Question

Honors Physics - Uniform Acceleration

Answer 1

Benny 'The Jet' Rodriquez hits a pop-up straight up. Sketch a position vs. time graph, a velocity vs. time graph, and an acceleration vs. time graph for the motion of the baseball from the instant it leaves his bat until the instant before it is caught by the catcher.

Answer 2

@Vocaloid Can I bother you for a bit with some dumb questions?

Answer 3

Depressingly, I saw the answer key afterwards but I just didn't understand at all. ):

Answer 4

hm. physics isn't really my strongest subject but I can try

position just seems like it would be a downward facing parabola (the ball goes up, reaches a peak, then drops again)

Answer 5

See, I just don't understand why. :( The parabola was indeed the right answer, but I don't know why.

Answer 6

if you think of x (position) as "distance from the starting position" the ball goes away from the person, then reaches a point where it can't go any farther, then drops back down again

if the ball kept going up forever that would mean it somehow escaped the gravitational pull of the earth

Answer 7

I also don't understand the difference between velocity, speed, and acceleration. :/

Answer 8

well first we have to understand distance vs. displacement

distance = the total amount traveled by an object
displacement = the net amount traveled by an object. this is found by subtracting (ending position - starting position). this might be more clear with an example:

if you walk 3 miles east and 4 miles north the distance = 7 miles (3+4) but the displacement is 5 miles (I'll show you why)

Answer 9

I understand the displacement. It's like the slope, correct?

Answer 10

hm, I don't think so, displacement wouldn't be the slope of the graph; on a position vs time graph the slope would be velocity

Answer 11

well in short

speed = total distance/total time
velocity = net change in distance/total time, this also includes direction
acceleration: net change in velocity / total time

Answer 12

Displacement in light red?

Answer 13

yup good

Answer 14

do you want to go back to your baseball thing or do you want to talk more about speed/velocity/acceleration?

Answer 15

Stupid question #2: What is net change lol

Answer 16

like, the shortest distance between the final and initial positions would be the "net" change

Answer 17

so in the example you drew, the red line = net change in distance while black line = total change in distance

Answer 18

OH WAIT LMAO IS NET CHANGE AND DISPLACEMENT THE SAME

Answer 19

yeah pretty much; net change in distance ---> displacement

Answer 20

tweaked.
So if making a formula, speed is total d/t
velocity is change in t over total time, but if it was like from up to down or something it would be negative.
I just don't get acceleration at all

Answer 21

velocity = displacement/time
acceleration = change in velocity/time

so if a car changes from 0 m/s to 20 m/s in 1 second, the acceleration is [20 m/s - 0 m/s ] / 1 second = 20 m/s/s

Ihavenoideahowcarsworkorifthatsevenareasonablenumber

Answer 22

Negative just refers to the direction. In Vocaloid's example, say it was a change in velocity from 0m/s to 20m/s west, it would be 20m/s/s as the acceleration. But if it was 0 to 20m/s east, the acceleration would be -20m/s/s.

Answer 23

ah, yes, since velocity includes direction then acceleration does too

Answer 24

just keep in mind there has to be a reference point; he defined acceleration west as positive and acceleration east as negative; it shouldn't matter as long as you are consistent with your definition

Answer 25

I hope that makes a bit more sense; the calculations for acceleration are very similar to velocity, just using velocity/time instead of displacement/time

Answer 26

It does, and I thank you for that. Learning with you is much better than wasting 50 minutes in his class lol.
In the answer key, the velocity graph is a negative linear line, how?

Answer 27

when he throws the ball upwards, he's giving it a positive velocity so your line starts with some positive value

as the force of gravity affects the ball, it will pull downwards, slowing the ball down. at the top of the position parabola, the velocity reaches 0 and the ball momentarily stops, then the velocity becomes negative as the ball goes back down to earth

Answer 28

the only accelerating force on the ball is gravity, and gravity is constant, so the slope of the velocity graph should be constant which is why it's linear

Answer 29

Oh my lord, I understand it. Somehow I thought it was a parabola...again because I thought velocity went up, then went down, but either way it's going down because the moment you hit it the velocity gets slower. Why wouldn't be this though :/

Answer 30

Cause wouldn't the velocity get faster as the ball reaches the ground?

Answer 31

hm. Idk if this'll make sense but

yes, the velocity does keep increasing in the negative direction, but the rate of increase is constant b/c acceleration is constant.

Answer 32

@Shadow if you have anything to add I'd appreciate it ;_;

Answer 33

You guys are on the acceleration graph right (velocity over time)

Answer 34

no not yet, we're still on the velocity graph; lowkey wants to know why the velocity graph has a constant slope even as the velocity of the ball increases

Answer 35

"position vs. time " this one?

Answer 36

so the original problem is a person throwing a baseball up in the air and they want to know why the velocity graph is this



rather than this

Answer 37

Okay, yeah. That's just how I like to think of them, since velocity over time is technically acceleration. It helps for these kinds of problems. But Vocaloid is right, the graph would be constant since the only force on the ball is gravity, which is -9.8m/s/s (a constant rate).

Answer 38

@lowkey are you confused in any way

Answer 39

@Shadow yeah, everything now

Answer 40

so hold on, velocity/time is acceleration right? so acceleration is the displacement over time over time
and then the gravity constant thingy is 9.8, but it's going down
WAIT WHAT

Answer 41

I can't LMAO

Answer 42

A velocity vs time graph is the change in velocity over time (acceleration). This is straight line since it goes from 15m/s to 0m/s because the force of gravity, then from 0m/s to -15m/s from gravity as well. All at the same rate of 9.8m/s^2

Answer 43

displacement over time is velocity, not acceleration

Answer 44

Yeah wait but no this is so confusing lemme get my kermit out

Answer 45

oVeR tImE oVeR tIMe

Answer 46

oh yeah

Answer 47

If that helps you think of it

Answer 48

Not really, i'm confusing myself.
I just don't see why the velocity that is decreasing as the ball goes up also equals the 9.8 oml

Answer 49

when the velocity is positive the ball is moving up (the positive sign indicates upwards direction)

when the velocity reaches 0 the ball is at the peak

when the velocity is negative the ball is moving downwards again. the velocity becomes more and more negative but the velocity is still increasing, just in the negative direction

Answer 50

so, like, 40 m/s and -40 m/s are the same speed but opposite directions

Answer 51

Wait but you said earlier that as the ball moves up the velocity decreases, that's why it curves on the parabola

Answer 52

that's true

something can be moving upward but still be slowing down

Answer 53

oh wait nvm

Answer 54

I understand what you mean, so the velocity as it hits the ball slowly decreases as the ball goes up, and that decrease is what causes the ball to turn back (vertex on the position and time graph) but on the velocity time graph, it is on the positive side becayse it's going up (hopefully that made sence) and so the vertex is the zero at the line. but if you think of it conceptionally, the line will most likely be symmetric on the position vs time graph as shadow said

Answer 55

I just still don't get what 9.8 has to do with this
or negative 9.8.
I know this is the constant used to find acceleration, but i'm completely lost.

Answer 56

for the sake of this problem we will treat acceleration upwards as positive and acceleration downwards as negative

the force of gravity is -9.8 m/s/s so a free-falling object will change its velocity at a rate of (-9.8 m/s) per second which is why your velocity/time graph has a constant, negative slope

Answer 57

oh

Answer 58

Thank you, I got the velocity graph!
And then since the slope of the velocity and time is acceleration, the acceleration graph is this, correct?

Answer 59

yup

Answer 60

Ahhhh god bless you guys LMAO ty <333