Big Idea 13: Force and Motion

1. Big Idea 13: Force and Motion By Mrs. Shaw

2. Force and Changes of Motion Main Ideas: A. It takes energy to change the motion of objects. B. Energy change is understood in terms of forces, pushes or pulls. C. Some forces act through physical contact, while others act at a distance. SC.6.P.13.1 : Investigate and describe types of forces including contact forces and forces acting at a distance, such as electrical, magnetic, and gravitational. SC.6.P.13.2 : Explore the Law of Gravity by recognizing that every object exerts gravitational force on every other object and that the force depends on how much mass the objects have and how far apart they are. SC.6.P.13.3 : Investigate and describe that an unbalanced force acting on an object changes its speed, or direction of motion, or both.

3. Part 1 – Lesson 1 Describing Motion

4. You need a reference point to describe an object’s position. • A reference point is the starting point you choose to describe the location, or position, of an object • Position describes an object’s distance and direction from a reference point. • Position always includes a distance, a direction, and a reference point. How do you describe an object’s position? Three things you need to know to explain position.

5. Explaining Position Practice • Position depends on a reference point. • If the reference point in the figure below is the goal, or point A, the player’s position is 10 m in front of the goal. • If the reference point is center field, point B, the player’s position is 40 m toward the goal. 41.2 m C 10 m D 40 m B A 10 m The location of the player has not changed. Only the description of the position changed because the reference point changed.

6. The distance an object travels is calculated following the path it travels. • Displacement is the difference between the initial, or starting, position and the final position • an object’s displacement and the distance it travels are not always equal. How are distance and displacement different?

7. Distance vs. Displacement Practice • During one play in the soccer game, the player runs 41.2 m from position D to position C. Then she runs 10 m to position B. The dotted lines show her path. • The total distance she travels is 41.2 m + 10 m = 51.2 m • The player starts at point D and finishes at point B. Her • displacement is 40 m in front of her initial position. 41.2 m C 10 m D 40 m B A 10 m

8. Motion is the process of changing position. • Speed is the distance an object moves in a unit of time. What is Motion? What is Speed?

9. Constant speed: When an object moves the same distance over a given unit of time. • Changing Speed: When the distance an object covers increases or decreases over a given unit of time. • Average Speed: Average speed is equal to the total distance traveled divided by the total time. How is speed described?

10. Calculating Average Speed Practice Average speed = Distance traveled / time For example, a bus carrying students to a soccer game traveled 10 km in 30 min. What was the average speed of the bus in km/hr? Step 1: Change minutes to hours. 30 min = 0.5 h Step 2: Replace the terms in the formula with the given terms. average speed = 10km 0.5 h Step 3: Divide to get the answer. average speed = 10km So average speed = 20 km/h 0.5 h

11. Now it’s your turn . . . On a hike, you travel 2,800 m in 2 h. What is your average speed in m/s? Step 1: Change hours to seconds. (360 seconds = 1 hour) 360s X 2h = _________s Step 2: Replace the terms in the formula with the given terms. average speed = 2,800m ____s Step 3: Divide to get the answer. average speed = ___________m/s

12. Velocity is the speed and direction of a moving object. • Speed includes explaining the distance traveled in a time period BUT velocity also includes the direction the object is traveling. What is Velocity? How are Speed and Velocity different?

13. Velocity vs. Speed Constant Velocity Changing Velocity Constant Speed Changing Speed Changing Direction

14. Acceleration is a measure of how quickly the velocity of an object changes. Positive acceleration = when an object is speeding up, the acceleration is in the same direction as the velocity. Negative acceleration = when an object is slowing down, the acceleration is in the opposite direction as the velocity. What is acceleration? What is the difference between positive and negative acceleration?

15. Positive vs Negative Acceleration Acceleration Animations: 1. Acceleration http://www.physicsclassroom.com/mmedia/kinema/acceln.cfm 2. Positive vs. negative acceleration http://www.physicsclassroom.com/mmedia/kinema/avd.cfm

16. Speed is the rate of change of position. Expressed as distance traveled per unit of time (m/s). • Velocity is speed with a direction (north, south, east, west, up or down) (Ex: m/s N ) • Acceleration is the rate of change of velocity. (m/s2) How are the terms speed, velocity, and acceleration related?

17. Part 2 – Lesson 2 Graphing Motion

18. When you study motion, you need to know how position changes as time passes. How can you graph an object’s motion?

19. Setting up your distance-time graph y-axis x-axis Distance is always put on the y – axis (left side of the graph) Time is always put on the x – axis (bottom of the graph) Now just plot the data you have collected and draw your connecting line.

20. A distance-time graph tells us how far an object has moved with time. • A speed-time graph show us whether an object has constant or changing speed, velocity, or acceleration. How can a graph help youunderstand an object’smotion?

21. Reading distance time graphs The steeper the graph, the faster the motion. A horizontal line means the object is not changing its position - it is not moving, it is at rest. A downward sloping line means the object is returning to the start.

22. Reading Distance – Time Graphs Time is increasing to the right, but its distance does not change. It is not moving. We say it is At Rest. Time is increasing to the right, and distanceis increasing constantly with time. The object moves at a constant speed. Constant speed is shown by straight (not curving) lines on a graph.

23. Analyzing Motion on Distance-Time Graphs Both of the lines in the graph show that each object moved the same distance, but the steeper dashed line got there before the other one: A steeper line indicates a larger distance moved in a given time. In other words, greater speed. Both lines are straight, so both speeds are constant.

24. Analyzing Motion on Distance-Time Graphs Graphs that show acceleration look different from those that show constant speed. The line on this graph is curving upwards. This shows an increase in speed, since the line is getting steeper: In other words, in a given time, the distance the object moves is changing (getting larger). It is accelerating.

25. A distance-time graph tells us how far an object has moved with time. • The steeper the graph, the faster the motion. • A horizontal line means the object is not changing its position - it is not moving, it is at rest. • A downward sloping line means the object is returning to the start. Summary of distance-time graphs

26. Practice reading distance time graphs The distance-time graphs represent the motion of a car. Match the description with the correct graph. Descriptions: 1. The car is stopped. _____ 2. The car is traveling at a constant speed. _____ 3. The speed of the car is decreasing. ______ 4. The car is coming back. ____

27. Speed-Time graphs are also called Velocity-Time graphs. Speed –time graphs Speed-Time graphs look much like Distance- Time graphs. Be sure to read the labels!! Time is plotted on the X-axis. Speed or velocity is plotted on the Y-axis.

28. Reading Speed-time graphs A straight horizontal line on a speed-time graph means that speed is constant. It is not changing over time. A straight line on a speed-time graph does not mean that the object is not moving! This graph shows increasing speed. The moving object is accelerating This graph shows decreasing speed. The moving object is decelerating.

29. Analyzing Motion on Speed-Time Graphs Both the dashed and solid line show increasing speed. Both lines reach the same top speed, but the solid one takes longer. The dashed line shows a greater acceleration.

30. A speed - time graph shows us how the speed of a moving object changes with time. • The steeper the graph, the greater the acceleration. • A horizontal line means the object is moving at a constant speed. • A downward sloping line means the object is slowing down. Summary of Speed-time graphs

31. Practice reading speed time graphs The speed-time graphs represent the motion of a car. Match the description with the correct graph. Descriptions: 1. The car is stopped. _____ 2. The car is traveling at a constant speed. _____ 3. The car is accelerating (positively). ______ 4. The car is slowing down (accelerating negatively). ____

32. Homework assignment http://graphs.mathwarehouse.com/distance-time-graph-activity.php Go to the website listed above and play the game to bring the spacecraft to the mother ship. Have fun!!

33. Part 3: Lesson 3 Forces

34. A force is a push or a pull on an object. • Force has size and direction. • Force is expressed in Newtons (N) • There are two types of forces; contact and noncontact. • Contact force: push or pull one object applies to another object that is touching it. • Noncontact force: A force that one object applies to another object without touching it. What is force? What are the different types of forces?

35. Examples of Types of Forces Contact Non-contact Friction: is a contact force that resists the sliding motion of two surfaces that are touching. Air resistance is the frictional force between air and objects moving through it. Gravity: is a non-contact attractive force that exists between all objects that have mass.

36. Practice with friction Jorge has raked leaves into a heavy pile on top of a tarp. He wants to pull the tarp up an incline. The sketch below shows the main forces (other than Jorge’s) acting on the tarp. What would have to be increased to make it easier for Jorge to pull the tarp up the hill? the tarp’s area the tarp’s mass the force of the wind the steepness of the hill

37. Answer What would have to be increased to make it easier for Jorge to pull the tarp up the hill? the tarp’s area the tarp’s mass the force of the wind the steepness of the hill

38. The amount of Gravity exerted on an object depends on 2 things: mass and distance Mass Distance If the distance between two objects decreases the gravitational force between them increases. The opposite is also true. Distance = Gravitational pull Distance = Gravitational pull If the mass of an object increases, the gravitational force increases between it and another object.

39. Practice with gravity

40. What happens when forcescombine?

41. How are balanced andunbalanced forces relatedto motion?

42. Balanced versus Unbalanced Forces

44. Speed with Net Force Increasing speed with net force: If the forces on an object are unbalanced, the motion of the object will change. Decreasing speed with net force: a net force can also slow down something that is moving.

45. Gravity is a Universal Force!! Gravity is a result of all mass and all matter is affected by gravity. All objects experience an attraction to all other objects. Weight is the measure of the gravitational force on an object. This force is usually the Earth’s gravitational pull but can also be the moon or other planets.

46. Weight as a measurement of Gravitational Force Gravity is a force, and weight is a measure of gravity, so weight is also measured in Newtons. The SI unit of mass is kilograms (kg) but can also be in grams (g) or milligrams (mg). Mass is the amount of matter in an object but Weight is the mass plus the gravitational pull of the earth.

47. Gravity and Acceleration Objects fall to the ground at the same rate because the acceleration due to gravity is the same for all objects. Acceleration due to gravity - for every second that an object falls, the objects downward velocity increases by 9.8 m/s.

49. Gravity and Falling Objects Velocity of Falling objects: You can calculate the change in velocity with the equation below: ∆v  g  t If an object starts at rest, this equation yields the velocity of the object after a certain time period