# Physics Test Help. (teacher recommended internet)

xsrewop27x
Some PHYSICS Problems.

Okay, let me give a little introduction. My 11th grade Physics teacher gave us a 6 page take home test and said we can use collaboration from book, internet help, or parents to complete the test. (I believe this is because he cant teach it). And since my teacher knows little about teaching a subject nobdy in the class understands the higher level problems. I am here for some help. I have about 10 on this test that I simply cant find the answer to but I will start off on here with 3 of the easier ones. If you can help me farther on this I would owe you greatly, in that case Email me: id43rd1@earthlink.net or talk to me on Aim: xSrewop27x to talk about the rest of the problems.

1) An object is on a frictionless inclined plane. The plane is inclined at an angle of 30degrees with the horizontal. What is the objects accelerations? (prly easiest, but confused me)

2)A lantern of mass (m) is suspended by a string that is joined to two other strings. What is the Tension in each string if they make equal angles of 35degrees from the support beam?

3)A child sits on a sled at rest on a snow-covered hill making an angle with the horizontal. If the coeffient of friction is .10, what is the maximum angle at which the sled remains at rest? Approximate the hill as an inclined plane.

(I HAVE PICTURES to go along wit hthe LAST 2. Email me or let me know if I should scan them for you!)

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xsrewop27x
Okay, let me give a little introduction. My 11th grade Physics teacher gave us a 6 page take home test and said we can use collaboration from book, internet help, or parents to complete the test. (I believe this is because he cant teach it). And since my teacher knows little about teaching a subject nobdy in the class understands the higher level problems. I am here for some help. I have about 10 on this test that I simply cant find the answer to but I will start off on here with 3 of the easier ones. If you can help me farther on this I would owe you greatly, in that case Email me: id43rd1@earthlink.net or talk to me on Aim: xSrewop27x to talk about the rest of the problems.

1) An object is on a frictionless inclined plane. The plane is inclined at an angle of 30degrees with the horizontal. What is the objects accelerations? (prly easiest, but confused me)

2)A lantern of mass (m) is suspended by a string that is joined to two other strings. What is the Tension in each string if they make equal angles of 35degrees from the support beam?

3)A child sits on a sled at rest on a snow-covered hill making an angle with the horizontal. If the coeffient of friction is .10, what is the maximum angle at which the sled remains at rest? Approximate the hill as an inclined plane.

(I HAVE PICTURES to go along wit hthe LAST 2. Email me or let me know if I should scan them for you!)

1) An object is on a frictionless inclined plane. The plane is inclined at an angle of 30degrees with the horizontal. What is the objects accelerations? (prly easiest, but confused me)

For a particle on an inclined plane, the weight component parallel to the plane is proportional to the sine of the angle of inclination.

Weight component down the plane = mg sin &theta;

From Newton's Second Law you know that F = ma, therefore

mg sin &theta; = ma

The masses cancel here, so

a = g sin &theta;

You should know that the sine of 30 degress is one half, so

a = &frac12;g

2)A lantern of mass (m) is suspended by a string that is joined to two other strings. What is the Tension in each string if they make equal angles of 35degrees from the support beam?

I don't understand this question, I'll need the diagram.

I'll get back to you on question 3 in a minute. It doesn't appear as simple as I first thought.

Re question 3.

I was working without pen and paper before, but now I've solved it.

The sled will remain at rest until the component of its weight exceeds the force of friction with the slope. My mechanics book says that when the object is just about to move -- when the force of friction is only just enough to hold it -- it is in a state of limiting equilibrium. I told you earlier that the component of an object's weight acting down the plane when the plane is inclined at an angle, &theta;, was mg sin &theta;

The force of friction that an object has with a surface is equal to the coefficient of friction multiplied by the normal reaction of the object with the slope.

F = &mu;R

The reaction force is given by

R = mg cos &theta;

Seeing as the sled is in a state of limiting equilibrium

mg sin &theta; = &mu;(mg cos &theta;)

Therefore

sin &theta; = &mu; cos &theta;

By re-arranging

(sin &theta;)/(cos &theta;) = &mu;

If you know a bit about trigonometry, then you know that

&mu; = tan &theta;

Therefore,

&theta; = arctan &mu;

arctan (0.1) = 5.7&deg;

Last edited:
xsrewop27x
Thank you very very much for the help so far AD. I guess I was just having a brain fart on the first one, but I did understand it after your great explanation. I will send you a ramdition of the diagram for #2 but I need your email. I also have 7 problems after this that I have been slaving over all afternoon. I will post those after I complete these 3.

xsrewop27x
Okay, you've got mail.

Hmmm. I haven't received the e-mail. I'll just re-PM you to make sure I gave you the right e-mail address.

Is the diagram less than 1MB in size? If it is larger than 1MB, Hotmail will not let it through.

xsrewop27x
You can now check your PM's I just wont send it to Hotmail.

xsrewop27x
4)An object of mass 6000 kg rests on the flatbed of a truck. It is held in place by metal brackets that can exert a maximum horizontal force of 9000N. When the truck is traveling 15 m/s, what is the minimum stopping distance if the load is not to slide foward into the cab?

5) A stack of books rests on a level frictionless surface. A force F acts on the stack, and it accelerates at 3.0 m/s2. A 1.0 kg book is then added to the stack. The same force is applied, and now the stack accelerates at 2.0 m/s2. What was the mass of the original stack?

6) A 10-kg mass slides down a flat hill that makes an angel of 10degrees with the horizontal. If friction is negligible, what is the resultant force on the sled?

7) A student pulls a box of books ona sooth horizontal floor with a force of 100N in a direction of 37degrees above the horizontal. If the mass of the box and the books is 40 kg, what is the acceleration of the box?

8) The coefficeient of static and kinetic frictions between a 3.0 gp box and a desk are 0.40 and 0.30, respectively. What is the net force on the box when a 15 N horizontal force is applied to the box?

I hand picked these from the 60 questions and narrowed it down to a few of the above which I was never taught. I am going to re-read my book now so i will be back in about 10 min. Thank you

xsrewop27x, I got the diagram for the second problem.

The two ropes supporting the lamp bear its weight equally. The angles they make with the beam are equal and the tension in each rope is equal.

The vertical component of the tension in each rope is equal to half the weight of the lamp.

So for each rope,

T sin &theta; = &frac12;mg

Just re-arrange to solve for T.

Have you attempted these problems yourself?

NateTG
Homework Helper
You should really put this in the homework help section, and you should really show what ideas you have about doing the problems.

All three of the poblems are fairily straighforward, and should be similar to homework that you've already done.

xsrewop27x
I have done 50 problems in the 60 problem pack and the above problems I read and started to solve them but I came up with almost nothing. Its a long and grueling task for me to complete as I have been at it for 2 days now and it is due tomorrow. I am going to read some more in the book and see if i come up with anything but i would really appreciate help because I cant find anything so far that will help me understand the above problems.

xsrewop27x
Some PHYSICS Teasers

Hey you guys, here are some teasers for you all! :)

1)An object of mass 6000 kg rests on the flatbed of a truck. It is held in place by metal brackets that can exert a maximum horizontal force of 9000N. When the truck is traveling 15 m/s, what is the minimum stopping distance if the load is not to slide foward into the cab?

2) A stack of books rests on a level frictionless surface. A force F acts on the stack, and it accelerates at 3.0 m/s2. A 1.0 kg book is then added to the stack. The same force is applied, and now the stack accelerates at 2.0 m/s2. What was the mass of the original stack?

3) A 10-kg mass slides down a flat hill that makes an angel of 10degrees with the horizontal. If friction is negligible, what is the resultant force on the sled?

4) A student pulls a box of books ona sooth horizontal floor with a force of 100N in a direction of 37degrees above the horizontal. If the mass of the box and the books is 40 kg, what is the acceleration of the box?

5) The coefficeient of static and kinetic frictions between a 3.0 gp box and a desk are 0.40 and 0.30, respectively. What is the net force on the box when a 15 N horizontal force is applied to the box?

Good luck.
Post your answers in the thread so others can correct you or agree to see if you got the ansrew right. Ill give you all the correct answers tomorrow!

xsrewop27x
5 Problems -- I have repeatedly attempted but I am STUMPED

I had about 50 problems to do and these are the 5 lefts unanswered. I read my book over and me and my dad also looked over it but we cant figure the problems. If some help could be given it would be very very greatly appreciated. Note: This is due tomorrow. (Monday)

Aim Messenger: xSrewop27x
or. Just post on here.

1) A stack of books rests on a level frictionless surface. A force F acts on the stack, and it accelerates at 3.0 m/s2. A 1.0 kg book is then added to the stack. The same force is applied, and now the stack accelerates at 2.0 m/s2. What was the mass of the original stack?

2) A 10-kg mass slides down a flat hill that makes an angel of 10degrees with the horizontal. If friction is negligible, what is the resultant force on the sled?

3) A student pulls a box of books ona sooth horizontal floor with a force of 100N in a direction of 37degrees above the horizontal. If the mass of the box and the books is 40 kg, what is the acceleration of the box?

4) The coefficeient of static and kinetic frictions between a 3.0 gp box and a desk are 0.40 and 0.30, respectively. What is the net force on the box when a 15 N horizontal force is applied to the box?

Note: This is due tomorrow. (Monday)

xsrewop27x
5 Problems -- I have repeatedly attempted but I am stumped

I had about 50 problems to do and these are the 5 lefts unanswered. I read my book over and me and my dad also looked over it but we cant figure the problems. If some help could be given it would be very very greatly appreciated. Note: This is due tomorrow. (Monday)

Aim Messenger: xSrewop27x
or. Just post on here.

1) A stack of books rests on a level frictionless surface. A force F acts on the stack, and it accelerates at 3.0 m/s2. A 1.0 kg book is then added to the stack. The same force is applied, and now the stack accelerates at 2.0 m/s2. What was the mass of the original stack?

2) A 10-kg mass slides down a flat hill that makes an angel of 10degrees with the horizontal. If friction is negligible, what is the resultant force on the sled?

3) A student pulls a box of books ona sooth horizontal floor with a force of 100N in a direction of 37degrees above the horizontal. If the mass of the box and the books is 40 kg, what is the acceleration of the box?

4) The coefficeient of static and kinetic frictions between a 3.0 gp box and a desk are 0.40 and 0.30, respectively. What is the net force on the box when a 15 N horizontal force is applied to the box?

Note: This is due tomorrow. (Monday)

To solve the first:

F = ma

So

F = 3m

A 1 kg mass is added, so

F = 2(m+1) = 2m + 2

Therefore

3m = 2m + 2

and

m = 2kg

Doc Al
Mentor

Originally posted by xsrewop27x
I had about 50 problems to do and these are the 5 lefts unanswered. I read my book over and me and my dad also looked over it but we cant figure the problems. If some help could be given it would be very very greatly appreciated. Note: This is due tomorrow. (Monday)
You'd better get busy! Give the problems a shot, show your work, and someone will help.

Why don't you tell us what you know, so we know where to begin helping you?

Re question 3

Do you know how to decompose vectors? You just need to find the component of the force in the direction of the acceleration, i.e. along the floor.

The force acting in the direction of acceleration is the total force multiplied by the cosine of the angle that the direction of the force makes with the floor. Simply divide by mass to get the resultant acceleration of the box.

ShawnD
1) A stack of books rests on a level frictionless surface. A force F acts on the stack, and it accelerates at 3.0 m/s2. A 1.0 kg book is then added to the stack. The same force is applied, and now the stack accelerates at 2.0 m/s2. What was the mass of the original stack?
Express the problem as algebra.
For the original stack, it looks like this
F = m(3)
For the changed stack it's this:
F = (m + 1)(2)
Since both are equal to F, they equal each other:
m(3) = (m + 1)(2)
3m = 2m + 2
m = 2

Now check if it's right. If the mass is 2 and the acceleration is 3, the force is 6.
6 = (2)(3)
Now for the changed stack, it looks like this:
6 = (2 + 1)(2)
6 = (3)(2)
6 = 6 so it checks out

2) A 10-kg mass slides down a flat hill that makes an angel of 10degrees with the horizontal. If friction is negligible, what is the resultant force on the sled?
I don't really understand the question too well so I'll assume they want the force from the hill on the mass. The thing to remember is what trig function is at a maximum when the angle is 0. cosine. Then I express the force of a surface as F = maCOS(x). Now just fill in
F = (10)(9.8)cos(10)
F = 96.5N is the force that the hill is applying to the box perpendicular to the hill

3) A student pulls a box of books ona sooth horizontal floor with a force of 100N in a direction of 37degrees above the horizontal. If the mass of the box and the books is 40 kg, what is the acceleration of the box?
The floor is a flat horizontal plane with no slant, that makes this question fairly easy. The function that is maximum when the angle is 0 again is cosine. To find our horizontal force on the books, it's just Fx = Fcos(a)
Fx = (100)cos(37)
Fx = 79.86N
a = F/m
a = (79.86)/(40)
a = 1.9965m/s
a = 2m/s

4) The coefficeient of static and kinetic frictions between a 3.0 gp box and a desk are 0.40 and 0.30, respectively. What is the net force on the box when a 15 N horizontal force is applied to the box?
What is gp? Before anything can be done, the mass or weight of the box has to be known.

Originally posted by ShawnD
I don't really understand the question too well so I'll assume they want the force from the hill on the mass.
Don't you think they might want the vector sum of the normal reaction and the weight?

xsrewop27x
Sorry, I mistyped this problem. You guys are GREAT!

4) The coefficeient of static and kinetic frictions between a 3.0 kg box and a desk are 0.40 and 0.30, respectively. What is the net force on the box when a 15 N horizontal force is applied to the box?

ShawnD
hmmm that's very possible.

in that case, we vector sum 96.5N at 100 degrees (10 + 90) with (10)(9.8) at 270 degrees.
sum of x = 96.5cos(100) + 98cos(270)
sum of x = -16.757N

sum of y = 96.5sin(100) + 98sin270
sum of y = -2.966N

Now to get our angle, let's just use some tan.
tan(a) = y/x
a = tan^-1(-2.966/-16.75)
a = 190 degrees

now find the force of that sum
f = sqrt(16.757^2 + 2.966^2)
f = 17N

resultant = 17N at 190 degrees

I might have a few errors in there, supper's on the table and I'm hungry