1. Interactions

2. Interactions – Forces and Momentum Try the following… MAP tutorialMAP tutorial This suggests a tentative definition: An interaction is a physical relationship that changes the motion of an object

3. Forces… Ways of describing forces: powers of 10 appletpowers of 10 applet –Contact –Long-range Modern Discussion of Forces…MAPForces…MAP –Comparison of Coulomb and Strong-nuclear forceComparison of Coulomb and Strong-nuclear force

4. Four Fundamental Interactions Gravitational Interaction –Binds stars and planets together –Controls large-scale structure of the cosmos Electromagnetic Weak Nuclear Strong Nuclear

5. Aristotelian Thinking Diagnostic Test –ExampleExample Sample Problems:

6. Forces from Motion

7. Key Ideas… The Kinematic Chain Net-Force Diagrams  Draw FBD  Express forces in vector form  Combine  Net force implies acceleration

8. 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?

9. 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?

10. 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