Keynote; ch. 9; speed & accelerations

Science Notes: Ch. 9; Motion & Energy

Position & Motion An object’s position is defined by a reference point and a reference direction. A reference point is something used for comparison to determine if an object is in motion. An object is in motion If the object changes position relative to a reference point.

Reference Point

Distance Distance is the length of a path between two points.

Displacement Displacement is the length and direction an object has moved from its starting point. Don’t confuse distance with Displacement.

Speed An object’s speed is the distance it travels in a given amount of time. Equation; Speed = distance/time S = d/t

Practice problems... Click for constant speed Word Problems...

Average speed Equation In most real-world situations, speed is not constant but changes, requiring several calculations. Average speed = Total distance/Total time. Average S = total d/total t Sa = Dt /Tt Dt = 32km + 13km = 45km T t = 2hr + 1hr = 3hr Sa = 45km/3hr, = 15km/hr

Instantaneous Speed The speed at which an object is moving at a particular point in time.

Graphing speed Constant speeda... Increasing speeda...

Slope... The slope of a d vs. t (d-t) graph is the rise over the run (y over x). Active art Here The slope of a d-t graph represents the speed.

Graphing speed, cont... Reducing speeda Final speeda

More d-t graphs...

Which graphs are impossible?

Vectors A Vector is a quantity that has both a magnitude and a direction. Example: 45 mph Northwest.

Velocity Velocity and speed are different! Velocity is speed in a given direction. IMPORTANT; Changes in velocity may be due to change in speed, change in direction, or both!!

Velocity equation Velocity is change in speed or direction so... Initial speed minus final speed... V = S1 - S 2 = meters/sec = change in m/s. If S1 - S2 is positive its speeding up. If S1 - S2 is negative its slowing down. A velocity always has a direction with it: “3m/s in a N-W direction”

D-T graphs, slope, & velocity...

Acceleration... Change in speed or direction is called acceleration. Increase in speed is acceleration, decrease in speed is deceleration. ***An object can still be accelerating even if its speed is constant if it changes direction. For example the seats in a Ferris wheel are accelerating even though the speed may be constant because they are moving in

Here’s what acceleration looks like...

Acceleration equation... So... acceleration is change in velocity per unit of time. Here’s the formula... A = V1 - V 2 = meters/sec = m/s 2 T 1 - T2 sec V1 is the initial velocity and V2 is the final. and...

Acceleration equation... T1 - T2 is the total elapsed time... T1 - T2 is always positive (can’t go back in time if V1 - V2 is positive the object is accelerating if V1 - V2 is negative the object is decelerating. (note this is not how the book presents it...

Graphing Acceleration If there is a curved line on a d-t graph the object is either accelerating or decelerating. Smile = accelerating Frown = decelerating

This is acceleration on a d-t graph Smile up = accelerating, smile down decelerating.

This is acceleration

Graphing Acceleration on a v-t graph A sloped line (either positive or negative) on a v-t graph represents acceleration.

Equation for graphing acceleration on a V-T graph... Only sloped portion is accelerating

V-T versus D-T graphs D-T Graph... V-T Graph... slope = velocity Straight slope = accel.

Speed vs. time graph... On an s-t graph, acceleration is always present when there is a sloped line (the graph below shows constant acceleration not deceleration).

Acceleration is also... Related to mass and force (Einstein)(Ch. 10) Force equals mass X acceleration: F = (m)X(a) So... Acceleration equals Force/Mass: A = (f)/(m)

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Keynote; ch. 9; speed & accelerations

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