Chapter 6—Forces in Motion

1. Chapter 6—Forces in Motion Material on the Final Exam

2. Falling Water • How does gravity affect our world?

3. Section 1—Gravity and Motion • Objectives • Explain the effect of gravity on falling objects • Explain the effect of air resistance on falling objects • Explain why objects in orbit are in free fall and appear to be weightless • Describe how projectile motion is affected by gravity

4. Section 1—Gravity and Motion • Aristotle was wrong again! • Aristotle thought that the rate at which on object falls depended on the object’s mass • Corrected by GalileoGalilei • Experiment with cannonballs • Supported by many scientists • NASA supports Galileo • Mythbusterssupport Galileo

5. Section 1—Gravity and Motion • Gravity and acceleration • Objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects • A golf ball may have more mass… • …but it will also be harder to accelerate

6. Section 1—Gravity and Motion • Acceleration due to Gravity • Acceleration is the rate at which velocity changes over time • Due to gravity, all objects accelerate toward earth at a rate of 9.8 m/s/s, or 9.8 m/s2 • Every second, velocity increases by 9.8 m/s

7. Section 1—Gravity and Motion • Velocity of Falling Objects • Formula Δv = g x t • Δv = change in velocity • g =gravity • t = time

8. Section 1—Gravity and Motion • Math practice! • A penny at rest is dropped from the top of a tall stairwell. What is the penny’s velocity after it has fallen for 2s? • Write the formula • Identify the variables • Plug in the numbers • Solve and circle

9. Section 1—Gravity and Motion • Math practice! • The same penny hits the ground in 4.5s. What is the penny’s velocity as it hits the ground? • Write the formula • Identify the variables • Plug in the numbers • Solve and circle

10. Section 1—Gravity and Motion • An acorn at rest falls from an oak tree. The acorn hits the ground with a velocity of 14.7 m/s. How long did it take the acorn to land? • Write the formula • Identify the variables • Plug in the numbers • Solve and circle

11. Section 1—Gravity and Motion • An rock falls off of a cliff on Mercury. The rock takes 5 seconds to fall and has a velocity of 18.5 m/s when it lands. What is the gravity on Mercury? • Write the formula • Identify the variables • Plug in the numbers • Solve and circle

12. Section 1—Gravity and Motion • Air Resistance and Falling Objects • Air resistance can affect the speed at which something falls • Air resistanceis a force that opposes the motion of objects through air • The amount of air resistance acting on an object depends on the size, shape, and speed of the object

13. Section 1—Gravity and Motion • Acceleration Stops at Terminal Velocity • At some point, air resistance (upward force) and gravity (downward force) equal and cancel out • Net force is 0N; object stops accelerating

14. Section 1—Gravity and Motion • Terminal velocity is the constant velocity of a falling object when the force of air resistance is equal in magnitude and opposite in direction to the force of gravity

15. Section 1—Gravity and Motion • Free Fall occurs when there is no air resistance • CAN ONLY OCCUR IN A VACUUM OR OUTER SPACE! • Free fall is the motion of a body when only the force of gravity is acting on the object

16. Section 1—Gravity and Motion • Orbiting Objects are in Free Fall • Astronauts float in orbiting space crafts because of free fall

17. Section 1—Gravity and Motion Projectile Motion • Projectile motion is the curvedpath an object follows when it is thrown or propelled near the surface of the Earth.

18. Section 1—Gravity and Motion Projectile Motion • It has two independent (have no effect on each other) components • Vertical motion –force exerted from gravity. • Horizontalmotion – the force exerted on the object. • The combination of the two motions form the curved path. • Examples: a frog leaping, water sprayed by a sprinkler, an arrow from an archer. Projectile motion video clip

19. Section 1—Gravity and Motion • Objects going out horizontally and objects being dropped will hit the ground at the same time video demo Projectile motion video clip 1

20. Section 2—Newton’s Laws of Motion • Objectives • Describe Newton’s First Law of Motion • Explain how the first law relates to objects at rest and objects in motion • State Newton’s Second Law of Motion • Explain the relationship between force, mass, and acceleration • State Newton’s Third Law of Motion • Describe and give examples of force pairs

21. Section 2—Newton’s Laws of Motion Newton’s First Law of Motion • An object at rest remains at rest, and an object in motion remains in motion at a constant speed and straight line unless acted on by an outside or unbalanced force. • Also known as (AKA) the Law of Inertia

22. Section 2—Newton’s Laws of Motion • Examples of Newton’s First Law • An object will not move unless you push it. • An object will stay in motion unless friction, or some other force changes the objects velocity. • A car at a red light will stay stopped until you let go of the brake. A moving car will continue to move until you hit the brakes. INERTIA = LAZINESS

23. Section 2—Newton’s Laws of Motion • Inertia is the tendency of an object to resist a change in its motion. • Mass is a measure of inertia. • An object that has a small mass has less inertia than an object that has a large mass.

24. Section 2—Newton’s Laws of Motion Newton’s Second Law of Motion • Force = mass x acceleration F = ma SI Unit = Newton (N) F = ma video clip

25. Section 2—Newton’s Laws of Motion • What is the acceleration of a 7 kg mass if a force of 68.6 N is used to move it toward Earth? • You try! • What force is necessary to accelerate a 1,250 kg car at a rate of 40 m/s2?

26. Section 2—Newton’s Laws of Motion • You try! • Zookeepers carry a stretcher that holds a sleeping lion. The total mass of the lion and the stretcher is 175 kg. The lion’s forward acceleration is 2 m/s2. What is the force necessary to produce this acceleration?

27. Section 2—Newton’s Laws of Motion • The more mass an object has the slower it will move. • In order to speed up an object with a large mass, a greater force must be applied.

28. Section 2—Newton’s Laws of Motion Newton’s Third Law • Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first. • All forces act in pairs. • For every action, there is an equal and opposite reaction

29. Section 3--Momentum • Objectives • Describe and calculate the momentum of moving objects • Explain the law of conservation of momentum

30. Section 3--Momentum • What is momentum? • The momentum of an object depends on the object’s mass and velocity. • The more momentum an object has, the harder it is to stop the object or change its direction. • If velocity increases, its momentum also increases.

31. Section 3--Momentum • Momentum symbol is p • Unit for momentum is kg· m/s • Formula p = mv

32. Section 3--Momentum Momentum math! • A bowling ball has a mass of 10 kg and is traveling at 5 m/s. What is the momentum of the bowling ball? • A bullet travels 300 m/s and has a mass of 50 g. What is the momentum? • Which has greater momentum? The bowling ball or the bullet?

33. Section 3--Momentum • Law of Conservation of Mass • States that when two objects collide, their combined momentum remains the same after the collision. • This law applies whether the objects stick together or bounce off each other after they collide. • Because action and reaction forces are equal and opposite, momentum is neither gained or lost in a collision. Conservation of Momentum… In Space!

34. Section 3--Momentum