Download Lecture 5

“If at first you don't succeed, then
skydiving definitely isn't for you.”
Anonymous
Course web page
http://sdbv.missouristate.edu/mreed/CLASS/PHY12
3
Cell phones put away when class begins please.
Announcements
Reading: Chapter 2, sections 1-3;
Chapter 3 section 1; and Chapter 4, all.
HW2 is on WileyPlus now and due Friday
by 5pm.
Sample problems also on WileyPlus.
Test 1 is a week from Wednesday.
Chapter 4:
Forces and Newton's
Laws of Motion
Newton’s 2nd Law
F = ma
where:
•F = force in Newtons
•m = mass in kilograms
•a = acceleration in
2
m/s
Forces have direction. What is the
net force on this box?
What's the first thing to do?
Forces have direction. What is the net
force on this box?
Decide on axes and break into
components.
+Y
162
+X
Forces have direction. What is the net
force on this box?
Break everything into components.
+Y
F1x=-258.7
F2x=263.3
F3x=0
162
F1y=84.1 +X
F2y=282.3
F3y=-441
Forces have direction. What is the net
force on this box?
Solve the components
+Y
162
Fx= -258.7+262.2+0=4.9
Fy= 84.1+282.3-441=-74.9
+X
Forces have direction. What is the net
force on this box?
Combine into resultant force (vector).
+Y
162
F = (4.92+(-74.9)2)1/2
=74.8 N
+X
q = tan-1(-74.9/4.9)
= -86.3o
Note that Y is negative
and X is positive.
Forces have direction. What is the net force
on this box?
What's the acceleration of the box?
Use Newton's 2nd law: F=ma → a=F/m
+Y
162
F = (4.92+(-74.9)2)1/2
=74.8 N
+X
q = tan-1(-74.9/4.9)
= -86.3o
Forces have direction. What is the net
force on this box?
What's the acceleration of the box?
1.7 m/s2 in the same direction.
+Y
162
F = (4.92+(-74.9)2)1/2
=74.8 N
+X
q = tan-1(-74.9/4.9)
= -86.3o
•Do heavier objects
fall faster since they
have a greater
mass?
No. Galileo proved that
g=constant.
•For gravity, we write F=mg since
2
g=-9.8m/s for us on Earth, always.
•Weight is a force caused by
gravity.
•Weight depends on the strength
of gravity.
•Mass is the amount of 'stuff' an
object is made of, and does not
change when gravity does.
Forces
• If anything is changing its velocity,
it has acceleration and it is being
acted upon by a force.
• And therefore follows F=ma
Fundamental Forces
1) Gravity
2) Strong Nuclear Force
3) Electroweak Force
But we will mostly use gravity and other
forces which are a result of 3.
Forces we will use this semester:
1) Gravity
2) Normal
3) Friction
4) Tension
5) Spring
Let's see how they work!
Newton’s 3rd Law
•For every action
there is an equal
and opposite
reaction.
Thought
experiment
•Gravity is acting
upon you. Why
don't you fall
through the floor?
Catalog of Forces
• Weight – gravitational pull of the Earth on
an object and is always directed toward
the Earth. It is a long-range force.
• Normal Force – reaction force of a surface
to counter gravity. Normal forces are
forces that can oppose other forces acting
downward and put the object in
equilibrium (in relation to the surface).
Identical stickmen stand on the ground. Weight
presses them down, the normal force of the ground
holds them up. One is supporting some of his weight
on a rope. Is the force from the ground the same on
each stickman?
Identical stickmen stand on the ground. Weight
presses them down, the normal force of the ground
holds them up. One is supporting some of his weight
on a rope. Is the force from the ground the same on
each stickman?
NO The
ground only
applies as
much force
as is
necessary.
Gravity (weight) pulls the boxes down,
but they cannot go through the surface:
the normal force pushes back. Always
perpendicular to the surface.
There is a net resultant force on the box
on the right. Will the box slide down the
slope?
There is a net resultant force on the box
on the right. Will the box slide down the
slope? Not if there's friction to oppose
the net force.
Ffs
Friction
• Friction is the force between an object and
the surface it is on. Static friction is the
force that keeps objects “stuck” in place
and acts in the direction opposite the
object would move. Kinetic friction is
friction that opposes motion once the
object is moving and acts in the opposite
direction the object moves.
• Friction always acts parallel to the surface.