Why Do These Basic High School Physics Problems Confuse Me?

[MR_COW]

These are the questions I don't get, I did all my other exam review. It's high school physics, like really easy stuff, so you will probably think this is stupid.

4) A ball is thrown at an angle of 20 degrees at 20 m/s. How high did it go? What is the horizontal distance?

5) An object is shot horizontally from a roof 100m high at a velocity of 5m/s. How far from the base will the object strike the ground.

7) A 10kg box is pulled 20 degrees with a force of 40N at a constant speed. What is the coefficient of friction?

8) A 10kg box is resting on a 40 degree incline. What is the Normal and Parallel force on the box?

Homework Equations

a=(vf-vo)/t
d/t=.5(vf+vi)

The Attempt at a Solution

4) I don't know how to set this up. I know how to do the trig work once I get the maximum height, I can't think of how to get there though.

5) Once again, I don't even know how to set this up. I don't think our teacher ever even taught us this. Whats the base the object will strike at mean?

7) Is the force expression Fnet = Fh - Ff? If so how do you get both of them

8) Is this Fparallel = 63.014? If so, what is Fn?

 

[Doc Al]

Show your work for each problem and point out where you got stuck.

 

[MR_COW]

I'm sorry, I fixed it. I just don't know how to set some up and need guidance. I thought I've been doing them right, but I got these wrong on a previous test and my teacher is like the oldest guy ever and doesn't really help. And our book sucks for self teaching.

 

[mgb_phys]

For the first two - the trick is to know that the horizontal and vertical velocity and accelerations can be treated separately. What do you know about the acceleration vertically and horizontally for a thrown/falling object?

For the second two - draw a force diagram, you won't get anywhere until you can draw what's happening.

 

[MR_COW]

I made a little graph for 4, its http://img440.imageshack.us/img440/7086/80597745qb9.png" .

5 I seriously have no idea what to do. I don't even get what its asking.

 

[Delphi51]

Cow, here is a reliable method for all these trajectory questions.
Write two headings: HORIZONTAL VERTICAL

Any given numbers like initial velocity that are at an angle must be resolved into their horizontal and vertical parts. Forget the original; use only the parts.

Ask yourself what kind of motion (uniform or accelerated) is going on in each.
Write the appropriate formulas under each heading. Just d = vt for uniform motion, but use a d = formula AND a v = formula (usually v = vi + at) for accelerated motion.

Put the known numbers in all three formulas. You should find that you can then solve one of the formulas because it has only one unknown. You can then find other things by plugging this result into the other formulas.

 

[Doc Al]

5 I seriously have no idea what to do. I don't even get what its asking.

As has been pointed out, treat the horizontal and vertical motion separately.

Hint: What's vertical component of the initial speed? If you dropped something from a height of 100m, how long before it hits the ground? During that time, how fast has it been moving horizontally?