Unit 4 Free Particle Model

This unit differs from the others in that instead of starting with an actual experiment you start with a thought experiment.  From the reading it is really clear that this is an important concept  and some time needs to be spent on the at rest condition.  Some things to discuss during this initial conversation.

·         Are there forces on the book sitting on a desk?

·         Come up with a list of what a force is

·         Come up with an operational definition.  Make sure the students know that it may be refined

·         A force must be from an object so cross off anything that is not an object.  An object is something I can hit you with or buy in a store.

First rule of forces—You must name the physical object applying the forces.  I think it is going to be really helpful to have students label the forces with the object that is acting on the object in question.

Types of forces

Contact forces	Non contact forces (spooky)
Everything else  The key is contact	Gravity
	Magnetism
	Electrostatic
	Nuclear forces (strong and weak)

We then developed a graphical representation for forces, a force scheme as a way to keep track of the forces that are present solid line for contact forces dashed line for non-contact forces. Then transition to force diagrams (free body diagrams) use a dot to represent the object.  Use arrows and the name of the object exerting the force.  Introduce the metric unit of force the newton.

I’m a little unclear as to how to introduce the idea that if the velocity is zero then the forces are balanced without just giving them that on authority (use spring scales to measure forces).  This is also where the idea of the surface force is developed using the springs and the sagging white board.  “How does the table know how much force to exert?”

Inquiry lab—What is the relationship between the force the earth pulls and the mass of the object?  Come up with a relations ship between kg and N.

 The concept of the surface exerting a force is a very important concept to cover so that some very common preconceptions can be over come.  To deal with this we used springs and sagging surfaces.

Force rules (these will be different because the students will be coming up with operational definitions but this is what I’m shooting for)

1.       When naming forces you must use the name of the object (and they must be made of matter)

2.       If the Dv =0 then the forces balance

3.       Surfaces push with a force perpendicular to themselves

4.       Strings pull in the direction parallel to the string

5.       Acceleration and unbalanced force are always in the same direction

6.       Surface forces will compensate to balance the forces (like a spring)

Use a hover disk to answer the question what is the force that keeps the disk moving   This is really important to counter the preconception that a force is needed to keep an object moving (impetus) So now the rule can be change from v=0 to delta v=0) A related question that can be asked is why do you need to keep pedaling on a bike?  What gives you a clue that the wind is exerting  a force in this case?

The idea of components is then introduced but we used ‘shadow arrows’ which I think will be a little clearer when forces are not inline.  Having taught pre-calculus I think that I will definitely relate things to components to help those student who will take pre-calculus.  Depending on the level of the class we can delve into the trigonometry if needed otherwise just focus on the qualitative concept of the forces balancing.

Some other concepts that were explored to help overcome some preconceptions dealing with Newton’s 3^rd law force pairs.

2N block on 4N block which block pushes more?

A critical competitor 4N on 2N

All forces come in pairs that are equal in strength opposite in direction and act on the other object.

Ask the students to tell you how to walk (or some other simple task involving an equal force in the opposite direction.

Come up with Newton’s Third Law force pairs.

Bike swimming canoe helicopter bullet in gun

What do you notice about each pair?

Force of bike on ground—Force of ground on bike

We also used the force probes and looked at the relation between the forces between two carts in various situations.

Some Extras that we covered during this unit

·         Praise the effort not the intelligence growth mindset verses fixed mindset.

·         Use demos for entertainment and as discrepant events.

Some Problems I can forsee

After 8 weeks or so of me doing little talking there might be some rebellion at the first part of this unit which involves a lot of discussion.  Hopefully the students can be brought into the discussions to be active participants.