Interactions

Interactions – Forces and Momentum

• Try the following… MAP tutorial• This suggests a tentative definition:

An interaction is a physical relationship that changes the motion of an object

Forces…

• Ways of describing forces: powers of 10 applet

– Contact– Long-range

• Modern Discussion of Forces…MAP– Comparison of Coulomb and Strong-nuclear force

Four Fundamental Interactions

• Gravitational Interaction– Binds stars and

planets together– Controls large-scale

structure of the cosmos

• Electromagnetic• Weak Nuclear• Strong Nuclear

• Aristotelian Thinking Diagnostic Test– Example

Sample Problems:

Forces from Motion

Key Ideas…

The Kinematic Chain

Net-Force Diagrams Draw FBD Express forces in vector form Combine Net force implies acceleration

( ) ( ) ( ) ( )d dr t v t v t a tdt dt

Practice Questions…

Graph shows motion of two trains on parallel tracks and who beside each other at time t = 0 s. Describe their motion.

Do the trains ever have the same speed? If so when?

Do the trains ever have the same acceleration?

Practice Questions… A car rounds the top of a circular

hill at a constant speed. Draw a force diagram and identify

all forces Draw the net-force How will the force diagram change

as the speed changes? Is it possible for the normal force to

be zero? What would “zero normal force” mean?

Is there a maximum speed for which the car can drive over this hill and still remain in contact with the road?

Key Ideas… There are 4 fundamental forces Forces can play many roles:

A force can cause a centripetal acceleration A force can cause a linear acceleration A force can be a normal force

We often describe forces in terms of what they do: Normal force, tension force, thrust, lift, frictional force, …

Forces are vectors and can be described by special vector diagrams called Free-Body Diagrams (FBDs)

Forces can be short range (contact) or long range (act over a distance)

All forces can (in principle) be defined operationally as F = ma