Download PES 1110 Fall 2013, Spendier Lecture 11/Page 1 Today

PES 1110 Fall 2013, Spendier
Lecture 11/Page 1
Today:
- Newton’s 3rd law
- Quiz 2
- Practice exam: Is longer than the exam in one week! Please read the instruction on the
cover sheet carefully since these are the rules that will be applied. I am not able to give
the exam next week due to a on campus visitor. The exam will be proctored by Prof.
Adams.
Newton’s 2nd law:


 F  ma
net
Example
A worker applies a constant horizontal force of 80.0 N to a block of ice on a smooth
horizontal floor. The frictional force is negligible. The block starts from rest and moves
11.0 m in 5.00s. a) What is the mass of the block of ice?
b) If the worker stops pushing at the end of 5.00 s, how far does the block move in the
next 5.00 s.
PES 1110 Fall 2013, Spendier
Lecture 11/Page 2
We need a third law because we still have unexplained motions!
I promise that Newton’s laws explain every motion (besides the two special cases I
mentioned in lecture 9).
How does anybody walk up steps? You push on the steps. Which way are you pushing on
the steps? You push down. What does the second law say about this? Not the third law
yet. This is why we need a third law. The second law says, if I push down on the steps
that downwards force tries to make the steps accelerate downwards. We don’t succeed
because how much mass do these steps have? A lot! The second law is about what’s
being pushed. If I push down on the steps this attempts to make the steps accelerate
downwards. It says nothing about you or me walking up the steps. But it is all about me.
So far what happens to me is unexplained. What does happen to me when I push down on
the steps? You know this since you have walked many steps in your life. When you push
down on the steps you yourself get also pushed up. That is what explains the motion here.
By pushing down on the steps you are also pushed up by the steps.
(There is a small acceleration on the steps, i.e. when you walk on old wooden steps they
sometimes make a sound when you step on them. This is the result of you pushing down,
your downwards force on them. In this case you get a bit more of a noticeable
acceleration.)
Newton’s Third Law
For every action, there is an equal but opposite reaction.
I have two things A and B, it could be me and the steps. One of them reaches out and
exerts a force on the other one
How do these forces compare? They are always equal in magnitude but opposite in
direction.


Third Law: FB on A = - FA on B
(as we know the negative sign flips the vector by 180º)
This notation is a bit tedious but it is the best there is!
Note in order to walk I am not just pushing on the floor; I am pushing backwards on the
floor in order to get a forward push. Basically all forms of propulsion are third law. How
do you swim? You push on the water. Which way do you push on the water? Backwards,
so the water pushes you forward. For a rocket, we have trust force. The rocket is
PES 1110 Fall 2013, Spendier
Lecture 11/Page 3
expelling hot gas. By pushing the hot gas backwards the rocket is pushed forwards. So
we see that the third law is all over the place.
But the problem becomes when you are not careful. What happens when I add equal and
opposite forces together? I will get zero. So it is easy to get yourself in this mental loop
that nothing can ever move. What is the way out of this?
Action and Reaction are applied to different objects!
I push down on the steps (steps are one object) and the steps push on me (I am the second
object).
Example:
Whenever Lionel Messi kicks a soccer ball, which of the following is a true statement?
(a) Messi exerts a larger force on the soccer ball than it exerts on him.
(b) The soccer ball exerts a larger force on Messi than he exerts on it.
(c) Messi exerts an equal force to the one the soccer ball exerts on him.
(d) Sometimes the force exerted by Messi is larger than what the football exerts on him.
It depends on how hard he kicks it.
(c) Messi exerts an equal force to the one the soccer ball exerts on him.
If this is true, why does the ball not just stay at its initial position? We know that the ball
will not stay! This is a case when the free body diagrams are useful. Because I have two
objects here I am interested in I have the man and the soccer ball. Each of the get there
own free body diagram because they are separate things.
(air resistance is put into friction). Notice that we drew the arrow for KMonB much longer
than the arrow for fB since we know that the ball will accelerate.
Messi has soccer shoes on so the friction is very large (exactly the same size as the
backwards kick force). The amount of friction between Messi and the ground is a lot
larger than the friction between the ball and the ground. The friction does not have to be
PES 1110 Fall 2013, Spendier
Lecture 11/Page 4
the same because it is acting on two different things. Also notice that the normal force is
a lot bigger because Messi has a lot more mass than the ball.
Net force on the man: zero  he stays at rest
Net force on the ball: forward  forward acceleration.
End result: Ball moves, Messi does not.
Example
An apple sits on a table in equilibrium. What forces act on it? What is the reaction force
to each of the forces acting on the apple? What are the action-reaction pairs?
Right now we are all pulling up on the earth. We all do not exert enough force to get a
noticeable acceleration since the mass of the earth is so huge!