Question

What speed must you toss a ball straight up so that it takes 4 s to return to you? Show your work.

Answer 1

I don't even know how to solve this, don't even know the formulas. Please walk me through this, thank you.

Answer 2

Ignoring friction from the air, the planet's rotational speed and all that other good stuff, the problem is straightforward:

The ball takes 4 seconds to return.

Its trajectory is symmetric, meaning that it takes half of that 4 seconds to go up, and half of it to come down. It also means that the speed that it's going when it reaches your hand on the way back down, is the same speed it was going when you released it, headed up.

2 seconds into its flight, it was stationary (halfway between going up, and coming back down -- velocity == 0)

The simplified question is therefore, how fast is a ball moving, 2 seconds after you drop it.

Can you solve it now?

Answer 3

Oh.My.Goodness.What?? >.<
Uhmm...I think I'm understanding what you're saying but I'm still not aware of how to solve that. I mean, I don't know how to get the "speed" of the ball if that's what the question's implying... is there a formula I'm supposed to plug the velocity (0) and seconds (2) into?

Answer 4

So, do you have the basic equations for the relationship between velocity and acceleration in hand?

Speed, which is what they're looking for, is simply velocity without taking direction into consideration, and the question is sufficiently simplistic that it implies you're supposed to ignore such complications, so it's fair to just think "speed" when the formula says "velocity".

I realize I'm not answering your question, but it's not going to help you, if I just give you a number.

Answer 5

Is Velocity = Displacement + Time ?
That's alright.

Answer 6

Nope -- Think about it. Your instructors probably get all excited about making your units work out, which is a reasonable thing to do, but it's often harder than just thinking through the problem.

Velocity is how fast (and what direction) something is going. If you have a car driving along at 60 miles per hour, does its velocity increase the longer it's driving? If there was a "plus time" component in the equation, then cars would have to work that way - the longer you drove, the faster you'd go.

Similarly, does how fast the car is going, depend on how far away it is (displacement?) If it did, then cars further from you, would always be driving faster than cars near you.

So - what makes a car go faster?

Answer 7

No it doesn't increase... I assume it would stay the same...
I don't drive so I'm just going to throw this out there lol.
Uhmm, the pressure on the gas pedal?

Answer 8

Sorry, for taking so long to grasp this btw... I'm a bit slow and I don't know anything at all about physics.

Answer 9

Pressure on the gas pedal. That's a good place to start. The gas pedal is also called?

And, no problem on the learning curve. Everyone has to start somewhere.

(I'm curious though - the question is a fairly basic basic-physics class question, but not something that ought to pop up randomly in daily life - if you don't mind, what are you up to, that you need to solve it?)

Answer 10

... must step out of the office - will check back later tonight...

Answer 11

if our acceleration is 0, then our velocity is a constant (never varies)

if our acceleration is constant (non zero), then our velocity increases at a steady rate

when tossing an object, lets assume the only acceleration acting on it is gravity at a nice -9.8 m/s^2

without trying to get to deep into the mathing:

if a = -9.8, then v = -9.8t + c

when time = 2 seconds, v = 0 giving us:

0 = -9.8(2) + c; then c = 19.6
--------------------------------

v = -9.8t + 19.6 , now we can determine the velocity when t=0; the start if it all

Answer 12

@willray An accelerator... so it's acceleration, and I'm going over the questions that I missed on my school assignment.

Answer 13

@amistre64 would the answer be 19.6? I suck at math too...

Answer 14

at t=0, v = 19.6 m/s yes

Answer 15

Is that supposed to be the answer e.O

Answer 16

the answer to what? i thought i was typing a letter to my grandmother in connecticut becuase that what i do .. i randomly pick some post in an obscure place online and just start typing to my grandmother in connecticut. she often likes to here me ramble on about velocities and accelerations of people throwing balls up and down. Its just mere coincidence that the place i chose to write to my dear old grandmother in connecticut was right here in this very post that just happened to mention something along the lines of tossing a ball into the air ....

Answer 17

Lmaoooo ok ok thank you

Answer 18

:)

Answer 19

I would have rather you got to the numeric answer without someone handing it to you on a plate, but --

The point of my original direction on this, was to help you see that physics isn't really hard. It's just a matter of breaking the problem down into bite-sized pieces that you can recognize as something you already understand.

You got to realizing that you already know that velocity is related to acceleration. The next question would be what kind of relationship does velocity have to acceleration -- so that you can write down an equation where velocity = <something in terms of acceleration>.

If you think about that question a bit, you'll arrive at the very same equation that your book try to teach you that you should memorize. You don't need to memorize it, it's obvious from the things you already know about the world around you, if you just take the time to tackle the question in bite-sized pieces.

Until you get up into some truly strange things that go on in the quantum realm, physics is pretty much all like that - it's a puzzle game, where the trick to winning is figuring out the pieces of the problem that you can already solve based on simple intuition, plugging those in and repeating as necessary, until the problem is solved.

Answer 20

Oooooh I see,,, thank you, I'll try to work on the problems with a different mentality using this method. Much appreciated... thank you. [:

Answer 21

Best of luck. School, textbooks, etc. have done a lot to make Physics look like it's difficult, but in reality, that's not true. Physics is what makes the world around you work, and pretty much everyone who can walk around without constantly falling over, has a good common-sense grasp of how the world works.

Use that knowledge to break down physics questions based on "what parts of this make sense to me", or "what thing that I know about, works this way", and you'll find physics to be both reasonably easy, and reasonably fun. There are a few places in upper-level College Physics that make you stand on your ear and just have faith that the numbers really work, but other than there, if you just keep telling yourself "this isn't really hard, its only pretending to be", you'll do fine.