Motion & Force Unit 1

Grade 8 Science Standards S8P3 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.

Relative 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. Relative Motion- An object’s change in position relative to the reference point.

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

Displacement Displacement is the direction and the distance in an object’s position from its starting and ending points. Example: Suppose you walked 200 meters East to the park from your house, what would be your displacement? Example 2: Suppose Dale Jr. drives the Daytona 500. Leaves from the start/finish line, completes the race and crosses the same start/ finish line. What would be his displacement?

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.

Formula for Speed Speed (m/s)=distance (m) / time (s) s = d t Average speed: divide the total distance by the total time taken to complete a change in an object position. Instantaneous speed: Speed of an object at a given instant in time. P.285

Time v. Distance Graphs Used to compare the speeds of two or more objects. Time is plotted on the X-axis. Distance is plotted on the Y-axis A straight horizontal line on a speed-time graph means the speed is constant. This means the speed is not changing over time.

Time/Distance Graph

Speed is a rate A rate is a ratio of two different variables. Speed is equal to the rate of distance divided by the time it takes to go that distance. Tim & Moby break it down...

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.

Speed vs Velocity Let's ask Mr. Panda Bear!

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 positive acceleration. Deceleration is a decrease in speed. It is negative acceleration.

Calculating Acceleration Acceleration (m/s2) = final speed – initial speed time a= sf – si t Calculate the acceleration of a car whose speed changes from 6 m/s to 12 m/s in 3 s. Calculate the acceleration of a bus whose speed changes from 30 m/s to 10 m/s in 5 s.

Speed & Time Graph

Where is Zero Acceleration?

Acceleration: Video Help Put the pedal to the medal!

Speed/Acceleration Summarizer Graphing Activity Look at the graph and answer the questions in your ISN. SPEED LAB (OUTSIDE)

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=d7iYZPp2zYY

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/collision-lab/collision-lab_en.html

Video Motion & Force