Can i get help with my F=ma problems?

[mrphobio]

Four questions that i don't know how to do at all.

1. A student standing on a scale in an elevator at rest sees that it reads 650 N.
As the elevator rises, the scale reads 700 N.
Find the acceleration of the elevator.

2. A sign in an elevator states that the maximum occupancy is 20 persons. Suppose safety engineers assume the mass of an average person is 70 kg. The elevator itself has a mass of 540 kg. The supporting cable can tolerate a maximum force of 32471 N. What is the greatest acceleration the elevator's motor can produce without snapping the cable?

3. A student pushes a 12.9 kg lawnmower. The handle makes a 28 degree angle with the ground.
If the friction is 68 N, find how much force the student must apply to the lawnmower in order to accelerate it at 3.7 m/s^2.

4. A 30 kg box is pushed across the level ground with a horizontal force of 172 N. As a result it accelerates at 5 m/s^2. How much friction must be present?

please explain every step because I am totaly lost and my teacher just threw us right into it without explain at all. Very much apperciated.

 

[netgypsy]

Start by drawing a diagram of the forces ON the person.

Next give of list of your given information including units of measure and give a variable for what you are tying to find

Then write the equation or equations you think you have to use to solve this

 

[e.bar.goum]

All of these problems are essentially the same. You've got Newtons second law, F = ma, two known values and one unknown in each question. Substitute in the known values (and in the case of question 3 do some trigonometry) and solve for F or a as required.

 

[netgypsy]

Come on - do it the long way which is easier. Do the diagram. List the info with the variables and then the formula in that nice neat format they give you on here. Once you see it, you can work it.

 

[mrphobio]

I've been trying to get it for the past few hours. ITs online homework so it tells you instantly if it is wrong or right. Just like on the last question. the question gives the force mass and acceleration, so using f=ma i just don't see how i can get teh friction out of it.

 

[e.bar.goum]

Well, in the instance of the last question, there should be a discrepancy if you evaluate F=ma.

Draw a free body diagram!

 

[mrphobio]

Discrepancy? How could i tell?

 

[e.bar.goum]

Well, in this case, you have values for F, m and a, does F = ma in this instance?

 

[mrphobio]

I guess not. 170≠150... So if i divided 170 by 150 would that be the answer? 1.14 more or less?

 

[e.bar.goum]

No, dividing would be precisely the wrong thing to do. If you're ever having to guess, try thinking of it dimensionally, Newtons/Newtons is dimensionless, but friction is a force.

So, how much force are you missing?

 

[mrphobio]

12 Newtons. ( i put 170 up there on accident)

 

[e.bar.goum]

150+12 ≠ 172, But I suspect you're on the right track. So, where do you think the extra force on the box went?

 

[mrphobio]

crap. i meant 22 Newtons. I am so tired.

 

[e.bar.goum]

Yep! So, where do you think the missing 22 Newtons went?

 

[mrphobio]

To the force of the friction? Is 22 the answer?

 

[mrphobio]

Yes it works! thanks so much. Now three more to go haha

 

[e.bar.goum]

Yes! It's friction!

And as to whether 22 is the answer, you did the math! Be certain!

(but yes, it should be)

 

[mrphobio]

As for the elevator problem. Would i set it up as 700N=65kg(a)?

 

[e.bar.goum]

That seems like the thing to do, doesn't it?

 

[mrphobio]

Alright. So 700/65=10.76... Which would mean 10.76=a. but it is saying that's not right. What else could i do?

 

[e.bar.goum]

Ah, I reread the question. The total force is 700 N, what part of that force is due to the acceleration of the elevator?

 

[mrphobio]

50n. so 50n=65kg(a)?

 

[netgypsy]

Did you draw the forces on the person standing in the elevator? What are they? How many of them are there?

Newton's second law is not F = ma It's the SUM of Forces = ma
If there's more than one force you have to add them and put signs on them according to their direction since they are vectors.

Very good and you can tell if you are accelerating up or down by the sign you get because if you accelerate down you'll suddenly weigh less than you did. (great way to lose weight)

Once more caution on elevator problems - most of the time you ride an elevator you are NOT accelerating. You just accelerate for a few seconds, then move at constant speed, then decelerate for a few seconds and stop and do the same in the down direction. So most of the time you weigh your normal weight.

 

[mrphobio]

so i would add 650 and 700 to get F?

 

[netgypsy]

yes and no. the 650 is in the down direction so it's negative. The 700 is in the up direction so it's positive and when you add them, you get the 50 that you used when you got it right (I'm talking about problem 1)

 

[mrphobio]

Alrighty. So 50 is the force right? and as for the mass, is it 65kg? or am i just making weird connections?

 

[e.bar.goum]

Yeah. The mass is pretty much 65kg (g = 9.8 m/s^2, but close enough).

 

[netgypsy]

You got it. 700n + - 650n = masstimes acceleration

700N is the upward force normal of the floor on your feeet. 650N is the downward force due to gravity on your body so you put a negative sign on it before you add them.

Plug in mass and get acceleration which you did correctly already. It's good to know why it works so you can get another one right.

 

[mrphobio]

The website where i do my homework just crashed. Ill try it once it gets back up. Yah thanks very much. That is what my teacher is lacking, he shows rather then explaining on why it works that way. i apperciate your guys help to much

 

[netgypsy]

Dont' use 9.8 m/sec2 unless the cable breaks. That's acceleration due to gravity. You're calculating acceleration in problem 1

don't you hate it when they "service" a website when you most need it.

 

[e.bar.goum]

Netgypsy - 650N/9.8m/s^2 gives the mass of the person on the balance.

 

[netgypsy]

duhhh right but I thought they were on an elevator?

 

[e.bar.goum]

On a balance on an elevator!

 

[netgypsy]

OK I hate it that I can't see the problem cause I'm on the second page. I should open the first in a different window but I'm too brain dead.

 

[mrphobio]

Now to number 3. I would do tan(28)=129/fx correct? Then fx-68 to find the answer?