Forces and Motion: Understanding the Interactions

1. Chapter 6 Force and Motion

2. An object that experiences a push or a pull has a FORCE exerted on it. • The object is called the SYSTEM. • The world around the object that exerts forces on it is called the ENVIRONMENT. • Forces have both magnitude and direction and are therefore _____________.

3. Contact vs. Long-Range Forces • Contact forces – Act on an object only by touching it. • Long-Range Forces – Exerted without contact. Ex. The force of gravity. • Each force has a specific, identifiable, immediate cause called the Agent. Ex. Gravity – Earth’s mass • VECTORS.

4. Newton’s Second Law of Motion • Fnet = ma or ∑ F = ma • Or a = Fnet/m • Recall: vectors are added and subtracted in one direction at a time only. Acceleration is a vector quantity. Mass is a scalar quantity.

5. Newton’s First Law • Known as the Law of Inertia. • Equilibrium – Net forces are zero! • Free-Body Diagrams • Net Forces = sum of all forces

6. Constructing Free-Body Diagrams • Sketch the Problem. • Choose a coordinate system. • Locate EVERY point where the environment touches the system. • Draw a motion diagram including the velocity and acceleration vectors. • Draw the Free-Body diagram. • Check your answer. –vector addition!

7. Review Ch. 6 Sec. 1 For the following ten items, Identify as: a. Contact Force b. Long-Range Force c. Not a Force Yes this is a quiz!

8. Weight • Mass • Inertia • Push of a hand • Air drag • Spring force • Acceleration • Friction • Tension • Mass times acceleration

9. 6.2 Using Newton’s Laws • Newton’s second law gives us a connection between the net force exerted on an object and its acceleration. The law identifies the cause of a change in velocity and the resulting displacement. • Aristotle’s followers believed that the heavier an object the faster the fall. Galileo hypothesized that all objects, no matter their weight, gain speed at the same rate.

10. What is the weight force Fg, exerted on an object of mass m? • Fg = mg • Scales – what do they measure?

11. Apparent Weight: • What would happen to the readings on the bathroom scale if you took readings in an elevator? • What would happen to the readings if the cable holding the elevator were to break? • Does this mean you have no weight?

12. Friction: • Imagine pushing on a crate and it not moving. You push harder and it still does not move across the floor. • Describe the forces acting on the crate. • The force opposing you moving the crate from rest is called STATIC FRICTION.

13. Static friction: • http://www.physclips.unsw.edu.au/jw/weight_and_friction.htm#coefficients • Kinetic friction: • http://www.physclips.unsw.edu.au/jw/weight_and_friction.htm#coefficients • Ffkinetic = μkFN • 0 ≤ Ffstatic ≤ μsFN • Some common coefficients are in table 6-3, pg 131

14. Causes of friction: • When two surfaces touch, they temporarily bond. In order to move the object one must break this bond. When objects are moving past each other, there is still an electrostatic attraction at the atomic level and this is the weaker kinetic friction.

15. 6.3 Interaction Forces • Identifying Interaction Forces. • System and Environment. Consider two systems whose motions you want to study. Recall that the environment is all the other systems whose motions are not being studied. • Ex. – Catcher catching a baseball. There are external forces acting on both systems and also interactions between the two systems.

16. F handon ball F ballon hand OR FAon B FB on A • Question? • Does this mean the ball causes the hand to exert a force? • The two forces either exist together or not at all.

17. Newton’s Third Law • An interaction pair is two forces that are equal in magnitude but opposite in direction. • *ALL FORCES COME IN PAIRS.

18. Fundamental Forces • Four Fundamental Forces. • Gravity • Electromagnetic Ex. Static cling, molecular bonds. • Strong Nuclear – acts between protons and neutrons. • Weak Nuclear – some kinds of radioactive decay. • Goal: To show that at some level, all four interactions are really one!

19. Forces on Ropes and Strings • Identify the Force Pairs.

21. If the rope breaks, the bucket will fall, so there must be a force holding the rope together. The force that the top part of the rope exerts on the bottom part is F TOP ON BOTTOM • Newton’s Third Law states that this is part of an interaction pair. So…. F BOTTOM ON TOP wouldbe the other half.

22. These forces are equal in magnitude but opposite in direction. • If the bucket is in equilibrium, the net force must be zero! So the tension on the top of the rope must be the weight of the bucket. • Tug of war example.

23. Frictionless pulley m1 m2 6.4 Pulleys • Examine the following diagram and label the forces.

24. Frictionless pulley m1 m2 • If m1 = 0.5 kg and m2 = 0.2 kg what happens to the system? • What does the pulley do?

25. PSS • Separate the system from the environment. • Draw a sketch of the problem with a coordinate system. • Identify all forces on the system and add them to your sketch • Draw a free-body diagram • Use Newton’s Second Law to equate the forces • Solve the problem

26. Online practice using FBD • http://www.glenbrook.k12.il.us/gbssci/phys/shwave/fbd.html • http://www.physicsclassroom.com/Class/newtlaws/newtltoc.html