement of two surfaces in contact and moving over each other.

To keep something moving, must apply a force larger than kinetic friction.

Two types of Kinetic Friction:

a) Sliding – the objects are sliding

b) Rolling – the objects are rolling.

How did they build the Pyramids?

Do we need friction to walk?

Why do you think yes or no?

Friction depends upon 3 things:

1. surface area in contact2. mass/gravity (weight) of object3. coefficient of friction – “mu”

Surface Area:

Why does an ice skater wear thin blades to skate with?

Of course! Less friction and they can go longer

Surface area affects friction (how much of each object is in contact with each other)

Weight:

What is harder to push from rest, a bookshelf full of books or an empty bookshelf?

Of course! Less friction and they can go longer

Mass x Gravity affects friction

The coefficient of friction

Mu – measured between two surfaces

Low mu – slippery

High mu – hard to move

Book friction… I need 2 strong volunteers!!

Friction is cumulative

Small amounts of friction in a system add up to become a large amount of friction.

And it can be measured. For example:

The force pulling a truck is 2,000 N. What is the approximate amount of static friction acting on the truck if the truck does not move?

Where is friction good?

Where is friction bad?

Homework

Explain two situations when you want friction, and two when you don’t. They cannot be opposites. Explain why you want friction or

don’t want it in each case. Full sentences, good grammar.