Forces

1. Forces

2. History • Archimedes was the first to write about forces. • Galileo envisioned a force as a push or a pull. • Newton developed equations dealing with forces and motion.

3. History • Modern physics includes the study of extremes in gravity, speed, and size.

4. Force • is best described as a push or a pull • acts in a single direction with a certain size or magnitude

5. Vectors are labeled with a boldface F or F. Force • is a vector quantity since it has size and direction

6. Force • The SI unit for force is the newton (N). • In this course, force is always outside the object (external).

7. Two Categories of Force • Contact • Field

8. Contact Forces (mechanical forces) a force between systems that acts only when one system touches another

9. Compression Compression Tension Tension Torsion Torsion Shear Shear

10. Field Forces (action-at-a-distance forces) a force which acts between objects that are not touching

11. Magnetic Magnetic Gravitational Gravitational Electric Electric

12. Field Forces Although gravitational force is exerted by all objects, it only becomes significant if the object is large like an asteroid, moon, or planet.

13. Field Forces • Do not confuse these field forces with the four fundamental forces: • strong nuclear • electromagnetic • weak nuclear • gravitational

14. Fundamental Forces Strong Nuclear Forces p n p n

15. Fundamental Forces Electromagnetic Forces

16. Fundamental Forces Weak Nuclear Forces Quarks U - “up” D - “down” proton

17. Fundamental Forces Gravitational Force

18. Balanced Forces • occur when the pushes and pulls cancel each other out • result in a total net force of zero

19. Balanced Forces FB FA no acceleration

20. Unbalanced Forces • occur when one force is larger than the rest • act in the direction of the larger force • result in a change in motion

21. Unbalanced Forces Fnet = FB - FA Fnet FA FB acceleration

22. Weight • is the result of the earth’s gravitational attraction on an object • is directly proportional to mass

23. Directly Proportional When one value goes up, the other does also.

24. 250 200 150 Weight (N) 100 50 0 0 5 10 15 20 Mass (kg) Slope: 9.81 N/kg

25. weight(in N) mass (in kg) proportionality constant Weight • is measured in newtons since it is a force w = k m The earth’s proportionality constant is 9.81 N/kg.

26. Sample Problem 1 If a car has a mass of 2123 kg, what is its weight in newtons? w = km w = 9.81 N/kg × 2123 kg

27. Sample Problem 1 Remember in elementary school, you solved: 3 × 4 = 12 Now solve: 9.81 N/kg × 2123 kg = 20826.63 N

28. Sample Problem 1 If a car has a mass of 2123 kg, what is its weight in newtons? w = km w = 9.81 N/kg × 2123 kg w = 20826.63 N w = 20,800 N 3 Sig Digs

29. The tallest building in New York has a weight of 3,248,311,039 N. What is its mass in kg? • 31,865,931,292.59 kg • 3,248,311.039 kg • 331,122,430.1 kg • 331,122.4301 kg

30. Weight is directly proportional to _____ • mass. • the proportionality constant. • balanced forces. • magnitude.

31. Who envisioned a force as a push or a pull? • Archimedes • Einstein • Newton • Galileo

32. Which of the following is out of place? • Shear • Compression • Magnetic • Torsion

33. Newton’s Laws of Motion

34. Early Thoughts on Motion • From the time of the ancient Greeks to the Medieval age, scientists believed that things tended to stop if left to themselves. This is true, but does it mean that objects stop by themselves?

35. Galileo investigated this idea with several experiments.

36. Early Thoughts on Motion • Galileo concluded that objects have no natural tendency to stop. • In order for an object to stop, it must be affected by some outside influence. • He called this property inertia.

37. Newton’s Laws of Motion • Newton had clocks that were accurate enough to make time measurements. • He extended Galileo’s studies and formulated his laws of motion.

38. First Law—Inertia • Objects at rest remain at rest. • Objects in motion continue in that motion. • Both of these statements change if a net unbalanced external force acts on the object.

39. First Law—Inertia • We cannot measure inertia directly. • We measure it indirectly by measuring the mass of an object.

40. First Law—Inertia • If no net force acts on an object, it is in equilibrium. • If any unbalanced force acts on an object, it will change its motion. • The force can be a contact force or a field force or a combination of both.

41. Summary: Summary: First Law—Inertia • Inertia is the property of all objects to have an unwilling-ness to change motion. • The more mass an object possesses, the more inertia it has.

42. Second Law—Accelerated Motion This law states the relationship between force, mass, and acceleration in the form of an equation. F = ma

43. Second Law—Accelerated Motion If both sides are divided by m F = ma m m F we get . a = m

44. Second Law—Accelerated Motion If both sides are divided by a F = ma a a F we get . m = a

45. increases increases Second Law—Accelerated Motion F = m a • If the force increases on an object (constant mass), what happens to the acceleration? Acceleration increases. This is a direct relationship.

46. increases increases Second Law—Accelerated Motion F = m a This is why a more powerful engine makes a vehicle go faster.

47. increases decreases Second Law—Accelerated Motion F = m a • If the mass of the object increases (constant force), then the acceleration … decreases. This is an inverse relationship.

48. increases decreases Second Law—Accelerated Motion F = m a An inverse relationship occurs when one variable going up causes another variable to go down.

49. increases decreases Second Law—Accelerated Motion F = m a This is why a truck with a load accelerates more slowly than when it is empty.

50. Second Law—Accelerated Motion F = m a If there is no net (unbalanced) force on the object, it does not accelerate.