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Forces

Forces. Chapter 6.1. What You Already Learned. Velocity is the ratio of the change in position of an object over a period of time. Acceleration describes a change in velocity of an object. Acceleration. What may cause an object to experience a change in velocity? Force

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Forces

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  1. Forces Chapter 6.1

  2. What You Already Learned • Velocity is the ratio of the change in position of an object over a period of time. • Acceleration describes a change in velocity of an object.

  3. Acceleration • What may cause an object to experience a change in velocity? • Force • Forces are often described as a push or pull. • A force may cause an object to speed up. • A force may cause an object to slow down. • A force may also cause an object to change direction.

  4. Types of Forces • Gravitational • Electromagnetic • Normal • Frictional • Tension

  5. Force • Force is a vector quantity, therefore it has both magnitude and direction. • The SI unit for force is the Newton. • The Newton is a derived unit where: 1N = 1kg•m/s2 • 1 Newton is the amount of force required to accelerate a 1 kg mass 1 m/s2.

  6. FN System Environment Fg The System and Environment • The object of interest is called the System. • The area around the object is called the Environment.

  7. Contact vs. Long Range • Some forces act over distances while others act only when two objects are in contact with one another. • Contact forces exist when two objects are in contact with one another. • Long-range (FIELD) forces act over distances without a need for direct contact. Electromagnetic forces and gravity are long-range forces.

  8. System FN Fg Free Body Diagram FN = Force of Desk on Book = Fg = Force of Gravity on Book Free-Body diagrams provide a means by which all the external forces acting on a system can be summarized and accounted for, and the resultant vectors determined.

  9. Newton’s 2nd Law • Newton determined that the acceleration of an object is directly proportional to the net unbalanced external force applied to move it and inversely proportional to the mass of the object. • a = Fnet/m Where F = Force in Newtons m = mass of the object a = acceleration of the object

  10. m2 Force m1 Acceleration Newton’s 2nd Law and Force vs. Acceleration • How does acceleration relate to force? • As the force on an object increases, the rate of acceleration will increase. Note: The slope of the line in a F vs. a graph will equal the mass of the object. The greater the slope, the greater the mass m2 > m1

  11. Velocity Time Newton’s 2nd Law and velocity vs. time (constant force) • Newton’s 2nd Law indicates that a constant force applied to an object will cause the speed to change at a constant rate. Note: Remember from kinematics that the slope of the line is equal to a.

  12. m1 Velocity m2 Time Newton’s 2nd LawForce Constant / Different Mass • If the applied force to a system is held constant and the mass is increased from m1 to m2, what would a velocity vs. time graph look like? m2 > m1 More Mass = Less Acceleration.

  13. F2 Velocity F1 Time Newton’s 2nd LawMass Constant / Different Force • If the mass is held constant and the force applied to a system is increased from F1 to F2, what would a velocity vs. time graph look like? F2 > F1 More Force = More Acceleration.

  14. Example: What is the rate of acceleration? • Two people are pushing a stalled car. The mass of the car is 1850 kg. One person applies a 275 N force while the other applies a 395 N force. A third force of 560 N acts in in the opposite direction compared to the two people. What is the acceleration of the car?

  15. Diagram the problem Fnet = Fperson 1 + Fperson 2– Fopposing force

  16. State the Known and Unknowns • What is known? • Mass (m) = 1850 kg • Force of person #1 (N) = 275 N • Force of person#2 (N) = 395 N • Opposing Force (N) = 560 N • What is not known? • Acceleration (a) = ?

  17. Perform Calculations • Fnet = Fperson 1 + Fperson 2 - Fopposing Where: • Fnet = ma • Substitute for Fnet and solve for a: • ma = Fperson 1 + Fperson 2 - Fopposing • a = (Fperson 1 + Fperson 2 - Fopposing)/m • a = (275 N + 395 N – 560 N)/1850 kg • a = 0.059 m/s2 • If there was no opposing force, how would the rate of acceleration change? • What do you think is the source of the opposing force? It would increase. Friction.

  18. Key Ideas • Force is a vector quantity that is a measure of the magnitude of a push or pull in Newtons. • Forces exist as contact or long range. • Use free body diagrams to represent forces when problem solving. • Newton’s 2nd Law of Motion states that the rate of acceleration of an object is proportion to the force applied and inversely proportional to its mass. • A constant force applied to an object will cause it to accelerate at a uniform rate. • As force increases, acceleration increases. • As mass increases, acceleration decreases.

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