Why Am I Getting the Wrong Answer in My 2D Motion Projectile Problem?

[Crazystuntsdude]

Yeah, I pretty much spent 5 hours today trying to figure this question out, yet for some reason I keep getting the wrong answer. I told my friends I'd do it and explain it to them, but I still can't do it right. Can some one please show me the steps using only the letters, not just numbers. Thanx.

A projectile is launched into the air from the top of an incline that makes an angle f =39° with the horizontal as show in the diagram below. The launch speed, v, is known to be 10.0 m/s but the launch angle, q, is unknown. However, the maximum height above the launch level that the projectile reaches is known to be h = 2.51 m. The trajectory is a perfectly parabolic. What is the angle, a (in degrees), with which the projectile strikes the incline (measured relative to the incline as shown)? Use g = 9.80 m/s2.
http://koso.champlaincollege.qc.ca/moodle/file.php/85/q5_4ProjectileOnIncline.png
(I don't know if the picture works, but give it a shot)

Homework Equations

Basic Motion Equations
Vf = Vi + A(D)T
(D)X = Vi(D)T + (A(D)T^2)/2
Vf^2 = Vi^2 + 2A(D)X
(D) is Delta
I know Calculus, but my friends don't yet.

(I would right down one of my attempt, but it would take way too long!)
I understand how you're supposed to do it(break it up horizontally and vertically) and whatever else(ex: hor. vel. doesn't change, but vert. does due to gravity), but for some reason all the things I tried never gave me the right answer. I filled nearly five pages of stuff!
Basically, I separated the problem into four points each with their x and y values(position, velocity, acceleration) and the change in time at each one. Using the 3 equations I'm able to find all the other data, but for some reason I keep messing up no matter what way I do it. I'm certain that q = arcsin(((root)2AH)/10), but I mess u psomewhere along the line. So yeah, any help would be great.

 

[chocokat]

I did this quickly, so you should double check, but I think your equation is close to correct but not quite. Should it be divided by 100 instead of 10? I think you originally had (10sinq)^2, so don't you have to square the 10 as well as the sinq? (Remeber, I could be wrong).

 

[ogb p]

Dear student,

It sounds like you have a good understanding of the basic motion equations and how to approach this problem. It can be frustrating when you spend a lot of time on a problem and still can't get the right answer. Here are some steps you can follow to solve this problem:

1. Draw a diagram: Draw a diagram of the situation, labeling the known and unknown variables. This will help you visualize the problem and keep track of your calculations.

2. Break the problem into parts: As you mentioned, it's helpful to break the problem into horizontal and vertical components. This means considering the motion of the projectile in the x and y directions separately.

3. Write down the equations: Write down the basic motion equations for both the x and y directions. In the x direction, the projectile will have a constant velocity, so the equation (D)x = Vix(D)t will be useful. In the y direction, the projectile will have a constant acceleration due to gravity, so the equation (D)y = Viy(D)t + (1/2)(A)(D)t^2 will be useful.

4. Solve for the unknowns: Using the known values and the equations, solve for the unknowns. In this case, you have two unknowns: the launch angle q and the angle of impact a. You can use the given information about the maximum height to find the launch angle q, and then use the known launch speed and the launch angle to find the angle of impact a.

5. Check your answer: Once you have found the values for q and a, make sure to check your answer by plugging them back into the equations and seeing if they give you the correct values for the known variables.

It's possible that you may have made a mistake in your calculations or used the wrong equations, which is why you were not getting the correct answer. Going through these steps and double-checking your work should help you find the right solution. If you are still having trouble, you may want to ask a classmate or your teacher for help.

I hope this helps and good luck with your problem!