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Forces

Forces. Chapter 4. A push or a pull. Gravitational Electromagnetic Weak Strong. Forces. Sir Isaac Newton. Newton’s First Law. An object in motion remains in motion and an object at rest remains at rest unless acted on by an outside force. Sometimes called the Law of Inertia Examples

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Forces

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  1. Forces Chapter 4

  2. A push or a pull Gravitational Electromagnetic Weak Strong Forces

  3. Sir Isaac Newton

  4. Newton’s First Law • An object in motion remains in motion and an object at rest remains at rest unless acted on by an outside force. • Sometimes called the Law of Inertia • Examples • Sudden braking • Flooring it • Seeing stars

  5. Inertia • A body’s tendency to keep doing what it’s already doing • Measured by mass • Does not need gravitational force.

  6. Newton’s Second Law • The acceleration of an object is directly proportional to the force on the object and inversely proportional to the mass.

  7. Units • Newtons • English equivalent is pounds • 4.45N=1lb

  8. Practice • A 2988kg elephant accelerates on a skateboard at a rate of 2.7m/s2. If the elephant is being pushed with a 10,000N force, what is the force of friction? • A 58.7N force is applied to a go kart to get it started. If the car starts from rest and accelerates for 4.0s covering a distance of 7.8m, what is the mass of the kart?

  9. Newton’s Third Law • When one object exerts a force on a second object, the second object exerts a force on the first of equal magnitude and opposite direction • Action-reaction pairs • Switch the nouns

  10. Weight • Weight and gravitational force are the same thing. • Since acceleration due to gravity (g) is known, we can define weight as: • In general, we will take care of directional information (positives and negatives) in the problem itself, so we will use +9.8m/s2 instead of -9.8m/s2

  11. Skydiving and Terminal Velocity

  12. Practice Problems • A skydiver reaches a terminal velocity of 55m/s when in free fall. If the skydiver has a mass of 78kg, how much force is air resistance providing? • When starting up the elevator, a rider experiences an acceleration of 1.2m/s2. If the force from the elevator floor is 650N, what is the mass of the rider?

  13. Normal Force • Force pushing up from a surface • When vertical acceleration is zero and the only two vertical forces are gravity and normal force, normal force and gravity are equal. • Scales read normal force • Elevators • Weightlessness (Newton’s Mountain)

  14. Friction • Depends only on the surfaces, not on velocity • Kinetic and static friction • Depends on two things, normal force of the object and the combination of surfaces • Coefficient of friction, µ equals the ratio of frictional force to normal force.

  15. Practice • A 16.8 kg crate is pushed horizontally along the floor with a force of 207N. If the coefficient of friction is 0.62, what is the acceleration of the crate?

  16. A sled is pulled across the snow with a constant velocity. If the sled has a mass of 32.1kg and the coefficient of kinetic friction is 0.14, what is the magnitude of the pull?

  17. A bunch of guys try to push a car out of the mud. They push harder and harder until it becomes unstuck and then push with a constant force. If the coefficient of static friction is 0.54 and the coefficient of kinetic friction is 0.40, what is the acceleration of the car once it starts moving?

  18. A crate of chickens is pulled with 145N of force with a rope that is inclined 30° up from the horizontal. If the crate has a mass of 35kg and the coefficient of friction between the floor and the crate is 0.43, what is: • The normal force on the crate? • The acceleration of the crate, assuming it accelerates horizontally.

  19. Tension • Upward force coming from a string or rope. • All vertical ropes supporting an object account for an equal amount of force unless specified otherwise.

  20. m2 m1 Atwood Machine Problems • If mass 1 is 5kg and mass 2 is 7kg, what is the acceleration of the system? • Assumptions • a1= a2 • FT1= FT2 • Pulley switches which direction is positive. • Consider the direction of motion of the box to be the positive direction.

  21. Box 1 a FT Fg

  22. Box 2 FT a Fg

  23. Putting it together

  24. Putting it together cont.

  25. m2 m1 Practice Problem • m1=8kg • m2=4kg • Find FT

  26. Sign Problems • Equilibrium-the state where all forces are balanced and acceleration is zero. • Includes both stationary and constant velocity cases.

  27. 1st Example • If the sign has a mass of 15kg, what is the tension in the string? What is the force the beam exerts on the sign? • Assume equilibrium. • One component at a time.

  28. Set up equilibrium case FT Fy 23 Fx Fbeam Fg

  29. Equilibrium case for vertical

  30. Equilibrium case for horizontal

  31. Practice Problem 2 • Find the tension in each rope. T2 T1

  32. Fy1 Fy2 Fx1 Fx2 Fg Break up Components F1 F2

  33. Fy1 Fy2 Fx1 Fx2 Fg Horizontal Pieces • Since the sign isn’t accelerating horizontally, we can use:

  34. Fy1 Fy2 Fx1 Fx2 Fg Vertical Pieces • Since the sign isn’t accelerating vertically, we can use:

  35. Putting the two together

  36. Putting the two together cont.

  37. Putting the two together cont.

  38. Ramp Problems

  39. Practice Problems • A box slides down a frictionless ramp. If the box has a mass of 12kg and the ramp is inclined at an angle of 32 degrees, what is the acceleration of the box? • A 76kg box slides down a ramp inclined at 21 degrees. If the acceleration of the box is 1.2m/s2, what is the coefficient of friction between the box and the ramp?

  40. Random Practice Problems • A 28kg box is pulled at an angle of 18o up from the horizontal with a force of 145N. If the coefficient of friction between the floor and the box is .32, what is the acceleration of the box?

  41. A dead pig has a µk of .21 and a µs of .43 when sitting on a book cover, how far will the book cover need to be tilted to start the pig carcass moving? Once it starts moving, how quickly will it accelerate?

  42. m1=13kg • m2=9kg • μk=.11 • What is a?

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