Transcript 1.2 Newtons 3 laws

Isaac Newton England
mid-1600s
NewtoN’s
3 Laws of
Motion
This is really Galileo’s
Law of Inertia
Newton's First Law
"An object at rest remains at rest, and
an object in motion continues in
motion with constant velocity unless
it experiences a net external force."
Here the net
external force
is zero.
That doesn’t
mean that no
forces are
acting….they
could justadd
up to zero.
Here, the normal reaction force and
weight cancel
Does the normal force of the floor always
cancel your weight?
Only if the floor is horizontal !
Newton's Second Law
• "The acceleration of an object is directly
proportional to the net external force acting
on the object and inversely proportional to the
mass of the object.“
a=F/m
or as commonly known
F= ma
Newton's Second Law
• What would increase the rate at which this guy
accelerates backwards?
Newton's Third Law
For every action there is an equal
and opposite reaction.
Newton's Third Law
The amount of friction between two
surfaces depends on 3 factors:
A man pushes a 50 kg load on a cart with 90
Newtons of force . Find the cart’s acceleration.
Wto men each push a
50 kg load on a cart
with 90 Newtons of
force each. Find the
cart’s acceleration.
A man pushes a 100 kg load on a cart with 90
Newtons of force . Find the cart’s acceleration.
Free body diagram:
a drawing of all the forces
acting on a body. It is helpful to isolate the body first
by drawing a circle around it. All objects that cut the
circle provide a contact force. Invisible forces that
may be present are weight and friction.
The force of gravity I also called…..
Fgravity
W = mg
weight
W
Where g = 9.81 m/s2
and is called
“the acceleration of gravity”
The General Strategy
for solving problems involving force
1. Draw FBD
2. Say  F = ma
in one direction
at a a time.
3. Solve for
desired variable
A Static Problem: where  F = 0
1. Draw FBD
A 300 N flower basket is held up by two vertical
ropes. Find the tension in each rope.
2. Say  F = ma
in one direction
at a a time.
3. Solve for
desired variable
As you pull an object, friction
grows to stop you, until the rough
interlocking ridges between
surfaces finally gives. This
maximum value is called “static
friction”
After your moving, the rough
ridges don’t lock as tightly. You
have less friction…..
Called “kinetic friction”
A 4 kg block is pushed up with 50 N of
force. Find its acceleration.
Sally pushes an old fashioned lawnmower
at a constant speed of 3 m/s. She pushes
with 100 N at an angle of 65°.
Draw a FBD and find the
vertical and horizontal
components of her push.
How much friction must there be?
Draw FBD
NO FRICTION
Find the horizontal force on the box.
Find the vertical component of the pull of the rope.
If the box has a mass of 8 kg
Find its acceleration
Find its weight
Find the normal force that
the ground pushes up.