Big Idea 13: Force and Motion By Mrs. Shaw
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.
Part 1 – Lesson 1 Describing Motion
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.
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.
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?
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
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?
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?
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
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
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?
Velocity vs. Speed Constant Velocity Changing Velocity Constant Speed Changing Speed Changing Direction
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?
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
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?
Part 2 – Lesson 2 Graphing Motion
When you study motion, you need to know how position changes as time passes. How can you graph an object’s motion?
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.
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?
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.
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 distance is increasing constantly with time. The object moves at a constant speed. Constant speed is shown by straight lines on a graph.
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, higher speed. Both lines are straight, so both speeds are constant.
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.
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
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. ____
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.
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.
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.
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
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). ____
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!!
Part 3: Lesson 3 Forces
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 between two objectstouching. • Example: Friction – shoe & floor • Noncontact force: A force applied without objects touching. • Example: Magnetic, electrical, & gravitational What is force? What are the different types of forces?
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.
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 friction on the tarp the tarp’s mass the force of the wind the steepness of the hill
Answer What would have to be increased to make it easier for Jorge to pull the tarp up the hill? the friction on the tarp the tarp’s mass the force of the wind the steepness of the hill
ALL objects with mass exert a gravitational force on ALL other objects. • YES . . . .you have gravity!!! • Bigger the mass = bigger the force of gravity • Weight (N) = mass (kg) x gravity (m/s2) • Gravity is an acceleration that is measured on Earth at 9.8 m/s2 Law of Universal Gravitation
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.
Practice with gravity 1. Which ball has more gravitational force? Mass = 0.62kg Mass = 0.045kg Ball A Ball B Which ball exerts more gravitational force on Ball A . . . . . Ball B or Ball C? Ball A Ball B Ball C
Mass is the amount of matter in an object. Weight = Mass X Gravity *Which object has more gravity? Earth, Moon, Jupiter, or Sun Mass vs. weight
The sum of all the forces acting on an object is called the net force. • When forces act in the same direction you add them to get the net force. 25 N 20 N = 45 N • When forces act in opposite directions you subtract them to get the net force. 25 N 20 N = 5 N What happens when forcescombine?
When the net force acting on an object = Zero Newtons (0N) the forces are called Balanced. 25 N 25 N = 0 N • When the net force acting on an object are Greater than Zero Newtons (>0N) the forces are called Unbalanced. 25 N 20 N = 5 N How are balanced andunbalanced forces relatedto motion? **Motion or change in motion can only occur when net force is unbalanced.
Practice Calculating Net Force Write down these 5 problems in your notebook then calculate net force AND describe whether motion occurs. 100 N Up + 50 N Down = 50 N Up + 50 N Down = 62 N East + 50 N West = 150 N Forward + 150 N Forward = 22 N Forward + 12 N Backward (friction) =
Newton’s 1st Law: An object at rest stays at rest and an object in motion stays in motion unless acted on by an outside force. (also called the law of inertia) • Inertia is the tendency of all objects to resist any change in motion. • Mass and Inertia are directly related. Greater mass = greater inertia. • So, changing the motion of an object that has a small mass is easier than changing the motion of an object that has a large mass. Newton’s 3 Laws of Motion
Newton’s 2nd Law: The acceleration of an object depends on the mass of the object and the amount of force applied. Force = Mass x Acceleration F=ma OR F/m=a Newton’s 3 Laws of Motion
Newton’s 3rd Law: For every action, there is an equal and opposite reaction. • Our rockets used air pressure as the force that powered them to lift off. The larger the amount of air pressure the higher the rockets flew. Newton’s 3 Laws of Motion
Practice Questions 1. Like Earth’s orbit around the Sun, the Moon’s orbit around Earth is elliptical. The diagram below shows the Moon’s orbit around Earth and Earth’s orbit around the Sun.
Question 1 continued . . . 1. Which of the following best describes the force that holds the Moon close to Earth? A. the magnetic force of Earth B. the magnetic force of the Sun C. the gravitational force of Earth and the Sun D. the gravitational force of Earth and the Moon
Question 1 answered . . . 1. Which of the following best describes the force that holds the Moon close to Earth? A. the magnetic force of Earth B. the magnetic force of the Sun C. the gravitational force of Earth and the Sun D. the gravitational force of Earth and the Moon