# of Motion (and force!!) Μηχανική

14 Νοε 2013 (πριν από 5 χρόνια και 7 μήνες)

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Newton’s Laws

of Motion

(and force!!)

A
force

is a push or pull on an object

control over
movement:

desired motion

or preventing
undesirable
movement

Apply Force:

Contact Forces

muscles

machines/mechanical

ropes and wires

Friction

adhesion

EXAMPLES of forces:

EXAMPLES of FORCES:

Non
-
contact forces:

Nuclear

Electrical/magnetic

Gravity

weight

Non
-
contact
Forces

such as
gravity,
magnetic,
and electric
forces.

Like Velocity and Acceleration…..

FORCE has both magnitude

AND

Direction!!!

NET FORCE (unbalanced force)

results in motion

for an object at rest)

or acceleration (a change in velocity)

For an object already in motion

Equilibrant Force

Net Force = 0 …..means,

an object at rest remains at rest

OR

an object moving with constant motion
will continue at that constant
motion.

FORCE IS MEASURED IN
NEWTONS

One NEWTON (N)

1 kg x 1m

s
2

Think about it….

Force = mass X acceleration

that gives a 1 kg object, an acceleration of 1m/s2

Inertia

tendency of an object to
RESIST a change in motion

Example: velocity of an object remains
constant unless a force changes it

the greater the mass, the
greater the inertia

Newton’s First Law of Motion

An object continues in a state of rest or in
a state of motion at constant speed along
a straight line…

unless compelled to change that
state by a
net force
.

an object
moving at a
constant velocity
remains at that
velocity unless a
NEW FORCE
(>0) acts upon it

Inertia plays
a central role
in one type of
seat belt
mechanism.

Newton's First Law of Motion (inertia):

Two mousetrap cars with different masses.

which one has more inertia?

Construction Tip: Wherever possible, remove mass = less energy
required to overcome inertia and get your car moving. So More
energy available to propel your car forward.

Less mass = less inertia!

friction

Friction

force of adhesion

depends on the surface and
force acting between surfaces

EXAMPLES OF FRICTION: air,
fluid, solids

Examples of

contact forces

Describe what
role Newton’s
first law plays
a role in what
is occurring in
this picture

The Normal Force & Friction

The Normal Force & Friction

Newton’s Second Law of Motion

When a net force
F

acts on an object
of mass
m
, the acceleration
a

that
results is directly proportional to the
net force and is inversely proportional
to the mass.

If: net F =
m
X a then….

direction

of acceleration = direction of the net force

SI Unit of Force: kg • m/s
2

= newton (N)

m

F
a
Newton’s second law and mousetrap cars:

mass resists acceleration, and force causes
acceleration.

to double the acceleration of an object, double the
force or remove mass.

Force causes acceleration!

Need more pulling force?

Shorten the lever arm and move the
mousetrap closer to the drive wheels.

Shortening the lever arm implies
moving the string closer to the
hinge on the mousetrap.

ONCE AGAIN!!!

A NEWTON

….is
the amount of force
required to accelerate a

1 kg object 1 m/s
2

2
m/s

059
.
0
kg

1850
N

110

m
F
a

N

+110
=

N

560

N

395

+

N

275
+

F
m

= 1850 kg

With two guys pushing…….what are

The net forces???

If the airplane’s mass is 13 300 kg, what is the
magnitude of the net force that the catapult and
jet engine exert on the plane?

N

10
4.1
=
)
m/s

kg)(31

300

(13

5
2

ma
F
The Normal Force

GRAVITY (gravitational force)

the pull an object exerts on another object

the amount of gravitational force is
dependent on:

1) Mass of the two objects

2) distance between objects

the greater the mass, the greater the
gravitational pull on that object

Weight

measure of gravitational force

varies dependent on proximity to EARTH

Weight vs Mass

Mass is the measure of…

the matter of an object and

an object’s resistance to change in
motion

Newton’s Third Law of Motion

Whenever a
body exerts a
force on a
second body, the
second body
exerts an
oppositely
directed force of

equal magnitude
on the first body.

Newton’s 3
rd

Law……

action force = reaction force

for every action, there is an equal and
opposite reaction!!

tires of a car push against the road,
and the road in turn pushes back on the
tires,

a swimmer pushes the water backward,
and the water pushes swimmer forward

"A mousetrap car is propelled because the drive wheels push
on the floorand the floor pushes back on the car [wheels]”

The wheels can only push on the floor as hard as the floor can
push back. If the floor cannot push back with the same force
as the wheels push, then the wheels will spin in place and the
car will not accelerate to its fullest potential.

Newton’s Third Law

For every action there is an equal and
opposite reaction.

Fortunately for
sparky,

Zeke knew the
famous

“Rex Maneuver”!

Free Body Diagrams

A Free Body Diagram distills the
problem’s complexity down to only
those actions that are interacting with
the object of interest.

Consider the following situation:
You are standing in the middle
of the room.

Are there any forces acting on
you?

Is there net force acting on you?

Draw the forces acting on you.

Justify your answer by referring to
Newton’s Laws.

Now remove the floor and draw
the forces acting on you.

What is your state of motion
now?

Characterize it by stating
something that we can
calculate.

Distance Car Tips:

longer lever arms

larger drive wheel(s), ideally 1
-
2 feet in diameter

small drive axle

small power output

decrease mass and rotational inertia

remove ALL friction

Speed Cars

shorter lever arms

smaller drive wheel(s), ideally around 2
-
5" in
diameter but no larger than a compact disc. If the
wheel
-
to
-
axle ratio is too small, your vehicle will
[spin out and] waste energy.

Increase traction

Larger diameter drive axle. You can get more torque
with a thick axle
--
more torque means greater
acceleration!

Larger power output. For speed, the objective is [to
quickly convert potential energy into kinetic energy.]

Decrease mass and rotational inertia

Remove ALL friction

Friction slows and stops the mousetrap car

Energy moves the vehicle.

too much friction = energy consumed too quickly
and vehicle won’t travel far or accelerate fast.

Check moving components and decrease the
friction at each point.

The more moving components, the greater the
force of friction will be (usually.

The smaller the frictional force, the farther your
supply of energy will propel your vehicle.

Slow
-
moving cars have a smaller force of air
resistance and travel farther than faster
-
moving
vehicles