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Newtons Laws

- Isaac Newton born Dec 25, 1642

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.

Brief History Isaac Newton 1665 Newton 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

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.

Newtons 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

Inertia

- This tendency to continue in a given state,
- (1st law) is called Inertia.
- The more mass an object has, the more inertia it

has.

Inertia

I have more inertia

- The tendency of an object to stay at rest or in

motion. - Objects with greater mass have greater inertia

but we fall at the same rate

Inertia keeps astronauts in motion

Seat belts protect against inertia

Inertia Question

- Answer
- B
- As it emerges, it continues to follow the path it

was on. Inertia.

Inertia Question 2

- Answer
- B
- At first, the ball continues to move

horizontally, but gravity soon acts on it

creating the usual parabolic trajectory.

Force Unit Newton

- We are familiar with the English unit for force,

the pound, lb. - However, the typical metric unit for force is the

Newton, N. - In Earths gravity 1 kg 9.8N

Mass vs. Weight

- Mass The amount of matter in an object.
- Units kg, g
- Weight The force upon an object due to gravity.
- Units lbs, Newtons

Dont confuse mass weight!

Two Questions

- If you travel deep into space, does your mass

change?

No

If you travel deep into space, does your weight

change?

Yes

Newtons 2nd Law

- Fnet ma
- The 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.

- 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!

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es-maj/Perso/manipulations/acceleration/index.htm

Example

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

how much will it accelerate if a 50N force is

applied to it?

Fnet ma a Fnet/m a 50N/10kg a 5m/s2

Given Fnet 50N m 10kg Unknown a?

Net Force

- The net force is simply the resultant of all the

forces acting on an object.

Consider your paycheck if youve ever held a part

time job...

Net Force

Net Force

Net Force

Newtons 2nd Law?

- F net ma
- Whats a net force?
- The resultant of all the forces acting on an

object.

Equilibrium

- When 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.

Static Equilibrium

- When an object is in equilibrium, and not moving,

this is called static equilibrium.

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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 Universe

- Nuclear 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

Mechanical Forces

- Friction the contact force that acts to oppose

sliding motion between surfaces - Normal the contact force exerted perpendicular

to the surface contact - Tension the pull exerted by a cable when

attached to a body - Compression the push exerted to the object
- Weight force of attraction due to distance from

the center of the earth and mass of the object - Units are Measured in NEWTONS ( N )

Sample Force Problem 1

- Using Newtons 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?

Solution Problem 1

- F m a
- F 1500 kg x 3.00 m/s2
- F 4500 kg-m/s2
- or
- F 4500 N

Given m 1500 Kg a 3.00 m/s2 Unknown F ?

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?

10

15

12 Kg

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

Friction

- Whenever surfaces come in contact friction

results. - It results from microscopic irregularities in the

surfaces in contact.

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.

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.

P. 6 Workbook Problems

- 1. 20 lb force applied to a 64 lb object for 5 s

FN Weight

Fnet

Weight mg (F ma)

- Weight 64N therefore W mg solve for m ?

- m W/g ? m 64 lb/ 32

ft/s2 2 slugs

b. Fnet ma therefore a Fnet / m ? a 20

lb / 2 slugs

a 10 ft/s2

FN Weight

Fnet

Weight mg (F ma)

- 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

FN Weight

Fnet

Weight mg (F ma)

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

P. 6 Workbook Problems

- 2. A force applied to a 50 kg object to

accelerate-20 m/s2

FN Weight

Fnet

Weight mg (F ma)

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

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

- 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

P. 9 Workbook Problems

- A force of 400N to a 100 kg object to move _at_ cv

FN Weight

Ff

Fapp

Weight mg (F ma)

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

b. Since the box is moving _at_ constant velocity -

therefore Fapp Ff ? 400 N µ FN ? µ 400 N/

980N .41

- Fnet Fapp - Ff ? 600 N 400 N 200 N- now
- Fnet ma ? a Fnet / m ?(200N)/(100 kg)

2 m/s2

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.

- But why does it work that way?
- The cannon ball has more mass, thus it has more

weight, and gravity pulls on it more...

- 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.

Realistic Free Fall

- In reality, things dont 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.

- Weight and drag oppose each other.
- Friction always opposes motion.
- Subtract to get Fnet.

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.

Terminal Velocity

- Eventually, her weight is completely balanced by

air resistance and she doesnt 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.

- 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.

Newtons 3rd Law

- For every force, there is an equal and opposite

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

reaction.

Examples of Newtons 3rd Law

- In all these cases, there is a pair of equal

forces opposing each other. - Forces dont appear alone.

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f5b8c02c0e46513b98f9

Gravity

The attractive force between all objects in

the universe

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F M1 M2 d2

- More mass means greater gravity
- More distance means less gravity

My gravitational attraction is small no matter

what the distance!

Massive objects have lots of gravity

The earth exerts a force on the student and the

student exerts a force on the earth because of

GRAVITY!

Gravity warps the fabric of space-time

Gravity

gravity pulls it in inertia keeps it going

What forces are acting on the moon?

Inertia