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What is a Force?

What is a Force?. A force is a push or a pull causing a change in velocity or causing deformation. Dynamics . What is dynamics? Dynamics is the study of the relationship between force and motion. How are contact forces different from long range forces?.

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What is a Force?

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  1. What is a Force? A force is a push or a pull causing a change in velocity or causing deformation.

  2. Dynamics • What is dynamics? • Dynamics is the study of the relationship between force and motion.

  3. How are contact forces different from long range forces? • Contact forces - touching - example: hand on desk • Long range forces - act at a distance - gravitational forces, electrostatic, magnetic

  4. What is Newton’s First Law of Motion? • Law of Inertia: tendency to resist change • An object at rest remains at rest, and an object in motion remains in motion (same direction, constant speed) unless acted upon by an unbalanced force. • Forces are balanced: Equilibrium

  5. Newton’s 1st Law - Types of Equilibrium • 1. Static Equilibrium object is at rest • 2. Dynamic Equilibrium object is moving at constant velocity

  6. What is Newton’s Second Law of Motion? • Unbalanced forces produce acceleration • **Acceleration is directly proportional to force and inversely proportional to mass • Fnet = ma • a = Fnet/m in reference table • Units 1 kgm/s2 = 1 Newton

  7. Example: • An artillery shell with a mass of 5.5 kg is fired from a gun with a velocity of 770 m/s. The barrel of the gun is 1.5 m long. Find the force on the shell.

  8. Solution: m = 5.5 kg vf = 770 m/s vi = 0 m/s d = 1.5 m

  9. Special Case - weight • W = mg is a special case of F = ma, where weight is the force, and gravity is the acceleration (can be written as Fg) • Example: What is the weight of a 5 kg object?

  10. Two Kinds of Mass • 1. Gravitational mass - using a balance - comparing the gravitational force on two objects (one has a known mass). • 2. Inertial mass - using Newton’s 2nd law - find the force necessary to produce a specific acceleration for a given mass • Gravitational mass = inertial mass

  11. Graph: Inertial mass • Slope is mass F a

  12. Free Body Diagrams • Used to show forces on an object • Sketch and Equations: What is Fnet? Fnet is the sum of the forces acting in the direction of motion (vertical or horizontal)

  13. Free Body Diagrams: Vertical Lifted object (Ftens is tension)

  14. Example:A 50 Newton force is used to lift a 2 kg object. What is the acceleration of the object? Lift 50 N Weight

  15. Example:A 4500 kg helicopter accelerates upward at 2 m/s2. What lift force is exerted by the air on the propellers? Lift ? N Weight

  16. Alternate Solution to helicopter problem:

  17. What is Newton’s 3rd Law of Motion? • For every action, there’s an equal and opposite reaction. • The force on one object is equal and opposite to the force on the other. • Object resting on a table: weight pushes back on table. Table pushes back with an equal and opposite force called Normal force (FN). If no other vertical forces are present: normal force equals weight

  18. Falling objects: what are the forces on an object falling through the air? Object in freefall (no air resistance) Object in freefall (with air resistance)

  19. Falling Objects: • What are the forces on an object falling is the air? What happens when they are equal? • Drag force (Fair) - air resistance - depends on size/shape of the object, air density, speed. • When drag force = weight, forces are balanced (Fnet=0) • No net force means no acceleration! You have reached TERMINAL VELOCITY.

  20. Free Body Diagrams • Free body diagram - horizontal motion FN Ff FA Fg or W

  21. What if…. • FA is greater than Ff • acceleration is positive • FA is equal to Ff then • no acceleration (constant velocity) • FA is less than Ff • acceleration is negative (you can slow down, but friction won’t make you move backwards!)

  22. Free Body Diagrams • Free body diagram - horizontal motion with no friction:

  23. Example: • If you apply a 50 Newton force to a mass of 5 kg, what is the acceleration?

  24. Example (continued): • What if a frictional force of 20 N opposes the applied force? Find a.

  25. What is friction? Friction - force opposing motion between 2 surfaces in contact and parallel to the surface • How can you calculate frictional force? • Equation: Ff = frictional force (N) FN = normal force (N) coefficient of friction (no units)

  26. What variables affect friction? • Weight? YES • Surface area? NO • Type of surfaces in contact? YES • Velocity? NO • Normal force - force pushing objects together- is normal force always equal to weight? • NO - they are not equal if other vertical forces are present.

  27. What are two types of friction? • 1. Static - objects are NOT in relative motion - static friction opposes the START of motion • 2. Kinetic (sliding) - force between surfaces in motion - resists motion • Which is greater? • Static friction is greater - it is harder to start motion that it is to keep an object moving.

  28. Example #1: • A block is place on a table. It has a weight of 50 N. You must exert a force of 20 N to keep the block moving at a constant velocity. • 1. What is coefficient of sliding friction?

  29. 2. Place a 10 N brick on the block. What force is required to keep the block moving at constant velocity? Same surfaces, so  = 0.4

  30. Example #2 • A 10kg box made of wood is pushed on a wood floor with a force of 100 N. • (a.) Will the box slide? • (b.) What force must be applied to move the box at a constant velocity? • (c.) What force must be applied for the box to accelerate at a rate of 5 m/s2?

  31. Solution: (a.) Will it move? It will move if the applied force exceeds the static frictional force. From reference table: static = 0.42 Yes, 100N>41.2N, box will slide.

  32. Solution: • (b). To move box at a constant velocity….. Use KINETIC friction. From reference table: kinetic = 0.30 Since a = 0, then Fnet = ma =0

  33. Solution: • (c.) a = 5 m/s2, find FA

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