1. FORCES IN ONE DIMENSION CHAPTER FOUR

2. FORCE Is a push or a pull exerted on an object. The object is called the system. Everything that surrounds the object is called the external world. Forces cause objects to speed up, slow down, or change direction.

3. Force is a Vector Force is a vector. So…Force has magnitude & direction. Forces are represented in standard vector fashion. Forces (and other vectors) can be represented by arrow-tipped line segments, whose length is proportional to force magnitude and point in the proper direction. Coordinate systems with the direction convention used in a situation need to be clearly indicated.

4. Two Force Types: Contact Forces Non-Contact Forces or Field Forces

5. Contact Force(s) Exists when an object from the external world physically touches a system and pushes or pulls it. Examples:

6. Field Force(s) Field Forces are exerted without direct physical contact. Examples: Gravity Magnetism Electricity

7. Some Types of Forces ( p. 94 ) ForceSymbolDefinitionDirection FrictionFfFf Contact force. Opposes sliding motion between surfaces Parallel to surface and opposite the direction of sliding NormalFNFN Contact force. Exerted by surface on an object Perpendicular to and away from the surface SpringF sp A restoring force. The push or pull a spring exerts on an object Opposite the displacement of the object at the end of the spring TensionFTFT Pull exerted by a string, rope cable (etc.) when attached to body and pulled taut Away from the object and parallel to the string, rope, etc. At point of attachment ThrustF thrust Type of force that moves rockets, planes, cars, people In the same direction as the acceleration of the object, discounting resistive forces WeightFgFg A field force due to gravity attraction b/w 2 objects. The Earth and 2 nd object Vertically down toward center of Earth or some other large mass.

8. Modeling Force Interactions Indicate coordinate system for direction. Draw a pictorial model. Label important components. Circle the system. Identify object’s point of contact with external world. Using particle model, draw a free-body diagram. Show all forces pointing away from object. Use blue arrows. Make each arrow length proportional to size of the force. Draw a particle model, motion diagram w/ Δd, v, and a.

9. Force, Mass & Acceleration This relationship is determined experimentally. Acceleration is proportional to Force. When force is doubled, acceleration is doubled. When force is halved, acceleration is halved. Acceleration is inversely proportional to mass. a = F/m or F = m x a

10. Units of Force (Net) Force = Mass x Acceleration 1 newton = 1 kg x 1 m/s 2 (mks) 1 dyne = 1 g x 1 cm/s 2 (cgs) 1 pound = 1 slug x 1 ft/s 2 (Brit)

11. Combining Forces Many objects are affected by more than one force (2 or more) at one time. Examples: To find the overall effect of multiple forces, you need to add them up. The sum of the forces (ΣF) is referred to as the net force (F net ) aka resultant force (F R ). F net = m x a

12. Newton’s Second Law F net = m x a or a = F net /m The acceleration of an object is equal to the sum of the forces acting on the object, divided by the mass of the object. Sample problem(s):

13. Fg = Weight F net = m x a Fg (weight) = m x g

14. Questions: What is the motion of an object with no net force (F net = 0) acting on it? If Fnet = 0, then a = 0. What is an object’s “natural state”? Either: Object is at rest (v = 0) OR Object is moving with constant velocity. (Constant speed + direction) (a = 0)

15. Newton’s First Law “An object that is at rest will remain at rest, and an object that is moving will continue to move with a constant speed in a straight line, if (and only if) the net force acting on the object is zero”. This law suggests that acceleration occurs only when Fnet ≠ 0. Sometimes this first law is referred to as the “law of inertia”.

16. Inertia Newton’s first law is sometimes referred to as the law of inertia. Inertia is the tendency of an object to resist change in motion. Inertia is not a force. Inertia is a function of mass. Mass is a measure of the inertia of an object.

17. Equilibrium If the net force on an object = 0, then the object is in equilibrium. If the net force ≠ 0, an object’s state of equilibrium is disturbed and the object must speed up, slow down, and/or change direction.

18. More Force Facts: It takes 2 objects to have a force. You can’t have a force with only one object. This interaction, called force, is about give and take. Object A pushes/pulls on object B and Object B pushes/pulls on object A

19. Newton’s Third Law Every action force is accompanied by an equal and opposite reaction force. The force of A on B is equal in magnitude and opposite in direction of the force of B on A. F A on B = - F B on A

20. The Four Fundamental Forces of Nature