# Kinematics What is motion?

Mechanics

Nov 13, 2013 (4 years and 8 months ago)

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NEWTON’S LAWS
OF MOTION

Newton’s First Law

Law of Inertia

“An object at rest tends to stay at rest and an
object in motion tends to stay in motion with
the same speed and in the same direction
unless acted upon by an unbalanced force
.

Inertia

Galileo

Objects eventually stop due to a force called friction

Force is not needed to keep an object in motion

Law of Inertia

The
motion

of an object does not change
unless it is acted upon by a net force.

If v = 0, it remains 0

If v

is some value, it stays at that value

Another way to say the same thing:

No net force

velocity is constant

acceleration is zero

no change of direction of motion

Inertia

Inertia is the tendency of an object to remain at rest
or in motion with constant speed along a straight
line.

Mass is the quantitative measure of inertia.

Property of an object that measures how hard it is to
change its motion.

Loosely defined as the amount of matter in an object.

More mass means more inertia.

Mass

Gravitational mass

is the mass of an object
measured using the effect of a gravitational field
on the object.

Static measurement

Inertial mass

is the mass of an object measured
by its resistance to acceleration.

Dynamic measurement

Galileo’s Principle of Equivalence

2
nd

Law of Motion

The second law states that the acceleration of
an object is dependent upon two variables

the
net force

acting upon the object

Direct relationship

Increase in net force = increase in acceleration

the
mass

of the object.

Indirect relationship

Increase in mass = decrease in acceleration

Newton’s 2
nd

Law

Used for comparison of the same force exerted
on objects of different mass.

The force of the Earth pulls the apple down (“action”).

The force of the apple pulls the Earth up (“reaction”).

According to the 3rd law, the two forces are equal.

F
apple

= F
Earth

m
(apple)
*
a
(of apple to Earth)
=
m
(Earth)
*
a
(of Earth to apple)

F = m*a

Terminal Velocity

When Do We See These Laws In
Action?

What happens to the person when the car hits the
brick wall?

According to the 1
st

law, why does this happen?

What modern day technologies or safety features
have been created to limit the effects of
automobile accidents?

How do these technologies apply to Newton’s 2
nd

law?

Understanding the Second Law

The cause of acceleration is…

_________ resists acceleration

The greater the force, the ________ the

The greater the mass, the _________ the
acceleration.

Force.

Mass

Acceleration.

less

greater

Newton’s 3
rd

Law of Motion

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

Finger pushes on box

F
finger

box

= force exerted on box by finger

F
finger

box

F
box

finger

Box pushes on finger

F
box

finger

= force exerted on finger by box

Third Law:

F
box

finger

=
-

F
finger

box

Identify the action
-
reaction pairs

Rocket Pushes Gas

Horse and Cart

Classic Problem

How does the horse move the cart?

Horse
-
Cart

Action
-
Reaction Pairs

3 points of view

the cart, the horse, the horse
-
cart system

The Horse
-

Cart Problem

3
rd

Law
-

A horse pulls a cart and the cart
pulls back on the horse with an equal and
opposite force!

Internal forces

Contribute nothing to the acceleration of the
system

Can be ignored

Internal forces (yellow horse
-
wagon) are irrelevant

Interaction between the horse
-

cart
system and the ground

(red ground, blue ground, blue
horse & red wagon)

Think of pushing a car
-

you cannot just push on the dashboard and expect it to move; you
must interact with the ground

The Horse
-

Cart Problem

When the horse pushes backward on
the ground, the ground pushes forward
on the horse.

If the horse pushes the ground with a
greater force than it pulls on the cart,
there will be a net force and
acceleration will occur.

Newton’s Laws in Action

A bug with a mass of
5 grams flies into the
windshield of a
moving 1000kg bus.

Which will have the
greater force, the bug
or the windshield?

Newton’s Laws in Action

The force exerted
by the car on the
bug is THE SAME
AS than the force
exerted by the bug
on the car.

For every action there is an
equal and opposite reaction,
but a bug can't withstand the
same amount of force as a
windshield, so it squishes.

During the collision between the car and
the bug, which one experiences the
greatest acceleration?

1. The car has a greater acceleration.

2. The bug has a greater acceleration.

3. The accelerations will be the same.

correct

F

= m
a

bug

car

bug

car

=

bug

car

x

Newton’s Laws in Action

The force would be the
same.

Accelerations are different.

Force
(bug)=

m
(bug)

x
A
(bug)

Force
(bus)

=
M
(bug)

x
a
(bug)

Newton’s Laws of Motion

Newton’s first law
-

an object with no net force acting
on it remains at rest or travels at a constant velocity.

Newton’s second law
-

When an unbalanced net force
acts on an object there is acceleration which is
summarized by F = ma

Newton’s third law
-

when one object exerts force on
a second object, the second exerts an equal and
opposite charge.

Understanding

Which law allows a spaceship to accelerate by burning
rocket fuel?

3
rd

What law relates force to acceleration?

2
nd

Which is the law of action/reaction?

3
rd

Which law governs when a tablecloth is pulled out from
under a setting of china without damaging it?

1
st

Understanding

Which is the law of inertia?

1
st

Which law governs when two skaters push apart in a
rink?

3
rd

Which law relates acceleration to mass?

2
nd

Which law is in control of a spaceship cruising through
space at constant speed without using any fuel?

1
st

What is
F
net
?

25 N

5 N

What is
F
net
?

25 N

35 N

What is
F
net
?

25 N

25 N

What is
a
?

25 N

25 N

5 kg

What is
a
?

25 N

5 N

5 kg

What is
a
?

30 N

25 N

5 kg