# Need help showing work for these problems

• kiarrahannice
In summary, the first problem involves a skateboarder moving along a circular track with a radius of 4.00 m. At the bottom of the track, the normal force exerted on the skateboarder is calculated to be approximately 1.68 kN. The second problem deals with a 135-g rubber ball colliding elastically with a 1100-kg car. The final velocity of the ball after the collision is calculated to be approximately 70.16 m/s. It is also shown that the kinetic energy of the ball is conserved. In the third problem, a 245-g piece and a 275-g piece move along the floor in opposite directions after a plate breaks and falls vertically. The

#### kiarrahannice

Question Details:
1) A skateboard track has the form of a circular are with a 4.00 m radius, extending to an angle of 90 degrees relative to the vertical on either side of the lowest point. A 57.0 kg skateboarder starts from rest at the top of the circular arc. What is the normal force exerted on the skateboarder at the bottom of the circular arc?

2) A 135-g rubber ball is thrown with a speed of 12m/s at an oncoming 1100-kg car which is approaching at 35 m/s, and undergoes a one-dimensional elastic collision with the car.
a) What is the speed of the ball after the collision
b) Show that kinetic energy of the ball is conserved
Answers: a)Vf, ball=70.16 m/s b) KEi=KEf=1.35x10^6J

3) a plate falls vertically to the floor and breaks up into three pieces, which slide along the floor. Immediately after the impact, a 245-g piece moves along the x-axis with a speed of 2.50 m/s, a 275-g piece moves along the y-axis with a speed of 1.50m/s. The third piece has a mass of 100g. What are the magnitude and direction of its velocity?
Answer: V3=7.384 m/s and Theta = 34 degrees

I do not know how to solve these equations can someone please show me the work so I can learn the appropriate equations to use and how to use them so I can solve the rest of my questions on my own thanks!

You're supposed to do the work and show us where you get stuck and then we can give you a hint. This forum is not an answer machine.

To get you started:
1) Since it is a circular track, there is a centripetal force acting on the skateboarder, as well as a gravitational force.

2) Momentum is conserved. Use the equation for momentum and set the initial momentum equal to the final momentum of both objects.

3) Again, momentum is conserved.

I have the answers already as you can see. What I don't understand is how to get the answers. and if I thought it was an answer machine then I would have posted all 25 questions. If you can't honor the request do not reply to my question Thanks. No time for smart remarks.

kiarrahannice said:
Great..

What I don't understand is how to get the answers.
That's why I tried to get you started

and if I thought it was an answer machine then I would have posted all 25 questions.
Ok...

If you can't honor the request do not reply to my question. Thanks.
Look, I'm trying to help you. I'm trying to get you started and I'm saying that you shouldn't expect someone to solve the entire problems for you. If you start solving the problems and show us where you get stuck, or just tell us what you think you might have to do we can help you further. If you just throw a couple of questions at us you have a small chance someone will help you. It's ok with me, but I think that's not what you want.

No time for smart remarks.
...Do you know the equation for centripetal force? Do you know the equation for weight? Do you understand what (conservation of) momentum is?

Last edited:
Fine I'll bite: for the first problem here is what I got
Ui + Ki= Uf+Kf
mgh + 0 = 0 +1/2 mv^2---v^2= 2gh
sigma Fy=macp
N-mg=m v^2/r
N=mg+m 2gh/r= mg(1+2h/r)
(57kg)(9.8 m/s^2)(2(4m)/4m)--1.68 kN

second problem:Elastic collision means both momentum and total KE are conserved:

mv + MV = mv' + MV'

(1/2)mv^2 + (1/2)MV^2 = (1/2)mv'^2 + (1/2)MV'^2

Don't know how to solve to get answer.

third problem: Clueless even with ur hint

kiarrahannice said:
Fine I'll bite: for the first problem here is what I got
Ui + Ki= Uf+Kf
mgh + 0 = 0 +1/2 mv^2---v^2= 2gh
sigma Fy=macp
N-mg=m v^2/r
N=mg+m 2gh/r= mg(1+2h/r)
(57kg)(9.8 m/s^2)(2(4m)/4m)--1.68 kN

second problem:Elastic collision means both momentum and total KE are conserved:

mv + MV = mv' + MV'

(1/2)mv^2 + (1/2)MV^2 = (1/2)mv'^2 + (1/2)MV'^2Don't know how to solve to get answer.

third problem: Clueless even with ur hint

Very well! First answer is correct.

As for the second problem:
mv + MV = mv' + MV'
You know m, M, v and V. The two unknowns you're left with are v' and V'. You can express v in terms of V' (or the other way around). This can be plugged into:

mv^2 + MV^2 = mv'^2 + MV'^2
Leaving you with only one unknown.

Last edited:
Last problem:

Momentum in the x direction is conserved, and in the y direction. Draw the momentum vectors. You know that the momentum of the third piece is equal and opposite to the other two, respectively along the x and y axis.

Does that help?

I hope u are right about the first problem

The second problem although I can set it up I can't solve I am having difficulty plugging the numbers in I know m1 is mass of the ball m2 is mass of car v1 is speed of ball after collision and v2 is speed of car. But I still can't solve. algebra isn't my strongest subject

third problem
does this make sense? mvx + mvy +mv3=what
mv3=-mvx-mvy--v3=v(-x) + v(-y)
but again I am having difficulty plugging and chugging the numbers.

kiarrahannice said:
I hope u are right about the first problem
Why do you think it would be wrong? I calculated it myself and got the same answer.
The second problem although I can set it up I can't solve I am having difficulty plugging the numbers in I know m1 is mass of the ball m2 is mass of car v1 is speed of ball after collision and v2 is speed of car. But I still can't solve. algebra isn't my strongest subject
Could you write it out?

third problem
does this make sense? mvx + mvy +mv3=what
mv3=-mvx-mvy--v3=v(-x) + v(-y)
but again I am having difficulty plugging and chugging the numbers.
I'm not sure what you're doing.

This is what you got along the x-axis:

Sum of [ px, i ] = Sum of [ px, f ] = 0
px,1= mv = .245 * 2.50
px,2= mv = ?
px,1 + px,2 = 0

So you know that px,2 = -.245 * 2.50. The minus sign tells you that the x component of the velocity is opposite to the object flying in the positive x-direction.

Do the same for y, draw it and then you should be able to figure out from trig how to find the angle.

## 1. How do I show my work for these problems?

One way to show your work is by writing out the steps you took to solve the problem. This could include writing down any given information, identifying the necessary formulas or equations, and showing any calculations or substitutions made. You can also include diagrams or illustrations to further explain your work.

## 2. Why is it important to show work for math and science problems?

Showcasing your work allows others to follow your thought process and understand how you arrived at your answer. It also allows for easier identification of any mistakes or errors made along the way. In a scientific setting, showing your work also promotes transparency and reproducibility of results.

## 3. How can I make sure my work is organized and easy to follow?

One way to ensure organization is to label each step or calculation clearly. You can also use headings or bullet points to break down your work into smaller, more manageable sections. Additionally, make sure to write neatly and use appropriate spacing to avoid confusion.

## 4. Should I show all of my work or just the final answer?

It is best to show all of your work, even if it may seem simple or unnecessary. This gives a complete picture of your problem-solving process and allows for a better understanding of your approach. However, if a certain step is very basic and does not require any explanation, you can skip showing it.

## 5. Is there a specific format or style for showing work in science and math problems?

There is no one specific format or style for showing work, as it may vary depending on the type of problem and personal preference. However, it is important to be consistent and organized in your approach. You can also check with your teacher or instructor to see if there are any specific guidelines or requirements for showing work in their class.