# Homework Help: Physics 12U - 5 Force Problems

1. Mar 3, 2012

### umaza

Physics 12U - 2 Force Problems

1. The problem statement, all variables and given/known data
See attachment.

I am very confused with these two questions.

Question 1: I understand how to do the pulley. Would the force the guy at the top is being pulled at be the same as the force with which the guy needs to push with? How does a horizontal push stop something from falling?

Attempt:
FnetJOE = Ft, FnetBRIAN = -Ft + FgBRIAN.
a1 = mBRIAN*g / (mJOE + mBRIAN)
a1 = 4.2m/s/s

Ft = mJOE*a
Ft = 336N

I get stuck here, would the push require the same force as tension or same acceleration? Any hints?
Also why does it matter about the mass of the pusher (JOE)?

This part below I do not know if it is right or not (equal accelerations):
The pusher must accelerate the box at 4.2m/s/s, the mass is equal to the total mass of box + the two others in contact.

Fnet = mBox+John+Brian*a
Fnet = 588N

Question 2: I do not understand at all. How does a horizontal force lift something vertically?

Attempt:
FgB = mB*g
FgB = 980N

This part below I do not know if it is right or not:
For the barrel to move up, it needs a force of 980N+, but for minimum I stick to 980N.

Where the pipe touches the Barrel is 0.25m of the ground. From this point I make a triangle from point of contact to middle, being 1m (diameter = 2m), I treat this as hypotnuse and 0.75m as opposite side. From this I get an angle.

θ = sin-1(1/0.75)
θ = 48.6°

Now I use this angle with the vertical component being 980 (opposite side), and find horizontal component.

#### Attached Files:

• ###### Physics Forces S.pdf
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Last edited: Mar 3, 2012
2. Mar 3, 2012

### SammyS

Staff Emeritus
Hello umaza. Welcome to PF !

You might have better luck posting only one or two questions per thread. (That's a rule for this Forum.)

Also, you need to be more detailed about your progress toward solutions so we can give more meaningful responses.

3. Mar 3, 2012

### umaza

Thanks, it is great to be here.

The problem is that I do not know where to start I am really stuck. What I was looking for was really an explanation of the concepts and to know where to start. All other homework questions I was given I did with ease, but with these problems I am finding there is too little information.

The only formula that comes to mind for these questions is F = ma.

And about how many questions, did you mean only 1 to 2 problems or 1 to 2 help questions.

Thanks for advice, I got rid of three questions and have listed one attempt for both.

Last edited: Mar 3, 2012
4. Mar 4, 2012

### umaza

Anyone? Should I post the actual question on post?