newton's 1st law: inertia

Dynamics Notes Part 2: Newton's Laws

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November 02, 2017

Ch 5 Title

Newton's Laws of Motion

Ch.5p. 152

Newton's Laws

Newton's Laws of Motion1st Law: Inertia

2nd Law: F = ma

3rd Law: Action‑Reaction Pairs


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November 02, 2017

N1: Inertia & Fnet

• an object at rest will remain at rest, and• an object in uniform motion will remain in uniform motion ... unless acted on by UNBALANCED forces (Fnet)

Newton's 1st Law: Inertia

Fnet is the sum (addition) of all forces in a particular direction.

Ex: If Fg is bigger than FN, you'll fall through the floor. Fg

FN

a

N1: Inertia & Fnet

• You can think of the forces playing tug‑of‑war with the object. The larger force "wins" by the amount Fnet.

AT RESTv = 0 m/sa = 0 m/s2

UNIFORM MOTIONv = # m/sa = 0 m/s2

Examples:

Newton's 1st Law: Inertia

ACCELERATINGv = # m/sa = # m/s2


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November 02, 2017

N2: F=ma

Fnet = manetFnet is the sum of all forces in one dimension; anet is the resulting (ACTUAL) acceleration

For instance...

x‑direction: y‑direction:Fnet(x) = Fnet(y) = anet(x) = anet(y) =

FN

FG

FA Ff

a

Newton's 2nd Law:

Fnet causes objects with mass to accelerate in the direction of the bigger force(s).

Ex: What & Why?

Example #2: After letting go.

Example #1: While pushing a curling stone.

Depends on direction!Analyze x & y

components separately

Newton's 2nd Law:Draw a Free Body Diagram including all forces. Explain what will happen to the stone and why.


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November 02, 2017

Model Problem

Model Problem


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November 02, 2017

p. 163 #13

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2

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November 02, 2017

Model Problem

p. 168 #47

p. 168 #4‑7 Remember "a" is the key to move you between Forces and The Big 4. Use what they give you to find "a", then use it to get what you're asked to find.


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4

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November 02, 2017

N3: ActionReaction

Newton's 3rd Law:

Forces come in equal and opposite action‑reaction pairs.

For every action force on object B due to object A, there is an equal and opposite reaction force due to object B on object A.

A B

N3

Each force acts on a different body. The forces don't cancel each other because they would be drawn on separate Free Body Diagrams.Do

n't worry!

Newton's 3rd Law:...if this law is true, how can they accelerate?


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November 02, 2017

ActionReaction Pairs

Newton's3rd Law

Identify the Action‑Reaction Pairs:

Ex: Collisions

Applying Newton's 3rd Law ‑ CollisionsExample 1: An inattentive driver accidentally rear‑ends a parked 1500 kg car. The moving car has a mass of 1250 kg and was originally driving 30 km/h. The collision brings it to a stop in 0.75 seconds.a) Find the force the parked car puts on the moving car during the collision.b) Find the force the moving car puts on the parked car during the collision.c) How much does the collision cause the parked car to accelerate?


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November 02, 2017

Ex: Collisions

Ex: Coupled Objects

Applying Newton's 3rd Law ‑ Coupled ObjectsExample 2: A 10,000 kg train engine is hauling 2 boxcars, each of which have a mass of 6,000 kg. The force pulling the last boxcar forward is 16560 N. The coefficient of friction between the tracks and the train is 0.20.a) Find the acceleration of the train and the boxcars.b) Calculate the forward force on each hitch, between the objects.c) Calculate the force the engine must apply to move the entire train forward at this acceleration.


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November 02, 2017

Ex: Coupled Objects


Ex: Coupled Objects


newton's 1st law: inertia

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