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CHAPTER 4 FORCES and the LAWS of MOTION

CHAPTER 4 FORCES and the LAWS of MOTION. THE NORMAL FORCE. a force that acts on a surface in a direction perpendicular to the surface. F n. THE NORMAL FORCE.

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CHAPTER 4 FORCES and the LAWS of MOTION

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  1. CHAPTER 4 FORCES and the LAWS of MOTION

  2. THE NORMAL FORCE a force that acts on a surface in a direction perpendicularto the surface Fn

  3. THE NORMAL FORCE the television is in equilibrium, so the normal force from the table must be equal in magnitude and opposite in direction to the gravitational force exerted on the television. Fn

  4. THE NORMAL FORCE Fn is ALWAYSperpendicular to the surface, but NOT ALWAYS opposite to the Fg Fn = mg ∙ cos

  5. The Net Force • 4 forces acting on the car • Add them as vectors • The resultant is then the NET FORCE • If the NET FORCE is 0, then you know that the car is travelling at constant velocity (zeroacceleration) Is the vector sum of all forces acting on the body

  6. Free Body Diagrams (FBDs) are diagrams used to show the relative magnitude and direction of all forces acting upon an object in a given situation.

  7. Free Body Diagrams (FBDs) • 4 forces acting on this object, but there MIGHT be more or less forces • Represent the object as a SQUARE or a CIRCLE • You MUST show all the forces acting on the object

  8. Free Body Diagrams (FBDs) A puck is sliding on a sheet of ice to the right FA Fapp Ffriction How many forces are acting on the puck? Fgravity Label all of them!

  9. Free Body Diagrams (FBDs) A puck is sliding on a sheet of ice to the right FA Fapp Ffriction If the object is moving, you MUST show the direction of acceleration Fgravity

  10. Free Body Diagrams (FBDs) A puck is sliding on a sheet of ice to the right a FA Fapp Ffriction If the object is moving, you MUST show the direction of acceleration Fgravity

  11. Interpreting FBD (Quantitative = with numbers) NET FORCE?? a FA Fapp Ffriction Fgravity

  12. Interpreting FBD (Quantitative = with numbers) NET FORCE?? a Fnet= 0 N in verticaldirection 40 N 10 N 25 N Fnet= + 15 N in horizontaldirection 40 N

  13. Interpreting FBD (Quantitative = with numbers) a 47 N Fnet= 0 N in verticaldirection 100 N 100 N Fnet= 0 N in horizontaldirection 47 N If the Net Force is 0 (Fnet= 0 N), the object is in EQUILIBRIUM

  14. 4.1 The Newton’s FIRST Law: Tells us what happens when all existing forces acting on a body are balanced If the Net Forceis 0 (Fnet= 0 N), the object is in EQUILIBRIUM 47 N If the forces acting on a body are balanced, then the acceleration of the body will be 0 m/s2 100 N 100N 47 N

  15. 4.2 The Newton’s Second Law: Tells us what happens when an unbalanced force acts on a body If the Net Forceis NOT 0 (Fnet≠ 0 N), the object is NOT in EQUILIBRIUM a 47 N If an UNBALENCED force acts on a body, the body will ACCELARATE 100 N 200N 47 N

  16. 4.2 The Newton’s Second Law: Tells us what happens when an unbalanced force acts on a body The rate at which the body will ACCELERATE depends DIRECTLY on the UNBALANCEDFORCE (Fnet) and INVERSELY on the mass of the body. ∝ a ∝ Fnet a ∝ 1/m

  17. 4.2 The Newton’s Second Law: Tells us what happens when an unbalanced force acts on a body The rate at which the body will ACCELERATE depends DIRECTLY on the UNBALANCEDFORCE (Fnet) and INVERSELY on the mass of the body. ∝ a = Fnet/ m Fnet = a · m

  18. First Law vs. Second Law

  19. FBD • Known? • Unknown? • Formula(s) Fg = 45.0 N µk = 0.333 Fapp = 25.0 N Fk = ??? FN = ??? A 45.0 N block is being pushed along a floor, where the coefficient of kinetic friction is 0.333. If a horizontal force of 25.0 N is applied, at what rate will the block accelerate? • Fk = ??? Fapp = 25.0 N Fnet = a · m m = Fg/g = 45.0/9.81 = 4.59 kg Fg = 45.0 N Fg = m · g Fk = µk· FN

  20. FBD • Known? • Unknown? • Formula(s) A 45.0 N block is being pushed along a floor, where the coefficient of kinetic friction is 0.333. If a horizontal force of 25.0 N is applied, at what rate will the block accelerate?

  21. Classwork Page: 112 Problems: all problems

  22. QUIZ On Monday: 15 minutes Problems: from 4.1 and 4.2

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