S 8 P 3 Relationship between the

S 8 P 3 – Relationship between the cause and effect between force, mass and the motion of objects

Grade 8 Science Standards • S 8 P 3 Students will investigate relationships between force, mass and the motion of objects. • A. Determine the relationship between speed, velocity and acceleration. • B. Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction.

Motion/ Reference Point • Motion is defined as an object having a change in position. • But how do you know if something has moved or not? P. 282 • Reference Point- A stationary point of comparison used to measure the distance an object has moved. • Position- The point or area occupied by a physical object.

Frame of Reference l 1 minute!!! Discuss with the person next to you: Is the speed of the ball different relative to the pitcher, the truck driver, and the jet pilot? Why or why not?

Distance • Distance is the measurement from one point to another. • Distance may refer to a physical length, a period of time, or an estimation.

Distance l Distance (d) – how far an object travels. l Does not depend on direction. l Imagine an ant crawling along a ruler. 0 1 2 3 4 5 6 7 8 l What distance did the ant travel? cm d = 3 cm 9 10

Distance l Distance does not depend on direction. l Here’s our intrepid ant explorer again. 0 cm 1 2 3 4 5 6 7 8 9 l Now what distance did the ant travel? l d = 3 cm l Does his direction change the answer? 10

Displacement • Displacement is the direction and the distance in an objects position from its starting and ending points.

Displacement l Let’s revisit our ant, and this time we’ll find his displacement. - + 0 cm 1 2 3 4 5 6 7 8 l Distance: 3 cm l Displacement: +3 cm l The positive gives the ant a direction! 9 10

Displacement l Find the ant’s displacement again. l Remember, displacement has direction! - + 0 cm 1 2 3 4 5 l Distance: 3 cm l Displacement: -3 cm 6 7 8 9 10

Displacement l Find the distance and displacement of the ant. - + 0 cm 1 2 3 4 5 l Distance: 7 cm l Displacement: +3 cm 6 7 8 9 10

Displacement vs. Distance l Example of distance: l The ant walked 3 cm. l Example of displacement: l The ant walked 3 cm EAST. l An object’s distance traveled and its displacement aren’t always the same!

Distance vs. Displacement l You drive the path, and your odometer goes up by 8 miles (your distance). l Your displacement is the shorter directed distance from start to stop (green arrow). start stop

Practice Problem 1 l An athlete runs around a track that is 100 meters long three times, then stops. l What is the athlete’s distance and displacement? l Distance = 300 m l Displacement = 0 m l Why?

Practice Problem 2 l A whale swims due east (from 0 km) a distance of 5 km, turns around and goes due west for 2 km and finally turns around again and heads 4 km due east. l What is the total distance traveled? l What is the displacement?

Practice Problem 3 Motion An object moves from point 1 to point 4 then reverses and ends at point 2 An object moves from point 1 to point 5 then reverses to point 2 An object moves from point 1 to point 3 then reverses to 0 An object moves from point 3 to point 5 and then reverses to point 1 An object moves from point 2 to point 4 and reverses to point 2 Distance Traveled Displacement X

Rate can be defined as how quickly omething changes. n physics, we calculate the rate of change y dividing the quantity of change by the me it takes for it to occur.

Speed l Speed (s) – Rate at which an object moves speed = distance / time s=d t l Speed (m/s)=distance (m) / time (s) l Units: m/s OR km/h l Like distance, speed does not depend on direction.

Speed • Speed is a measure of the distance an object moves in a given amount of time. • During a typical trip to school, your car will undergo a series of changes in its speed.

Measures of Speed l Constant speed - Speed that does not change (same distance is travelled in the same amount of time) l Instantaneous speed – Speed at a given instant in time (what the speedometer says right now!) l Average speed-- The total distance traveled divided by the total time it took to travel that distance.

When completing a formula you MUST show all your work! l Step 1 – write formula l Step 2 – plug in the information you are given l Step 3 – solve l Step 4 – add your units

Speed Practice Problems 1. Melissa shot a model rocket 360 m into the air. It took the rocket 4 s to fly that far. What was the average speed of the rocket? 2. If Jessica ran 5 meters in the first second, 8 meters the next second, and 5 meters the third second to her house, what was her average speed? 3. What was her instantaneous speed at the end of 2 seconds?

Can I determine time given distance and speed? How? l A rocket is traveling at 10 km/s. How long does it take the rocket to travel 30 km? Can I determine distance given time and speed? How? l A racecar is traveling at 85. 0 m/s. How far does the car travel in 30. 0 s?

S=d/t D S T

Calculating Speed, Distance or Time l If you know any two of the variables, you can calculate the missing variable.

Velocity • Velocity is an object’s speed in a particular direction. If the direction changes, the velocity will also change. • Velocity always includes speed and direction. • The velocity of an object can change if it: – Speeds up – Slows down or – Changes direction

Is this speed or velocity? • A man drives 30 km/h to catch a train. Speed • The train was traveling at 10 km/h east. Velocity • The train slowed down to 1 km/h as it was pulling into the station. Velocity • The train followed the tracks to the right as it sped up and turned to leave the station. Velocity

You tell me review! Vs Velocity An object is moving in a circle at a constant speed of 10 m/ s We say that it has a constant speed but its velocity is changing. Why? Direction of Motion The direction of the object keeps changing.

Graphs show relationships D Distance l A good way to show a relationship between two variables is to use a graph. l A graph makes it easy to see if changes in one variable cause changes in the other variable (the effect).

The distance vs. time graph used to graph the speed of an object or compare speeds of multiple objects Distance l To graph data, you put time on the horizontal (x) axis – this is your independent variable. l Distance goes on the vertical (y) axis – this is your dependent variable.

The distance vs. time graph l Distance vs. time data tells you the runner’s position at different points in time. l The runner is at 50 meters after 10 sec. , 100 meters after 20 sec. and 150 meters at 30 sec. Distance

A straight, diagonal line indicates… l Constant Speed Why is the line in a constant speed graph straight and diagonal? • The object is traveling the same distance in the same amount of time.

An Object At Rest l Object’s AT REST are not moving l A horizontal line on a Distance vs. Time graph has NO SLOPE = 0 speed

A curved line indicates… • Changing Speed

Distance-Time Graph and Changing Speed What do the different lines indicate when an object is changing speed? Downward Curve l Slowing down Horizontal Line • Stopping Upward Curve • Speeding Up

Time/Distance Graph

Terry, Jade and Jerome raced each other. Plot their data on a distance vs. time graph!

Comparing speeds on a Distance – Time Graph 1. Find the average speed of each line. 2. Compare the steepness of each line. The steeper slope indicates: Faster Speeds

Distance vs. Time Graphs l Which graph shows the faster moving object?

Math Connection What does the slope tell you on a distance vs. time graph? l Rise = Distance (y axis) l Run = Time (x axis) l Rise/Run = Speed or velocity

Force • A push or pull that causes an object to move, stop, or change direction • A force will cause an object with mass to accelerate.

Forces can affect motion in several ways: • • • → They can make objects start moving. → They can make objects move faster. → They can make objects move slower. → They can make objects stop moving. → They can make objects change direction. • ***These are ALL forms of acceleration!!

Acceleration & Deceleration • Acceleration is an increase in speed. It is a positive quantity. • Deceleration is a decrease in speed. It is a negative quantity. • Acceleration can also occur because of a change in direction.

Types of acceleration l Increasing speed l Example: Car speeds up at green light l Decreasing speed screeeeech l Example: Car slows down at stop light l Changing Direction l Example: Car takes turn (can be at constant speed)

Calculating Acceleration (m/s 2) = final speed – initial speed time a= sf – si t 1. Calculate the acceleration of a car whose speed changes from 6 m/s to 12 m/s in 3 s. 2. Calculate the acceleration of a bus whose speed changes from 30 m/s to 10 m/s in 5 s.

Graphing Acceleration Speed – Time Graphs l. Shows how SPEED changes over TIME l. X axis = TIME l. Y axis = SPEED

Object at Rest • The speed is zero and does not change • Horizontal line (like a distance – time graph y = 0 )

Constant Speed • The line will be HORIZONTAL • The further the line is from the X - AXIS the FASTER SPEED It is moving.

Speeding Up • The line on a speed – time graph is CLOSER to the x-axis in the BEGINNING of the time period when it has a LOWER speed. • The line slants UPWARD toward the right side of the graph as the speed INCREASES.

Slowing Down • When initially starting to slow down the point representing speed is FAR from the x-axis. • As speed DECREASES the points representing speed get CLOSER to the x-axis. • The line on a speed-time graph slopes DOWNWARD to the right. • When the line touches the x-axis, the speed is ZERO and the object stopped.

Mass & Inertia • Mass is the amount of matter in an object. • Inertia- the tendency of an object to resist a change in its motion.

More on Inertia • As an object’s mass increases…so does it’s inertia. WEAR YOUR SEATBELT!!!! • https: //www. youtube. com/watch? v=d 7 i. YZ Pp 2 z. YY

Momentum • Momentum- A measure of how hard it is to STOP a moving object and it depends on the mass and velocity of the object. • Which would be harder to stop…. – A 68 kg. guy going 10 m/s on a bicycle or a 2 g. bullet fired from a rifle at 1200 m/s? Why ? Momentum = mass X velocity (g or kg x m/s ) = g or kg x m/s

Law of Conservation of Momentum • In any collision between objects, momentum is transferred from one object to another. (example…playing pool or bowling. ) • The total momentum will stay the same unless acted on by an outside force like gravity or friction. • Let’s make predictions based on size and directions: small- big, Big-small, same size

Momentum Simulation • http: //phet. colorado. edu/sims/collisionlab/collision-lab_en. html