Forces and Motion: Understanding the Effects and Types of Forces in Physics

1. Chapter 12 – Forces and Motion

2. 12.1 Forces Force – A push or pull that acts on an object What is a Force?

3. 12.1 Forces In other words a force causes an object to accelerate What effect does a force have on an object? 1. Can cause an object to speed up or slow down 2. Can cause an object to change direction

4. 12.1 Forces Named in honor of Sir Isaac Newton The SI Unit for measuring force is the newton (N) A newton is the force that causes a 1 kg mass to accelerate at a rate of 1 m/s2 1 N = 1 kg · m/s2 Force is a vector – has both magnitude and direction

5. 12.1 Forces Since force is a vector, it can be represented with an arrow The direction of the arrow represents the of the force direction The length of the arrow represents the strength, or, the of the force magnitude

6. 12.1 Forces Forces in the same direction add together Forces in the opposite direction subtract Forces can be combined by vector addition The net force is the overall force after all forces have been combined

7. 12.1 Forces force of dog on toy force of man on toy = The dog and man pull with the same amount of force, but in opposite directions When the forceson an objectarebalanced, the net force is zero and there is no change in the object’s motion

8. 12.1 Forces force of stick on ball When an unbalanced force acts on an object, the object accelerates

9. 12.1 Forces What is friction? friction – a force that opposes the motion of objects that touch as they move past each other friction acts at the surface where objects are in contact 4 main types of friction 3. Rolling friction 1. Static friction 4. Fluid friction 2. Sliding friction

10. 12.1 Forces pushing force on crate static friction on crate = The crate won’t move unless the pushing force overcomes the maximum static frictional force static friction – the friction force that acts on objects that are not moving

11. 12.1 Forces sliding friction on player sliding (kinetic) friction – a force that opposes the direction of motion of an object as it slides over a surface friction always acts in a direction opposite the motion of an object

12. 12.1 Forces

13. 12.1 Forces sliding friction is always less than static friction

14. 12.1 Forces static and sliding friction have two primary causes friction always acts in a direction opposite the motion of an object

15. 12.1 Forces rolling friction – caused by the change in shape at the point of rolling contact friction always acts in a direction opposite the motion of an object

16. 12.1 Forces air resistance is a type of fluid friction fluid friction – opposes the motion of an object moving through a fluid friction between submarine and water friction always acts in a direction opposite the motion of an object

17. 12.1 Forces gravity – a force that acts between any two masses an attractive force – pulls objects together

18. 12.1 Forces

19. 12.1 Forces gravity – a force that acts between any two masses Earth’s gravity acts downward toward the center of the Earth A field force that can act over large distances

20. 12.1 Forces normal force normal force gravitational force gravitational force An upward force balances the downward force of gravity (normal force)

21. 12.1 Forces as objects fall to the ground they accelerate air resistance air resistance opposes the motion gravitational force Gravity (a force) causes objects to accelerate downward with greater speed, comes greater air resistance What is the direction of the net force on the squirrel? Why is the squirrel able to increase the distance covered when it jumps if the air resistance increases?

22. 12.1 Forces Gravity (a force) causes objects to accelerate downward as objects fall to the ground they accelerate air resistance air resistance opposes the motion gravitational force with greater speed, comes greater air resistance terminal velocity – the constant velocity of a falling object when the force of air resistance equals the force of gravity

23. 12.1 Forces What is the net force on the skydiver just before stepping out of the plane? How do the force of gravity and air resistance compare as the skydiver is falling and gaining speed? How does the force of air resistance change as the skydiver gains speed while falling? What happens to the skydiver’s speed when the force of air resistance becomes equal to the force of gravity? How does the force of air resistance change when the parachute opens? Terminal Speed

24. 12.1 Forces

25. 12.1 Forces

26. 12.1 Forces Projectile Motion projectile motion – the motion of a falling object (projectile) after it is given an initial forward velocity a projectile follows a curved path despite an initial forward velocity, both projectiles hit the ground at the same time

27. 12.1 Forces

28. 12.1 Forces When a sufficiently massive star reaches the end of its lifetime, it explodes in a violent event called a supernova. The core of the star collapses suddenly, releasing vast quantities of particles and energy that blow the star apart. As shown in this animation, an expanding shell of gas called a supernova remnant spreads away from such a stellar cataclysm. (The animation ends with an X-ray image of the supernova remnant Cassiopeia A.)

29. 12.1 Forces The first part of this time-lapse video shows how stars near the very center of the galaxy moved from 1992 through 1998. The area shown in these infrared images is about 0.12 parsec (0.38 light-years, or 24,000 AU) on a side. The second part of the video zooms in on the motion of one particular star called S2, and shows how this star is expected to move through 2006. The stars' motions indicate that they are orbiting around an unseen object at the position marked by the yellow cross. Using Newton's form of Kepler's third law, astronomers calculate that the mass of this object is about 3.7 million solar masses. This compact, invisible object is almost certainly a supermassive black hole.

30. 12.1 Forces

31. 12.2 Newton’s 1st and 2nd Laws of Motion Aristotle (384 B.C. – 322 B.C.) Proposed that force is required to keep an object moving at constant speed While right about many things he was INCORRECT about this Where else did we hear about Aristotle?

32. 12.2 Newton’s 1st and 2nd Laws of Motion Discovered that gravity produces a constant acceleration Galileo (1564 - 1642) Concluded that moving object not subjected to a force would continue to move indefinitely

33. 12.2 Newton’s 1st and 2nd Laws of Motion His important work in which all of this is published is called Principia Isaac Newton (1642-1727) Built on the work of Galileo Defined mass and force Introduced his laws of motion

34. 12.2 Newton’s 1st and 2nd Laws of Motion

35. 12.2 Newton’s 1st and 2nd Laws of Motion Inertia – The tendency of an object to resist a change in its motion. An object at rest remains at rest, and an object in motion continues in a straight line at a constant speed, unless acted upon by an outside force. Newton’s 1st Law of Motion The Law of Inertia – The state of motion of an object does not change as long as the net force acting on the object is zero.

36. 12.2 Newton’s 1st and 2nd Laws of Motion

37. 12.2 Newton’s 1st and 2nd Laws of Motion

38. 12.2 Newton’s 1st and 2nd Laws of Motion

39. 12.2 Newton’s 1st and 2nd Laws of Motion Newton’s 2nd Law of Motion Mass – A measure of the inertia of an object. Depends on the amount of matter the object contains. The Law of Acceleration - The force needed to give an object a given acceleration is equal to the object’s mass times the acceleration. F = ma kg · m/s2 N

40. 12.2 Newton’s 1st and 2nd Laws of Motion Newton’s 2nd Law of Motion The acceleration of an object is always in the direction of the net force.

41. 12.2 Newton’s 1st and 2nd Laws of Motion Newton’s 2nd Law of Motion The acceleration of an object is always in the direction of the net force. The acceleration is inversely proportional to the mass.

42. 12.2 Newton’s 1st and 2nd Laws of Motion Example 1 A 20.0 N net force acts on an object with a mass of 2.0 kg. What is the object’s acceleration? Example 2 Example 3 A box has a mass of 150 kg. If a net force of 3000 N acts on the box, what is the box’s acceleration? What is the acceleration of a 1000 kg car subject to a 500 N net force? Using Newton’s Second Law F = ma Page 367 Math Skills and Math Practice

43. 12.2 Newton’s 1st and 2nd Laws of Motion • You are stuck going to the grocery store...your job is to push the cart. If the cart has a mass of 15 kg and you can push with a force of 20 N, what acceleration can you give to the cart? • Just before leaving the grocery store, the cart is now loaded with 30 kg of stuff. Now, what acceleration can you give to the cart (with the same amount of force)? • Why did your answer for #2 change like it did? Explain.

44. 12.2 Newton’s 1st and 2nd Laws of Motion Weight is the product of the object’s mass and acceleration due to gravity W = mg g = 9.80 m/s2 mass and weight are NOT the same thing Weight – The force of gravity acting on an object. kg · m/s2 N mass is a measure of the inertia of an object, weight is a measure of the force of gravity acting on an object

45. 12.2 Newton’s 1st and 2nd Laws of Motion mass weight is measured with a balance is measured with a scale

46. 12.3 Newton’s 3rd Law of Motion and Momentum girl’s car on boy’s car boy’s car on girl’s car = A force CANNOT exist alone. Forces ALWAYS exist in pairs.

47. 12.3 Newton’s 3rd Law of Motion and Momentum Newton’s 3rd Law of Motion The Law of Reaction – Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. One of these forces is called the action force and the other is called the reaction force

48. 12.3 Newton’s 3rd Law of Motion and Momentum Force of nail on hammer Force of hammer on nail action – reaction forces DO NOT act on the same object!!!!

49. 12.3 Newton’s 3rd Law of Motion and Momentum Force gas on rocket Force of rocket on gas action – reaction forces DO NOT act on the same object!!!!

50. 12.3 Newton’s 3rd Law of Motion and Momentum Ronald pushes on bar Bar pushes on Ronald action – reaction forces DO NOT act on the same object!!!!