What are Forces?

1. What are Forces?

2. Standard • Standard - 3.2.7.B1: Describe how unbalanced forces acting on an object change its velocity. Analyze how observations of displacement, velocity, and acceleration provide necessary and sufficient evidence for the existence of forces.

3. Eligible Content • Anchor Descriptor - S8.C.3.1: Describe the effect of multiple forces on the movement, speed, or direction of an object. • PDFPrint • S8.C.3.1.1: Describe forces acting on objects (e.g., friction, gravity, balanced versus unbalanced).

4. Newton’s Second Law of Motion • Force = mass X acceleration • F = ma • The greater the mass of an object the less the acceleration if the same force is applied.

5. What is friction? • Friction is a force which opposes motion. • Friction is an important force in our lives.. -Athlete shoes =greater friction

6. From Glencoe Physical Science

7. What causes friction? • There are two factors which affect friction between two surfaces: • Surfaces in contact (rough or smooth) • Amount of force

8. From Glencoe Physical Science What is static friction? • Friction between two surfaces which are not moving past each other.

9. Static Friction • Intended direction Static friction • of motion

10. Glencoe Physical Science What is sliding friction? • Sliding friction occurs when a force is great enough to overcome the static friction.

11. Intended direction Sliding friction of motion

12. What is rolling friction? • Friction that allows wheels to turn and objects to move. Direction of Motion Rolling Friction

13. What is Fluid Friction? When an object pushes fluid aside, friction occurs. Fluid friction

14. Air Resistance – another type of friction?? • Air resistance is a force which opposes a moving object. • A penny and a feather dropped in air will not fall at the same rate, but a penny and feather dropped in a vacuum (no air) will.

16. What is terminal velocity? • Forces cause objects to accelerate • When the force of gravity on a falling object equals the force of the air resistance going against gravity, the forces balance out and the object stops accelerating. • The object will travel at a constant velocity – the terminal velocity.

17. What is Gravity? • Gravity is a force of attraction between two objects. • Law of Gravitation – any two masses exert an attractive force on each other. The amount of attraction depends upon two things: the mass of the objects and the distance between the objects.

18. What is gravitational acceleration? • When objects fall they accelerate toward the ground. • Using Newton’s 2nd Law we can see this relationship: F = ma • The force caused by gravity acts upon a certain mass to cause it to accelerate toward the ground at a constant rate. • Acceleration due to gravity is 9.8 m/s2

19. Gravity causes weight • The weight of an object is caused by the force of attraction between the earth and objects on the surface of the earth. Gravitation force is equal to weight, therefore…

20. How much does a person with a mass of 70.0 kg weigh on the earth? Weight = mass X 9.8 m/s2 Weight = 70.0 kg X 9.8 m/s2 Weight = 686 N Remember that 1 N = (1 kg) (1m/s2)

21. What is weight? • When an astronaut moves away from the earth they “lose” weight. Weight is caused by the force of attraction between the earth and objects on its surface. The greater the distance an object has from the earth, the less the force of attraction which would exist.

22. What is the difference between mass and weight? • Mass is the amount of matter in an object. This does not change in an object as it moves away from the earth. • Weight is caused by gravity. Therefore, the greater the distance, the less the force of attraction. The less the distance the greater the force of attraction. • Your weight is dependent upon the mass of the earth. If you were able to go to the planet, Jupiter, your weight would be 27 time greater, because the mass of Jupiter is 27 times greater.

23. What causes “weightlessness”? • Even in the space shuttle, there is a force of attraction exerted by the earth on the shuttle and its contents. • When the space shuttle temporarily “falls” toward the earth, the contents of the space shuttle appear to be weightless, but in fact they are falling with the space shuttle. This is called “freefall”.

24. What is projectile motion?

25. When an object is thrown into the air, two forces and two motions affect the movement of the object. The object may be thrown horizontally with a certain force, but the vertical force of gravity combines to affect the motion. Slow projectile - shoot a monkey Fast projectile - shoot a monkey

26. What is centripetal force? • “centripetal” means to move toward the center. • Centripetal acceleration is the acceleration of an object toward the center of a curve or circular path. • Centripetal force is a force toward the center of a circle which holds the moving object in its circular path.

28. What is Newton’s 3rd Law of Motion • For every action there is an equal but opposite reaction. • When one object exerts a force on a second object, the second one exerts a force on the first that is equal in size and opposite in direction. • Action-Reaction Law

29. Two objects in contact with one another and moving must use the action-reaction law. Rocket propulsion involves the action-reaction law. The explosion from one end of the rocket causes a force in that direction. This causes an unbalanced force in the opposite direction causing the rocket to accelerate. Video Clip!

30. What is momentum? • The momentum of an object is equal to the product of its mass and its velocity. The unit for momentum is kg m/s

31. What is the Law of Conservation of Momentum? • When object collide the momentum of one object is transferred to the other object. • View this video to see a demonstration of conservation of momentum. • Notice that the soccer ball has greater momentum since it has a larger mass. Its momentum is transferred to the smaller ball which has a smaller mass and will therefore have a larger velocity. Try this with a soccer ball and a tennis ball!

32. Newton’s Laws of Motion I. Law of Inertia II. F=ma III. Action-Reaction

33. While most people know what Newton's laws say, many people do not know what they mean (or simply do not believe what they mean).

34. Newton’s Laws of Motion • 1st Law– An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force. • 2nd Law – Force equals mass times acceleration. • 3rd Law – For every action there is an equal and opposite reaction.

35. 1st Law of Motion (Law of Inertia) An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.

36. 1st Law • Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless. These pumpkins will not move unless acted on by an unbalanced force.

37. 1st Law • Once airborne, unless acted on by an unbalanced force (gravity and air – fluid friction), it would never stop!

38. 1st Law • Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

39. Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force? It’s a force we sometimes cannot see – friction.

40. Objects on earth, unlike the frictionless space the moon travels through, are under the influence of friction.

41. What is this unbalanced force that acts on an object in motion? Friction! • There are four main types of friction: • Sliding friction: ice skating • Rolling friction: bowling • Fluid friction (air or liquid): air or water resistance • Static friction: initial friction when moving an object

42. Slide a book across a table and watch it slide to a rest position. The book comes to a rest because of the presence of a force - that force being the force of friction - which brings the book to a rest position.

43. In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever! (Or at least to the end of the table top.)

44. Newtons’s 1st Law and You Don’t let this be you. Wear seat belts. Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 m/hour.

45. 2nd Law F = m x a

46. 2nd Law The net force of an object is equal to the product of its mass and acceleration, or F=ma.

47. 2nd Law • When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N). • One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.

48. 2nd Law (F = m x a) • How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second? • Write the formula • F = m x a • Fill in given numbers and units • F = 1400 kg x 2 meters per second/second • Solve for the unknown • 2800 kg-meters/second/second or2800 N

49. If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.

50. Newton’s 2nd Lawproves that different masses accelerate to the earth at the same rate, but with different forces. • We know that objects with different masses accelerate to the ground at the same rate. • However, because of the 2nd Law we know that they don’t hit the ground with the same force. F = ma 98 N = 10 kg x 9.8 m/s/s F = ma 9.8 N = 1 kg x 9.8 m/s/s