Linear Motion.

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Presentation transcript:

Linear Motion

Jumpstart Calculate the acceleration of a car that is traveling from 80 m/s to 0/s in 20 seconds.

Kinematics Branch of mechanics that describes the motion of objects without necessarily discussing what causes the motion.

Motion is relative Measured in reference to another object or point. Depends on Frame of Reference. How fast is your body moving right now?

How far did you go? Distance Needs no frame of reference Separation between two points Only a length, no direction Scalar Quantity Units: Meters, kilometers, centimeters. . . .

What is your new position? Displacement Change in position. Where are you relative to some starting point? Magnitude and direction Vector Quantity Express direction with a sign or direction 45m to the North -82 cm A B C D

The difference between distance and displacement. . . . .

Speed How fast something is moving. Speed = distance/time Units: m/s, km/h, mph. . . Scalar Quantity Instantaneous speed Average speed Over the course of a trip, can instantaneous speed and average speed be different from each other?

Approximate Speeds in Different Units Miles per hour Meters per second Kilometers per hour 12 mph 6 m/s 20 km/h 25 mph 11 m/s 40 km/h 37 mph 17 m/s 60 km/h 50 mph 22 m/s 80 km/h 62 mph 28 m/s 100 km/h 75 mph 33 m/s 120 km/h

Velocity Speed in a given direction. Vector quantity Magnitude AND direction Velocity = displacement/time V = d/t Units = m/s, km/h, mph. . . Direction expressed with Signs: + or – Direction: North, South, East, West, Left, Right, Forward, Backward. . .

A change in velocity is called. . Constant Velocity constant speed and direction Changes in velocity are due to Increase/decrease in speed Change in direction If you drive around a circular track with a constant speed of 60 km/h, is your velocity also constant? A change in velocity is called. .

Velocity Instantaneous Velocity Average Velocity

Example Problems A team begins play on the 50 yard line; they lose 5 yards on a play and then gain 15 yards on the next play. What distance did the team travel? What was the team’s displacement? A truck travels 300km north in 10 hours. What was the truck’s average velocity?

Example Problems How long would it take you to drive 15 km a rate of 30 km/h?

Acceleration The rate of change of velocity. Equal to the change in velocity divided by time. a = (vf – vi)/t OR a = Δv/t Units: Meters per second per second (m/s/s) Meters per second squared (m/s2) If a car can accelerate from 0-60mph in 15 seconds, what is its acceleration? (60mph – 0 mph) / 15s = 4 mph / s2

Acceleration Vector quantity Can refer to: Has both magnitude and direction Direction expressed with a sign (+/-) Can refer to: Increase in speed Decrease in speed Change in direction

Acceleration If speed increases while you are moving forward, acceleration is positive. If speed decreases while you are moving forward, acceleration is negative Example 1: You are driving and increase your speed from 50 m/s to 75 m/s in 10 seconds. What is your acceleration? 75 m/s – 50 m/s = 25 m / s = (2.5 m / s2) 10 s 10 s

Acceleration Example 2: You are riding your bike and slow from 10 m/s to 5 m/s in 2 seconds. What is your acceleration? Example 3: How long will it take to accelerate from 20 m/s to 40 m/s if your velocity changes at a rate of 4 m/s2?

Position-Time Graphs Position on the y-axis, time on the x-axis. Position is measured with respect to a reference point; therefore position can increase or decrease. Slope of the line is velocity.

Position-Time Graphs Time (seconds) Distance (meters) 10 5 20 30 15 40 10 5 20 30 15 40 50 25 60 70 35 80

Position Time Graphs Time (seconds) Position (meters) 5 10 20 15 30 40 5 10 20 15 30 40 50 60 70 80 90 100 110 120 130

Position Time Graphs Time (seconds) Position (meters) 5 10 20 15 30 40 5 10 20 15 30 40 50 60 70 -10 80 -20 90 100 -15 110 120 -5 130

Position-Time Graphs Time (seconds) Distance (meters) 1 2 4 3 9 16 5 1 2 4 3 9 16 5 25 6 36 7 49 8 64

Position-Time Graphs Time (seconds) Distance (meters) 1 2 4 3 9 16 5 1 2 4 3 9 16 5 25 6 36 7 49 8 64 Average Velocity Instantaneous Velocity Instantaneous Velocity Instantaneous Velocity

Position-Time Graphs Curve = changing velocity Constant Acceleration = Curve Straight Line = constant velocity

Position Time Graphs Constant + v, slow Constant + v, fast

Position-Time Graphs Positive acceleration (increasing velocity) in the negative direction Negative acceleration (decreasing velocity) in the negative direction

Position-Time Graphs

Velocity-Time Graphs Time on the x-axis (s) Velocity on the y-axis (m/s) Slope is acceleration (m/s2) Area under graph represents displacement. It line is curved – the acceleration is changing

Velocity-Time Graphs Slope of the graph is important: Slope = 0 (Horizontal line) Velocity is constant Positive slope Velocity is increasing Negative slope Velocity is decreasing Shape of the graph is important Straight line: acceleration = 0 Velocity may be 0 Velocity may be constant Curve Acceleration is changing

Velocity-Time Graphs Increasing Velocity Constant +Acceleration Time (seconds) Velocity (m/s) 1 10 2 20 3 30 4 40 5 50 6 60 7 70 8 80 What was this object’s displacement?

Velocity-Time Graphs Decreasing Velocity Constant -Acceleration Time (seconds) Velocity (m/s) 80 1 70 2 60 3 50 4 40 5 30 6 20 7 10 8

Velocity-Time Graphs Increasing Negative Velocity Constant -Acceleration Time (seconds) Velocity (m/s) 1 -10 2 -20 3 -30 4 -40 5 -50 6 -60 7 -70 8 -80

What do you think? If you are given a table of velocities of an object at various times, how would you determine if the acceleration of the object was constant? Time (seconds) Velocity (m/s) 1 -10 2 -20 3 -30 4 -40 5 -50 6 -60 7 -70 8 -80

Velocity-Time Graphs Decreasing Negative Velocity Constant +Acceleration Time (seconds) Velocity (m/s) -80 1 -70 2 -60 3 -50 4 -40 5 -30 6 -20 7 -10 8

Velocity-Time Graphs Wed. 10/10/12 HOW COULD YOU DETERMINE: Average Velocity? Instantaneous acceleration? Velocity-Time Graphs Wed. 10/10/12 Time (seconds) Velocity (m/s) 1 2 4 3 9 16 5 25 6 36 7 49 8 64

Velocity-Time Graphs Increasing Velocity Positive Acceleration Time (seconds) Velocity (m/s) 1 2 4 3 9 16 5 25 6 36 7 49 8 64

Velocity-Time Graphs Time (seconds) Velocity (m/s) 1 2 4 3 9 16 5 25 6 1 2 4 3 9 16 5 25 6 36 7 49 8 64 Average Acceleration Instantaneous Acceleration Instantaneous Acceleration Instantaneous Acceleration

Velocity-Time Graphs Constant velocity (v=40m/s) Zero Acceleration

Acceleration-Time Graphs

Acceleration-Time Graphs

Acceleration-Time Graphs

Constant Rightward Velocity Website Animation

Constant Leftward Velocity

Rightward Velocity/Rightward Acceleration

Rightward Velocity/Leftward Acceleration

(10/10/12) Wednesday Sketch a position vs. time , velocity vs. time and an acceleration vs. time graph for each of the following two situations. Leftward velocity and rightward acceleration Leftward velocity and leftward acceleration

Leftward Velocity/Rightward Acceleration

Leftward Velocity/Leftward Acceleration

And now for the calculations . . . Assume acceleration is constant Either zero or some non zero value. Variables a = acceleration v0 = initial velocity v = final velocity d = displacement t = time

Recap: Displacement – change in position Δx = x2-x1 Velocity – a measure of how fast something moves from one point to another. Average Velocity - change in position change in time Vavg = Δx or vavg = (vf + vi) Δt 2

DERIVING THE BIG 4 KINEMATIC EQUATIONS Velocity of an object with constant acceleration (condition often true if air resistance ignored) Basic formula for acceleration Solve for vf (final velocity) Given any three of these variables, you can solve for the fourth. (vf – vi) = a t vf – vi = at vf = vi + at

The Big 4 Kinematic Equations vf = vi + at

Velocity and Displacement under average acceleration DERIVING THE BIG 4 Velocity and Displacement under average acceleration Δx = Vavg Δt Vavg = Δx AND Vavg = (vf + vi) Δt 2

Combine the equations for displacement and average velocity: DERIVING THE BIG 4 Combine the equations for displacement and average velocity: Δx= vt AND Vavg = (vf + vi) 2 vt = ½(vf + vi)t Δx = ½(vf + vi)t Sub in Vavg substitute Δx for tv

The Big 4 Kinematic Equations vf = vi + at Δx = ½(vf + vi)t

Displacement when acceleration and time are known. vf = vi + at Δx = ½(vf + vi)t Δx = ½(vi + at + vi)t Δx = ½ t (2vi + at) Δx = 2vit + at2 2 2 Δx = vit + ½at2 plug first formula into second formula for vf Simplify the resulting formula (ΔX is displacement)

Review 3 Kinematic equations from our earlier derivation vf = vi + at Δx = ½(vf + vi)t Δx = vit + ½at2

Displacement when velocity and acceleration are known Δx = ½(vf + vi)t v = vi + at t = (vf– vi)/a Δx = ½(vf + vi)(vf – vi)/a Δx = ½(vf2 – vi2)/a Δx = (vf2 – vi2) 2a (re-arrange to solve for Vf2 ) vi2 + 2aΔx = vf2

The Big 4 Kinematic Equations vf = vi + at Δx = ½(vf + vi)t Δx = vit + ½at2 vf2 = vi2 + 2aΔx

The Big Four Kinematic Equations v = vi + at x = ½(v + vi)t x = vit + ½at2 (starts at rest it becomes:) x = ½ at2 v2 = vi2 + 2ax Variables v, v0, a, t x, v, v0, t x, v0, t, a v, a, x, v0

Using the big 4 derived from the definition of displacement, velocity and acceleration – you can solve almost any 1D problem under constant acceleration.

How to solve a problem Read the problem carefully – try to visualize situation. Make a sketch. Define your variables. Write out your knowns and unknowns. Decide which formula contains each of these variables. Solve the formula for the variable you are trying to solve for. Plug in numbers and solve the problem Check your answers – to you have units? Do they make sense – did you answer the question? (seconds for time – meters for distance etc…)

Example 1 A race car starts at rest and accelerates at a constant +0.75 m/s2 for 10 seconds. What is the rider’s displacement during this time? What are the knowns? V0 = a = t = x =

Example 2 Starting from rest, a race car moves 100 m in the first 5.0 s of uniform acceleration. What is the car’s acceleration? What are the knowns? V0 = a = t = x =

Example 3 A car accelerates at a constant rate from 15 m/s to 25 m/s while it travels 125 m. How long does this motion take?

Example 4 A pilot stops a plane in 484 m using a constant acceleration of -8.0 m/s2. How fast was the plan moving before braking began?

What about the direction? Airplanes … Speed is how fast is something going. Speed = distance crossed / time elapsed Velocity is the speed and direction of an object. Velocity is a vector (has magnitude and direction). Speed is a scalar (has magnitude only)

Speed and Velocity Moving all the time equally fast (with respect to the ground) - called? Constant speed Moving all the time equally fast and also in the same direction? Constant velocity Do we always move equally fast? Do we always move in the same direction?

Average and Instantaneous Speed Trip from home to college Things to Consider: Stop signs Traffic lights Speeds allowed What does the car speedometer measure? What is the police concerned about?

Average and Instantaneous Speed Things to Consider for the each trip: Average speed? Maximum speed? Time What is the trip choice based on? Unit conversion 2.8 mil in 5 min 1.8 mil in 6 min Video 1

Average Velocity Things to Consider: Average Velocity? Distance and Displacement Average Velocity? Avg. Velocity same or different for both trips? Avg. Velocity after returning home 2.8 mil in 5 min Straight Shot 1 mile 1.8 mil in 6 min

Average and Instantaneous Velocity Things to Consider: Average Velocity and Instantaneous Velocity 2.8 mil in 5 min Straight Shot 1 mile 1.8 mil in 6 min

Reminder (Previous cycle): Distance and Displacement START FINISH Need to distinguish how long we traveled from how far away (and in what direction) we traveled.

Distance and displacement Distance and displacement are two quantities which may seem to mean the same thing, yet have distinctly different definitions and meanings. Distance is a scalar quantity (has magnitude only) which refers to "how much ground an object has covered" during its motion. Displacement is a vector quantity (has magnitude and direction) which refers to "how far out of place an object is"; it is the object's change in position. http://www.glenbrook.k12.il.us/gbssci/phys/Class/1DKin/U1L1c.html

Direction of the Average Velocity START FINISH

Direction of the Average Velocity Displacement is a vector - Distance is not Velocity is a vector - Speed is not START FINISH

Make sense: Velocity and speed How would you determine the average speed of an object on these paths: START FINISH START FINISH Average speed vs. Average velocity

Velocity is a vector Velocity is a vector Vectors are quantities that have magnitude and direction Speed is the magnitude of a velocity.

Average vs. Instantaneous Velocity

Gravity On earth, all objects fall with an acceleration of -9.8m/s2. varies slightly with location Pike’s Peak = -9.879 m/s2 Maine = -9.810 m/s2 Any object with mass will fall with this acceleration—if air resistance is ignored.

Gravity As an object falls, it gains 9.8m/s every second. As a thrown object rises, it loses 9.8 m/s every second. v = vo + gt

Draw the p-t, v-t, and a-t graphs for this situation Gravity Because the object’s velocity is increasing, it covers more distance each second. x = vot + ½gt2

Gravity The same uniform acceleration formulas apply to gravity Instead of a, use “g” v = vo + gt x = ½(v + vo)t x = vot + ½gt2 v2 = vo2 + 2gx

Example 1 Suppose a ball is dropped from a height of 100m. How long will it take for the ball to hit the ground? What will the ball’s velocity be when it hits the ground?

What do you think? A baseball is thrown straight up into the air. When the ball reaches its maximum height its velocity is equal to ______?

What do you think? What is the total displacement of the ball upon landing? (from previous slide)

What do you think? Ignoring air resistance, if a 2 kg ball and a 5 ton SUV were dropped from the top of the Empire State building (assume all pedestrians below had been cleared) – the acceleration of the 2 kg ball would be _______ the acceleration of the SUV. Greater than Less than Equal To

Example 2 You drop a stone off the side of a cliff. You hear it hit the ground 4 seconds later. How high is the cliff?

Example 3 You throw a ball straight up in the air. The ball leaves your hand with an upward velocity of 30 m/s. Draw a diagram showing the path of the ball and its velocity at the following times: 0.0s, 1.0s, 2.0s, 3.0s, 4.0s, 5.0s, 6.0s