Download Equilibrium & Newton`s 2nd Law of Motion

Equilibrium & Newton’s 2nd
Law of Motion
Do Now: What is the weight of a 500-g
ball on earth?
• 500g x 1 kg/103 g = 0.5 kg.
• 0.5 kg x 9.81 m/s2 = 4.9 N.
Review: Law Of Inertia
Newton's 1st Law - If there is no net
force acting on a body, then it will
continue in it's state of constant
velocity (which may be zero).
Mathematically.
Add all the forces acting on a body.
R = 0 - constant velocity or zero.
It is at equilibrium – acceleration = 0
What if there is Fnet.
• The object will accelerate in direction of
Fnet.
• We sketch free body diagrams to add up
the forces and find Fnet.
A car speeding up to the right on a
horizontal road. Friction is there too.
The net force acting on an object is the
vector sum of all the forces acting on it.
Find the net force on each – it is helpful to
treat horizontal (x) & vertical (Y) separately.
10 N
3N
3N
6 N, up
7N
3N
4N
4 N, right
What is the motion of each box?
10 N
3N
3N
6 N, up
7N
3N
4 N, right
4N
Accl up
Accl right
Objects not in Equilibrium will
accelerate in the direction of the net
force.
But how much?
“Law of Acceleration”
a =
Fnet
Fnet = m a
m
m - mass in kg
a - m/s2.
F - Newtons
1 N = 1kg m
s2
(N):
Fnet means sum the forces in one direction.
A constant net force causes
constant/uniform acceleration!
More massive objects require
more force to accelerate!!
Finding acceleration
• Sketch a free body diagram show
all forces on one object.
• Calculate magnitude & direction of
Fnet.
• Use a = Fnet/m to find a in any
direction where Fnet ≠ 0.
1. A 2000-kg car’s engine exerts a westward
force of 12500-N. Friction between the tires
and the road is 500-N.
• Sketch the free body diagram.
• Calculate Fnet
• Calculate the acceleration.
• a = (-12,500 – 500) N/ 2000 kg
• a = - 6 m/s2 or 6 m/s2 or west.
2. An 80-kg man who weighs 800-N fell out of
an airplane. As he falls air resistance pushes
him up with a force of 150-N.
• Sketch the free body diagram.
• Calculate his acceleration as he falls to Earth.
• Why is his actual acceleration different than g?
Fair.
•
•
•
•
•
•
•
Fg = mg = (80kg)(9.81 m/s2)
= 785 N down
Fnet = (785 – 150) N
Fnet = 635 N (down)
a = Fnet / m
635 N / 80 kg =
7.9 m/s2.
Fg.
3. Find the acceleration of the box below.
Assume the box has mass 2.5 kg.
3N
8N
4N
7N
3N
X & Y forces separately.
Vertically, we have +3N(up) and –3N
(down), so they add to zero.
Vertically, a = 0.
Horizontally, +8N +4N (right), -7N (left), so Fnet
is +12 – 7 = +5 N.
a = F/m
+5N / 2.5 kg = 2 m/s2.
Acceleration to the right.
example prb’s
Do Now:
The forces on the man in the
elevator are balanced. Which
of the following are possible?
He could be:
•
•
•
•
Fn
1. Moving down.
2. At rest
3. Moving up
4. In freefall.
Fg
Sketch the graph of:
• Acceleration on the X axis
• Force on the Y axis.
• What is the slope?
Graphs of Newton’s 2nd Law
• Write the equation:
• Fnet = ma
• Slope =
• mass.
Sketch the graph of:
• Force on the X axis
• Acceleration on the Y axis.
• What is the slope?
Graphs of Newton’s 2nd Law
• a = Fnet/m.
• Slope =
• 1/mass
Ex: Sketch the graph of acceleration vs. mass.
• a = Fnet/ m
• How would you find Fnet?
• Transform to linear, find slope.
• Acceleration vs. 1/mass. Slope = Fnet.
• At constant velocity, you feel normal.
• Because a requires Fnet,
• you feel acceleration!!
• There is Fnet on you, you feel pushed or pulled.
• Inertia makes it hard stop your state of motion!
Inertia is also noticed when one object has Fnet
and accelerates the another doesn’t
• Car goes forward. Coffee does not!
Hwk Handout
Review.
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Newton 1
Newton 2
Weight
Friction.
Normal force.
Prac Set
If you are not accelerating
toward the Earth, then some
force is balancing your weight!
You feel acceleration!!
If there is a net force on you, you
feel pushed or pulled.
If you are moving at a constant
velocity, you feel normal.
Newton’s 3 Laws 4.5 minutes.
http://www.youtube.com/watch?v=iH48Lc7wq0U
Newton 2
6 minutes.
• http://www.youtube.com/watch?v=Wzvhu
Q5RWJE&feature=relmfu
Or Weight with spring scale.
Equilibrium
• When all forces are balanced they are in
equilibrium.
• How can we determine 3 force could be in
equilibrium?
• See Starreview pg 83.
Ted Elevator Forces
• http://ed.ted.com/lessons/would-youweigh-less-in-an-elevator-carol-hedden