Download NEWTON`S LAWS OF MOTION

NEWTON’S LAWS OF
MOTION
The 2nd and 3rd Law
REVIEW
• NEWTON’S FIRST LAW OF MOTION:
Every object continues in its state of rest,
or uniform velocity in a straight line, as
long as no net force acts on it.
• MASS: measure of the inertia of an object
• FORCE: measure of the magnitude and
direction of the interactions
Newton’s Second Law of Motion
• Why objects change velocity?
• What happens if force acts:
-in direction of motion
-opposite the direction of motion
-sideways
• What is the relation between acceleration and
force?
• What is the relation between mass and force?
Statement of the 2nd Law
• The acceleration of an object is directly
proportional to the net force acting on
it, and is inversely proportional to its
mass. The direction of the acceleration
is in the direction of the net force
acting on an object

a

F

F
m
-vector sum of all forces acting on the object (net force)
• If the mass is
constant we can write


 F  ma
EQUATION OF MOTION
Force vector can be written in component form in rectangular coordinates as
F
x
 max
F
y
 may
F
z
 maz
Find the acceleration...
F2=5N
F1=10N
m = 5kg
…and some more
F1=10N
m = 5kg
Newton’s Third Law of Motion
• Whenever an object
exerts a force on a
second object, the
second exerts an
equal force in the
opposite direction on
the first.


F12   F21
Force on an object and by an
object
•
•
•
•
How is a car accelerating forward?
Which force makes us walk?
Why you feel hurt when you kick the wall?
How do birds fly?
IMPORTANT: only force exerted on an
object influences the motion, not the force
exerted by an object itself
Force exerted
by runner
on the ground
Force exerted
by the ground
on the runner
Direction of motion
Force by tyre on the ground
Direction of the tyre rotation
Force by ground on the tyre
Force that wall exerts on the person
Force that person exerts on the wall
Air pushes the wing up
Wing pushes the air down
Questions
• 1. While driving, Anna observed a bug
striking the windshield of her car.
Obviously, a case of Newton's third law of
motion. The bug hit the windshield and the
windshield hit the bug. Which of the two
forces is greater: the force on the bug or
the force on the windshield?
Rockets are unable to accelerate in space
because ...
A-there is no air in space for the rockets to
push off of.
B-there is no gravity is in space.
C-there is no air resistance in space.
D-... nonsense! Rockets do accelerate in
space.
• . A gun recoils when it is fired. The recoil is the
result of action-reaction force pairs. As the gases
from the gunpowder explosion expand, the gun
pushes the bullet forwards and the bullet pushes
the gun backwards. The acceleration of the
recoiling gun is ...
A-greater than the acceleration of the bullet.
B-smaller than the acceleration of the bullet.
C-the same size as the acceleration of the bullet.