# 3rd week of beginner physics, brain fried

• picklepie159
In summary, a 9th grader attempted to solve a problem involving projectile motion, but was confused by the question. He asked his uncle for help, who told him that to find the angle at which the ball lands, you need to use the inverse tan of y-vector/x-vector. To find the final velocity, you need to find the initial velocity and the time it takes for the ball to bounce 7 times.
picklepie159
1. Ok, hi everyone, I'm new to the forum. I'm hoping you can help me a bit because I'm in trouble now, and a few concepts need clearing up.
Ok, so there are two questions
#1 A ball is rolled off a table 1 meter high, and lands 4 meters away. what is the velocity for the x vector? What is the angle at which it hits the floor?

#2 A (hyperelastic? superelastic? perfectly elastic? I forgot) ball is rolled down the edge of a well at a horizontal speed of 4 m/s. The well is 4 meters deep and 0.5 meters across. What is the number of times the ball hits the wall?

## Homework Equations

What I was thinking of was these...
1 meter = 1/2g t^2 To find the time
4 meters= velocity x (t)
I am probably wrong, but to find the angle at which the ball lands at, I will have to get the y-vector/x-vector = inverse tan. Is that correct? If so, then what should I put for the magnitude for the y vector? Should I put the final velocity, or average velocity on the y-axis?

For #2, I was planning to find the amount of time that it takes for a 4 m/s ball to cross 0,5 meters= ie. 0.5 m = 4 m/s T

Then, I would use D= 1/2 g t^2 to find how much it would drop for the first bounce. After the first bounce, with the velocity accelerating, how would i find out the amount of bounces for the rest of the distances?

3. The Attempt at a Solution - sort of explained above. Please help me in simple terms- I am but a mere 9th grader. Thanks for all the replies!

picklepie159 said:
What I was thinking of was these...
1 meter = 1/2g t^2 To find the time
4 meters= velocity x (t)
Looks good so far...
(depending on what you mean by "velocity")

picklepie159 said:
... to find the angle at which the ball lands at, I will have to get the y-vector/x-vector = inverse tan. Is that correct?
Yes (kind of).
Actually y-vector/x-vector = tan(angle), so you'll need to take the inverse tan of y-vector/x-vector to get the angle.

picklepie159 said:
... then what should I put for the magnitude for the y vector? Should I put the final velocity, or average velocity on the y-axis?
Are you looking for the initial y-vector, or the final y-vector?

okay, maybe not magnitude, but then how would i be able to find out what the y vector's supposed to be? Now I'm confused about the question itself :(

Ok, just asked my uncle over the phone, and he told me that to get the angle, it was simply
1meter/4meter= inverse tan. Is that it? Just wondering

No, what is the trajectory the ball would take? It's not a straight line which is what your uncle is thinking.
Find the final velocity in the x and y components then in that instant you can use trig.

wait- so with a time of 0.45 seconds, X velocity of 8.8 m/s, and a final y velocity of 0.441 meters, that would make out to be around 2.862 degrees? Is that right? Hrrmmm...

hopefully you know this bu Ill say it anyway, the units of velocity are m/s with some direction not metres.
known:
Sx=4 and Sy=1 S is displacement
a_y=9.81 and ax=0
uy=0 and ux=? u is the initial velocity
S=ut+1/2at^2
4=ut and 1=1/2*9.81t^2 goes to t=0.45 seconds
so 4=u*.45 goes to u=8.86m/s in the x direction initial and final velocity (no acceleration)
v^2=u^2 + 2as v is final velocity
v^2=2*9.81*1
v=4.429m/s final velocity in the y direction

so now we know that the final velocity is 8.86x+4.429y
so tan(theta)=4.429/8.86
theta=63.43 degrees!

Hope you understand my procedure?

Yeah! Thanks alot- it was the final y velocity which messed me up. I tried to use
V(final)=V(initial) + gt, making it
V(f) = 0 + 9.8(0.45)= 4.41
Except that I accidently kept on putting 0.98 as g instead of 9.8, causing lots of confusion.

As for the other problem, I hope that the following procedure might make sense.
Since the ball doesn't lose speed, I would treat it as if it had rolled off a table 4 meters high, going at 4 m/s. The time would be around 0.89 seconds. I would then calculate displacement on the x-axis with D= VT, and that would be around 3.57 meters. 3.57 m divided by 0.5 meters is around 7 bounces, right? Maybe...

Thanks again!

Sorry, I am not sure about your second question, you might have to wait for someone else or go see your teacher/professor.

pat666 said:
so now we know that the final velocity is 8.86x+4.429y
so tan(theta)=4.429/8.86
theta=63.43 degrees!

How did you come up with this?

4.429/8.86 [itex]\approx[/tex] 0.49989(without rounding off values for Vx and Vy, this actually comes out to exactly 0.5)

What is [itex]tan^{-1}(0.5)[/tex]? (It's not 63.43 degrees!)

Whooops, I must have put it in my calc upside down! (ie tan^-1(8.86/4.429)
Sorry.

## 1. What are some common topics covered in the 3rd week of beginner physics?

The 3rd week of beginner physics typically covers topics such as motion, forces, and energy. This may include concepts like Newton's laws of motion, calculating acceleration, and understanding different types of forces.

## 2. How can I keep my brain from getting fried while studying physics?

Studying physics can be challenging, but there are a few strategies you can use to prevent your brain from getting fried. Some tips include taking breaks, practicing problems regularly, and seeking help from a tutor or teacher when needed.

## 3. What are some common challenges students face during the 3rd week of beginner physics?

Some common challenges students face during the 3rd week of beginner physics include understanding the concepts of motion and forces, applying mathematical equations to real-world scenarios, and memorizing key formulas and principles.

## 4. How can I improve my comprehension of physics concepts during the 3rd week of beginner physics?

To improve your comprehension of physics concepts during the 3rd week of beginner physics, try using different study methods such as watching educational videos, participating in group discussions, and practicing problems with classmates. Additionally, make sure to ask your teacher or tutor for clarification on any confusing topics.

## 5. What resources can I use to supplement my learning during the 3rd week of beginner physics?

There are many resources available to supplement your learning during the 3rd week of beginner physics. These may include textbooks, online resources such as Khan Academy and Crash Course, and educational apps. It can also be helpful to form study groups with classmates or seek out additional help from a tutor or teacher.

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