Download Lecture 4 - Newton`s 2nd law

Goal: To understand
Newton’s 2nd law.
1)
2)
3)
4)
6)
Objectives:
To understand Newton’s Force Law
To learn about how Weight works
To learn about the basics of Friction
To understand Some applications for
Friction
5) To understand Freefall
To understand Non-free fall motions
Law 2: Force Law
Force = mass * acceleration
Force is how much you are pushing or pulling on
something
Mass is how much stuff you have (NOTE: mass is
NOT weight!!!)
Acceleration is the rate at which you change your
speed.
Net force = mass * net acceleration
F=m*a
• If my mass is 100 kg and the gravitational
acceleration on the surface of the earth is
9.8 m/s2 then what is my “weight” in
Newtons on the surface of the earth?
• The moon has 1/6th the gravity as the
earth. What is my weight on the moon
and what is my mass on the moon?
Friction
• One type of force is friction.
• Nothing is perfectly smooth.
• Everything has microscopic pits and parts
that stick out.
• The smoother the surface the less of these
they have.
• Friction is the colliding of these two rough
surfaces.
Advantage friction
• Friction can be a hindrance – but it can
help also.
• When you walk, you are using friction to
your advantage!
• You are setting it up so that you get a force
applied to you – but more on that in
chapter 5.
2 types of friction
• There are 2 types of friction.
• The first is static friction.
• When objects are at rest then tend to
settle a bit and make it hard to push them
apart.
• This type of friction has a great force.
Sliding friction
• Once an object starts moving it can go over
most of the valleys of the other surface, but still
gets slowed a bit.
• This type of friction is slower.
• Concept question:
• Which object will have a greater static friction
force on it:
• A 5 lb hockey puck sliding across a floor at 50
miles per hour or a 5 lb hockey puck sliding
across the same floor at 100 miles per hour?
Air resistance
• Sort of like friction – basically it is friction
with the air (liquids do something similar,
but at a greater scale).
• It slows you down by applying a force to
you.
• The more your air resistance the slower
you fall (the force depends on air velocity).
Earth vs. space
• Ignoring friction or resistance, what is the
acceleration on a 10 kg wheel being pushed with
a force of 100 N in the horizontal direction?
• What is the weight of the wheel?
• What would the wheel weigh on the moon (1/6th
gravity)?
• Now for the clincher, if you applied a 100 N force
to the wheel on the moon in the horizontal
direction then what would its acceleration be?
Demo time!
• Need a volunteer.
• We will push off of each other on
skateboards (hopefully with equal force).
• Now, some questions?
• Which one of us will reach the wall first?
• Will one of us have a faster speed? If so
which one of us.
Acceleration
• Acceleration = net force / mass
• So, with equal forces, the smaller the
mass, the bigger the acceleration.
• Half the mass means double the
acceleration.
But
• Um but didn’t you say that gravity pulls
everything at the same rate?
• Yes!
• So, what does that tell you about the force of
gravity?
• If object A has gravitational force F applied to it,
then what is the gravitational force on object B if
B has twice the mass as A?
Follow up
• An astronaut is in orbit around the earth.
• The mass of the astronaut is 100 kg and the
mass of his spaceship is 5000 kg.
• What are the “weights” of the astronaut and
spacecraft (hint what is the net force on them?)?
• Suppose the astronaut applies a 1000 N force
on the spacecraft. What will the acceleration of
the spacecraft be?
• If the same force were applied to the astronaut,
what would the acceleration of the astronaut be?
• Why are the two accelerations so different?
Free fall
• If you can ignore wind resistance then as
you fall you experience “free fall”.
• That is just a fancy way of saying that you
are accelerated at the full rate of gravity
(10 m/s2).
• If a stone of mass 1 kg is in free fall then
what is the net force being applied to it?
Not free fall
• But what happens if you have wind
resistance?
• The wind resistance decreases the net
acceleration.
• How does it do this (hint, think net force
and think direction)?
Forces add
• If you have more than 1 force on something,
they add.
• If the forces are in opposite directions, you still
“add” them, but one is a negative number, so
you are adding a negative number.
• So, 5 N + (-2N) = 3N
• That is, if you had an object weighing 5 N and it
was falling with wind resistance of -2N in the
down direction on it (or +2N Force in the up
direction) then its NET force would be 3N.
Net force vs acceleration
• 5 N + (-2N) = 3N
• If the object weighs 5N on the surface of
the earth, what is its mass?
• If the net force is 3N down then what is the
acceleration on the object?
Now lets do it with friction
• You push an object with a force of 299 N.
• It does not budge.
• You push it just a little harder (300 N) and it
starts to accelerate at a rate of 1 m/s2.
• If the mass of the object is 100 kg then find:
• A) The force of kinetic friction
• B) What is the Net force on the object (no it is
not 300 N)?
• C) What is the force of sliding friction on this
object?
Conclusion
• We have seen from Newton’s 2nd law that force
= mass * acceleration
• Weight is just the net downwards force. If
gravity is the only force other than the force from
the ground pushing back then your weight is the
same as your gravitational force.
• Your acceleration depends on your mass –
smaller masses have bigger accelerations with
the same force.
• If you are in freefall you have no weight
• If you are not in freefall then your net
acceleration depends on your net force.