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Newton’s Laws

Isaac Newton born: Dec 25, 1642

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1st. Law of motion: The Law of Inertia

(Actually, this is not Newton's idea, it is a restatement of Galileo's work on inertia.)

The Law of Inertia states: Objects at rest tend to stay at rest while objects in motion tend to stay in motion in a straight line (constant velocity) unless acted on by an outside force.

Simply put: Things keep doing what they are already doing.

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Newton’s 1st Law:Every object continues in its state of rest, or

uniform motion in a straight line,

unless it is acted upon by an outside force

BAM!

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InertiaThis tendency to continue in a given state,

(1st law) is called Inertia.

The more mass an object has, the more inertia it has.

Large amount of inertia! Small amount of inertia!

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Seat belts protect against inertia

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Inertia Question:Answer:

B

As it emerges, it continues to

follow the path it was on.

Inertia.

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Inertia Question #2:

Answer:

B

At first, the ball continues to move horizontally, but gravity soon acts on it creating the usual parabolic

trajectory.

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Force Unit: NewtonWe are familiar with the English unit for force,

the pound, lb.

However, the typical metric unit for force is the

Newton, N.

In Earth’s gravity: 1 kg = 9.8N

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Mass vs. WeightMass: The amount of matter in an object.

Units: kg, g

Weight: The force upon an object due to gravity.

Units: lbs, Newtons

Don’t confuse mass & weight!

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If you travel deep into space, does your mass change?

If you travel deep into space, does your weight change?

No

Yes

Two Questions:

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Newton’s 2nd Law:

Fnet = maThe acceleration of an object is directly

proportional to the net force, and inversely proportional to the mass.

Notice that a Newton, N, is equivalent to a kgm/s2.

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This is common sense! A larger mass is more difficult to accelerate than a smaller one!

Also, a larger force accelerates a mass more than a smaller one!

http://www.espace-sciences.org/science/images/images-maj/Perso/manipulations/acceleration/index.htm

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Example:If a 10kg block rests on a friction-less surface,

how much will it accelerate if a 50N force is applied to it?

10kg50 N

a = ?

Fnet = ma

a = Fnet/m

a = 50N/10kg

a= 5m/s2

Given:

Fnet = 50N

m = 10kg

Unknown: a?

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Net Force:The net force is simply the resultant of all the

forces acting on an object.

Consider your paycheck if you’ve ever held a part time job...

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Net Force:

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Net Force:

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Net Force:

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Newton’s 2nd Law?

F net = ma

What’s a “net” force?

The resultant of all the forces acting on an object.

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EquilibriumWhen all the forces on an object balance out, or

cancel out, the object has a net force of 0.

This condition is known as equilibrium.

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Static EquilibriumWhen an object is in equilibrium, and not moving, this is called static equilibrium.

Tension in rope

Weight

Person hanging motionless...

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Dynamic Equilibrium:An object can be moving and still be in

equilibrium.

It could be moving at a constant velocity. There would be no net force or acceleration on it. This

is called dynamic equilibrium.

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Mechanical ForcesFriction: the contact force that acts to oppose

sliding motion between surfacesNormal : the contact force exerted

perpendicular to the surface contactTension : the pull exerted by a cable when

attached to a bodyCompression : the push exerted to the objectWeight : force of attraction due to distance

from the center of the earth and mass of the object

Units are Measured in NEWTONS ( N )

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Sample Force Problem #1Using Newton’s Second law of Motion ( the

acceleration of a body is directly proportional to the net force on it and inversely proportional to the mass, F = ma).

What net force is required to accelerate a 1500 kg car at 3.00 m/s2?

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Solution Problem #1

F = m aF = 1500 kg x 3.00 m/s2

F = 4500 kg-m/s2

orF = 4500 N

Given:

m = 1500 Kg

a = 3.00 m/s2

Unknown:

F = ?

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Static Balance (free body) Problem #2

Forces of 10 N and 15N are arranged according to diagram. What is the net force on this object. What is the rate of acceleration of the 12 Kg object?

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Forces Pop Quiz (free body)

Forces of 10 N and 15N are arranged according to diagram. Draw this diagram TO SCALE! Set up a scale that will work on your paper

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Forces Pop Quiz (free body)Now use the Fg formula to calculate the weight

of the object in Newtons. Draw this Force vector on your diagram TO SCALE straight down. Then add the Normal Force (due to the surface) to your diagram TO SCALE!

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Forces Pop Quiz (free body)What is the net force on this object. Hint:

What 2 forces are not balanced? What direction is this net Force?What is the rate of acceleration of the 1.1

Kg object? Hint: use Fnet = ma

Bonus: What would happen to the object if there were NO Net Force?

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Forces Pop Quiz (free body)Forces of 10 N and 15N are arranged according

to diagram. What is the net force on this object. What is the rate of acceleration of the 12 Kg object?

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Solution #2 Given:

Force 10 N and 15 N in opposite directions Mass = 12 Kg

Find : Net Force Acceleration

Net Force = F1 - F2 = 15N – 10N = 5 N

F = ma

5N = 12 Kg (a)

5N = a

12 Kg

0.417 m/s2 = a

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Friction:

Whenever surfaces come in contact friction results.

It results from microscopic irregularities in the surfaces in

contact.

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Direction of Friction:Friction always opposes motion.

If the crate moves at a constant velocity, the net force on it must be zero. Thus, there must

be 75N of friction countering the 75N push.

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Types of Friction: Static Friction: The friction between two

surfaces when they are stationary.

Sliding (kinetic) Friction: The friction between two moving surfaces.

Static friction is generally always higher than kinetic friction.

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Newton’s 3rd Law:

For every force, there is an equal and opposite force.

For every action, there is an equal and opposite reaction.

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Examples of Newton’s 3rd Law:

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In all these cases, there is a pair of equal forces opposing each other.

Forces don’t appear alone.

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While the heavier object may have more weight, it

also has more mass to move (more inertia).

Since the acceleration any object feels is F/m, the

two factors “cancel” out, yielding the same

acceleration.

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Realistic Free Fall

In reality, things don’t fall in a vacuum. There is significant air resistance, or drag.

The more air an object must plow through, the more resistance.

This diminishes an objects acceleration.

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Weight and drag oppose each other.

Friction always opposes motion.

Subtract to get Fnet.

drag

weight

Fnet

rock

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Less than free fall...As a skydiver jumps, at first, she accelerates at

nearly 9.8 m/s2.

As she goes faster, she must push more and more air out of the way. This reduces her

acceleration.

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Eventually, her weight is completely balanced by air resistance and she doesn’t accelerate

anymore.

This “top speed” that is reached is called

terminal velocity.

Once this speed is reached, a skydiver would continue the rest of the way at this top speed.

Terminal Velocity

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For a skydiver, terminal velocity is about 200 km/hr, although this can vary with weight, and

area.

Terminal velocity for a feather is only a

few cm/s.