Transcript Newton`s 1st, 2nd and 3rd Law

-Motion: Why?
-Newton’s Laws
Physics
Mrs. Coyle
Part I –Newton’s First Law
The Early Days

Greece

Aristotle (4th Century BC)
Natural and violent motion
Italy

Galileo Galilei (1564 AD)

Is a force necessary in order to keep an
object moving?
Demos
Galileo’s Experiment with Ball


A force is not required to keep the ball
moving on a horizontal plane.
In the absence of friction, the ball
would move perpetually.
Newton’s First Law

Isaac Newton -England (1561)
1st Law – Law of Inertia
An object at rest or moving at constant
velocity will continue to be at rest or at
constant velocity, unless acted upon by a
net force.

Inertia---Mass


Inertia: the resistance of an object to
a change in its state of motion
Mass: amount of matter in an object



a measure of inertia
unit: kg
scalar
Seatbelts!!!!!!!
(http://www.glenbrook.k12.il.us/gbssci/phys/m
media/newtlaws/cci.html)
An airplane drops a flare.
Which is the path of the flare?
v
A
v
B
C
Part II- Newton’s 2nd Law
Force: a push or a pull

Vector

Unit: Newton N
The 4 Fundamental Forces




Gravitational (attractive force
between masses)
Electromagnetic (caused by to
electric and magnetic fields)
Strong Nuclear Force (holds nuclear
particles together)
Weak Nuclear Force (involved in
radioactive decay of nuclei)
Friction:
a force acting when two surfaces rub
against each other.


Friction opposes motion.
Is caused by the irregularities in surfaces.
Newton’s 2nd Law
F=ma
Normal Force:
a force acting perpendicular to the
interface of contact between two
objects.
Weight:



the gravitational force with which the
earth pulls an object
vector
unit: N
Free Body Diagram:
Part III- Newton’s 2nd Law and
Kinematics
Example 1
A 40kg box is starting from rest and
accelerates to 2m/s in 3s.
a)Find the acceleration.
b)Find the net force acting on the object.
Example 2
A 2kg rock falls through a liquid for 3m. It
initial velocity was 0.5m/s and its final
velocity was zero.
a)Find the acceleration.
b)Find the net force acting on the object.
Example 3
A 1500kg car traveling at 5m/s hits a
muddy barrier and is stopped by a net
force of 2000N.
a) Find the car’s net acceleration.
b) Find the distance the car traveled while
being stopped by the barrier.
Example 4
A 0.1kg arrow is pulled back and then after
being at rest is released by a bow. During
the release the arrow traveled 0.3m. Its
final release speed was 12m/s.
a)
b)
Find the acceleration given to the arrow by
the bow.
Find the force with which the bow pushed the
arrow.
Part IV- Newton’s 3rd Law
Newton’s 3rd Law



When an object exerts a force on a
second object (action), the second
objects exerts an equal and opposite
force on the first object (reaction).
To every action there is an equal and
opposite reaction.
Forces occur in pairs.
What is the reaction force to:



Kicking a soccer ball?
The windshield of a bus hitting a
fly?
The swimmer pulling the water?
Can you hit a piece of paper
with a large force with your
hand?
What are the reaction forces to
the each of the forces shown?
Example 1
Snoopy has a mass of 15kg and kicks a 0.4kg
ball with a force of 15N.
a)Draw the free body diagram showing the
forces acting on each object.
b)With what force does the ball kick Snoopy
and why?
c)What was the acceleration of the ball?
d)What was the acceleration of Snoopy?
Ans: b) 15N, c) 37.5m/s2 d) 1m/s2
Example 2
Two blocks, one 8 times as massive as the
other, are connected by a compressed spring
(sitting on a frictionless surface). When the
spring is released the blocks fly apart.
a) Draw the free body diagram showing the
forces acting on each object.
b) What is the acceleration of the lighter block
relative to the heavier one?
Example 3
Two skaters, a girl of mass 45kg and a 65 kg
boy push off of each other. The boy
accelerates at 2m/s2. Find the girl’s
acceleration.
Example 4
A student of mass 55kg is standing on the
surface of the earth.
a)What is the force that the earth pulls
the student with?
b)What is the force with which the
student pulls the earth?
c)What is the acceleration of the earth
because of the pull of the student?