Ch 4 – Newton’s 2nd Law of Motion (N2)
4.1 Day 3
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If an object is subject to a net force, this will cause it to
accelerate. Remember – No net force; no acceleration. No acceleration; no net force.
Soooo…acceleration is a direct result of a net force. That means acceleration ~ force (acceleration is proportional to force).
Q: If you push a crate with 4 times as much force, what would be the resultant acceleration (assuming no friction)?
Q: If the crate is rough on the bottom, what would be the comparative acceleration?
4.2 Using Newton’s Laws
Q: How does a bathroom scale work?
A:
So, spring scales measure weight, not mass!
Mass and Weight
Acceleration depends on force and mass. Mass is directly related to inertia. The more mass something has, the more inertia it
has. Mass seems most like weight, but they are different.
DEF: Mass =
DEF: Weight =
Dumbells and mass vs weight
Mass and weight are directly proportional to each other. Here on earth, that proportionality constant = 9.8 N, and is called g.
Sound familiar?
One way to see which of two objects is heaviest, you might move them back and forth, or shake them in each hand. What you
are really testing is to see which has more inertia. Those two objects would offer the same relative resistance to change of
position in your hands whether you were standing on
or on the
Units of Force
Lb’s
Kg’s
N’s
Q: What is a Newton, again?
A:
Q: Weight is a force, so what would the SI unit for weight be?
A:
DEF: Apparent weight =
So, your weight on a scale is your apparent weight, given your location.
.
Q: What would be your apparent weight in free fall?
A:
So weightlessness doesn’t mean your weight is zero; it just means there is no _______________pushing back up on you, so your
apparent weight is zero.
DEF: Weightlessness =An object’s apparent _____________that results when there are no contact forces pushing UP on
the object.
The proper units for measuring mass are kg, and for weight are Newtons. Remember: 1 N = 1 kg∙m/s2.
The meaning of 1 N is the amount of force required to accelerate a 1 kg mass to a rate of 1 m/s/s.
Q: How do you convert lbs to kg?
A:
How do you convert lbs to N?
A:
Q: Why multiply by 9.81 m/s2?
A:
Try These: 
1. Does a 2-kg brick
 inertia
 mass
 volume
 weight
as a 1-kg brick?
have twice as much:
2. Would it be easier to lift a cement truck
(assume identical friction)?
on earth or to lift it on the moon? What about pushing it horizontally
3. Is it better to say F = ma, or F ~ ma? Why?
4. If F = ma, then a = Fnet / m. What does this really mean?
5. If a = ∆v / ∆t, and a = F / m, does this mean that acceleration is the ratio of force to mass? How do these two statements
inter-relate?
6. Which falls faster in a vacuum, a feather
or a coin
? Do equal forces of gravity act on each?
Drag Force and Terminal Velocity
DEF: Drag is
 friction between a solid and a fluid, or between two fluids
 dependent on the nature of the fluids involved
 does depend on __________
 does depend on __________
Think about the wake produced by a boat. That wake is a visual representation of the effect of the
drag between the water and the boat. The bigger the boat, the more the wake. The faster the boat, the more the wake.
Increases in size / speed pushes aside more water. There is a maximum value, at which shock waves become relevant, but this
is enough for now.
Q: What force does a
experience when you push it right with a force of 110 N, and F s is left with a force of 100 N?
A:
Q: A
moves with a constant velocity of 1000 km / h when the eastward thrusting force of its engines is constant at
100,000 N. What is its acceleration? What is the force of the air resistance on the jet?
A:
When Acceleration is Less Than g – Non-Free-fall
If acceleration due to free-fall in a vacuum is Fnet / m, do you think it would be:
Less
same
morr
when there is air resistance?
a = Fnet _ = _mg_ - R__ = g - _R_
m
m
m
Which is / are true for the value (magnitude) of acceleration in free-fall?
a could be < g
a could be = g
a could be morr than g
Q: What is pi? Is it the same no matter what size a circle is?
For a rock,
g = F_
m
For a leaf,
g = _F_
m
The amount of air an object has to forge its way through, as well as the speed at which it must travel through that air, both
affect the resultant air resistance, or drag. When an object has a large area facing the direction of motion (like a leaf falling
down), the ratio of its area to its weight is large, meaning it doesn’t have to fall very far before F net (drag versus g) = 0. When
Fnet = 0, the object is no longer accelerating, and has thus reached terminal velocity. We use velocity to mean not only speed,
but direction. For a leaf, terminal velocity may only be a few centimeters per second. For a rock, it may be 40 km / h,
depending, of course, on the size of the rock.
Q: If you were diving out of a plane
, how could you minimize your terminal velocity?
A:
Consider two people parachuting out of a plane with equal sized parachutes.
and the girl weighs 550 N.
1. What has to happen for either of them to reach their terminal velocity?
2. Who will reach their terminal velocity faster?
3. Who will fall faster, if either?
4. Who has a greater terminal velocity?
5. Who reaches the ground first?
Let’s say, the guy weighs 1100 N,
Fun Fact Terminal speed for a twice-as-heavy person will be about 41% greater than the lighter person’s terminal speed,
because the “retarding force” of air resistance is proportional to the speed squared v man2 / vwoman2 = 1.412 = 2
As a sky diver jumps from a plane, her acceleration:
decreases
stays the same
increases
as she travels downward though the air.
It must ______________________________________________________________________________________
So, what does this do to Galileo’s findings in his experiments regarding free-fall? What does that tell you about the height of
the
?
See Ex 1, p 97 and Ex 2, p 99.
Ex 1, p 97
Anudja is holding a pillow, with a mass of 0.30 kg, when Sarah decides that she wants it and tries
to pull it away from Anudja. If Sarah pulls horizontally on the pillow with a force of 10.0 N left and
Anudja pulls back with a horizontal force of 11.0 N right, what is the horizontal acceleration of the
pillow?
anet =
Ex 2, p 99
Your mass is 75.0 kg, and you are standing on a bathroom scale in an elevator. Starting from rest,
the elevator accelerates up at 2.00 m/s2 for 2.00 s and then continues at constant speed. Is the
scale reading during acceleration greater than, equal to, or less than the scale reading when the
elevator is at rest?
Fnet =
Fscale =
Fscale = Fw
Fscale =
When there is a net acceleration,
Fscale =
This means you__________________________________.
What this really means is that if the elevator pushes up against you as it accelerates up toward you, you will “weigh” morr, as
the scale will get squished up against your feet!
So what happens to your apparent weight if the elevator accelerates on the way down?
Decelerates on the way down? Decelerates on the way up?
Accelerates on the way up?