1 Newton’s Laws Isaac Newton born: Dec 25, 1642 2 Brief History: The Greeks: thought there were...

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Newton’s Laws

Isaac Newton born: Dec 25, 1642

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Brief History:The Greeks: thought there were two types of motion, natural and violent. They also thought that a force was required to keep objects in constant motion. Today we know they were incorrect. They had no concept of friction. The Greeks basically thought that objects wanted to return their "natural" resting place in the universe. If you tried to move an object against this natural motion it was called violent motion. Thus, there were two types of motion, natural and violent.

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Brief History:Isaac Newton: 1665Newton was an amazing individual. He did more for physics than almost every other scientist before of after him. Newton's ideas on motion were so unique that they started revolutions in Europe. The laws of motion actually gave the common person a new view on how the world worked. His laws of motion can be summarized as follows:

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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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Inertia

The tendency of an object to stay at rest or in motion.

Objects with greater mass have greater inertia

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Inertia keeps astronauts in motion

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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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4 Forces of the UniverseNuclear : Strong forces holding the

nucleus of an atom together (gluon)Weak : Forces as a result of some

atoms that decay the nucleus ( radioactive )

Electromagnetic : a force based on electricity, magnetism and light properties

Gravitational : The weakest force caused by the 2 factors, MASS and DISTANCE

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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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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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P. 6 Workbook Problems #1. 20 lb force applied to a 64 lb object for 5 s

a. Weight = 64N – therefore W = mg – solve for m

m = W/g m = 64 lb/ 32 ft/s2 = 2 slugs

Weight = mg (F = ma)

FN= Weight

Fnet

b. Fnet = ma – therefore a = Fnet / m a = 20 lb / 2 slugs

a = 10 ft/s2

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c. Kinematics problem now –

G – vi = 0 ft/s ; a = 10 ft/s2 ; t = 5 s

U – vf = ?

E - vf = vi + at vf = 0 ft/s + (10 ft/s2)(5 s) = 50 ft/s

Weight = mg (F = ma)

FN= Weight

Fnet

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d. Kinematics problem now –

G – vi = 0 ft/s ; a = 10 ft/s2 ; t = 5 s

U – d = ?

E - d = vit+ 1/2at2 d = 0 ft/s + (5 ft/s2)(5 s)2 = 125 ft

Weight = mg (F = ma)

FN= Weight

Fnet

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P. 6 Workbook Problems #2. A force applied to a 50 kg object to accelerate-20 m/s2

b. Weight = mg W = (50 kg)(9.8 m/s2) = 490 N

Weight = mg (F = ma)

FN= Weight

Fnet

a. Fnet = ma Fnet = (50 kg)(20 m/s2) = 1000 N

c. G – vi = 0 m/s ; a = 20 m/s2 ; t = 12 s U – vf = ?

E - vf = vi + at vf = 0 m/s + (20 m/s2)(12 s) = 240 m/s

d. G – vi = 0 m/s ; a = 20 m/s2 ; t = 12 s U – d = ?

E - d = vit+ 1/2at2 d = (10 ft/s2)(12 s)2 = 1440 m

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P. 9 Workbook Problems A force of 400N to a 100 kg object to move @ cv

a. Weight = mg W = (100 kg)(9.8 m/s2) = 980 N

Weight = mg (F = ma)

FN= Weight

Ff

c. Fnet = Fapp - Ff 600 N – 400 N = 200 N- now…

Fnet = ma a = Fnet / m (200N)/(100 kg) = 2 m/s2

b. Since the box is moving @ constant velocity - therefore

Fapp = Ff 400 N = µ FN µ = 400 N/ 980N = .41

Fapp

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Free Fall Revisited

If air resistance is negligible, which hits the

ground first, a heavy cannon ball, or a lighter

pinball?

They should tie! Both accelerate at the same

rate.

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But why does it work that way?

The cannon ball has more mass, thus it has more weight, and gravity pulls on it more...

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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.

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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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http://www.teachertube.com/view_video.php?viewkey=f5b8c02c0e46513b98f9

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Gravity

The attractiveforce between all objects in theuniverse

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F = M1 M2

d2

More mass means greater gravityMore distance means less gravity

My gravitationalattraction is smallno matter what the distance!

Massive objects have lots of gravity

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The earth exerts a force on the studentand the student exerts a force on the earth becauseof GRAVITY!

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