Warmup Sept 17 th - Think

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# Warmup Sept 17 th - Think - PowerPoint PPT Presentation

Warmup Sept 17 th - Think. You were kidnapped and knocked out. Some time later, you wake up in a dark warehouse. All there is in the room is a scale and a light, so you decide to weigh yourself. WOW!!! You gained a whole lot of weight last night…what happened?

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Warmup Sept 17th - Think
• You were kidnapped and knocked out. Some time later, you wake up in a dark warehouse. All there is in the room is a scale and a light, so you decide to weigh yourself. WOW!!! You gained a whole lot of weight last night…what happened?
• You remember seeing alien figures in a dream-state, and realize you must be on another planet. Is the planet smaller or more massive than Earth?
Weight and Mass…

Fg = mg (formula for weight)

Fg (weight)

is the force an object has due to gravity Gravity on the Earth 9.8m/s2

Weight and Mass…

What’s the mass of my 893N ex-husband that just happens to be in my basement?

Weight Problems! Fg=mg
• What does a 60kg tool weigh?
• What’s the mass of a crate of tools that weighs 234 N?
Weight and mass:
• What does a case of the new Chipotle Gatorade with a mass of 54.8kg weigh?
What would a 60kg tool weigh:
• On Mars (accel of gravity 3.7m/s2 )?
• On Jupiter (accel of gravity 24.8m/s2 )?
Now it’s time for Newton’s 2nd Law
• Newton’s 1st: Inertia. At rest, stay at rest; in motion, stay in motion.
• Newton’s 2nd: What happens when a force acts on an object?
• Acceleration.

Newton’s Second Law:Force = mass x acceleration

Force equals mass times acceleration.

F=ma

Newton’s Second Law:
• The acceleration of an object is directly proportional to the force applied to the object, and inversely proportional to the mass.
• -or – if F then A

if M then A

How? Like this:

My 4 kg cat, Jeter, ran at our kitten Shadow with an acceleration of 3.0 m/s2. He didn’t stop. Poor little kitten! After suffering a mild concussion, we wondered….how much force did Jeter hit him with?

Force = mass x acceleration, or…

4 x 3 = 12

12 what?

Remember Force has its own special unit…

1kg x 1m/s2 = 1 newton (1N)

F = ma

Learn it. Live it. Love it.

Force = mass(acceleration)

1. How much force is needed to push a lawnmower of 30 kg at 2.5 m/s2?

2. How much force must be applied to a bowling ball that has a mass of 5kg to achieve an acceleration of 25m/s2?

3. What is the mass of a baseball that has an acceleration of 125 m/s2 after a force of 44 Newtons has hit it?

FRICTION

Friction - a force that opposes motion, or “pushes back” when two surfaces are touching.

It’s always around and it affects everything.

FRICTION- works against you …..but it can be helpful! When do I want…
• A lot of friction?
• A little bit of friction?
• http://digg.com/video/the-science-of-figure-skating
• http://www.nbclearn.com/olympics
Friction types and parts…
• There are three Basic Types of Friction
• Static
• Kinetic
• Fluid
Types of friction:

1) Static friction: Friction that resists the start of sliding motion between two objects at rest.

Keeps things stopped

Types of friction:

2) Kinetic friction: The force that opposes the movement of two surfaces that are in contact and moving over each other.To make something move, you just overcome static friction by pulling with a force larger than kinetic friction.

Kinetic Friction:

Sliding or Rolling

Types of friction:

3) Fluid friction: The friction between layers of a fluid.

2 types- Liquid and Gas

Liquid- water, oil etc.

Gas - Air

Types of friction:

Described by viscosity.

Viscosity – the “thickness” of a fluid.

Low viscosity = “thin;”

High viscosity = “thick” or “viscous.”

The less viscous a fluid is, the easier it moves.

Does Size Matter?

I’m talking about Mass or surface area folks!

Friction DOES depend upon:

• 1. surface area in contact
• 2. mass/gravity (weight) of object
• 3. the texture of the surface
Friction is cumulative
• Small amounts of friction in a system add up to become a large amount of friction.