Understanding Force and Acceleration in Physics Problems

In summary, the conversation revolves around a physics problem where a 10.0kg rock is pulled upwards with a force of 100N. The conversation includes discussions on drawing a free body diagram, using Newton's Second Law, and the correct units for acceleration. The final answer should be 10.0 m/s^2, but there is confusion about the answer being marked wrong. The conversation ends with a question about the possibility of acceleration being negative upwards.
  • #1
anderrod
4
0
Ok...so I am physics phobic! I read these physics questions and honestly have no idea what they are even asking. I have no idea how to start or what to do. I understand that this forum is for assistance for homework, but what do you do when you haven't a clue on how to even begin?

So I am looking for help on how you start with a problem like this:

You have a 10.0kg rock with a rope tied around it. What acceleration would you give the rock if you pull upwards on the rope with a force of 100 N?

How can I get an answer when I don't even know how far the rock traveled? Wouldn't I need to know that in order to answer the question?

Please help!
 
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  • #2
start out by drawing a free body diagram for the rock.. mg will be downwards and the 100 N will upwards. So the difference ( 100N - mg) will be equal to ma. and solve for a
 
  • #3
[tex]\vec{F}_{net} = \Sigma \vec{F} = m \vec{a}[/tex]

Newton's Second Law should give you everything you need.

[tex]\Sigma \vec{F} [/tex]
Would be the force the rock was pulled up with minus the weight of the rock
[tex]\Sigma \vec{F} = 100 - (10*(9.8))[/tex]
then on the other side
[tex]m \vec{a}[/tex]
would be the mass of the rock (10 kg) and then you could solve for a (acceleration)
 
  • #4
By the way, you asked about knowing distance. Acceleration could not care less for the distance something goes, acceleration is just how quickly something is changing speed and this is caused by being pushed. If you push something for a longer time, it's not going to change acceleration.
Does that help at all?
 
  • #5
Thanks for the help, but I still am not getting it. I thought the answer was 10 m/s but when I submitted that answer, it was marked wrong. When I use your equations, I get .2 m/s and that just seems all wrong.

What am I not getting here?

I appreciate the help.

Thanks
 
  • #6
When something is being pulled (assuming the rope is massless) the force that it is being pulled with is not the only force acting on it, teh mass and gravity does work on the rock as well, making it harder to pull up with a faster acceleration. The sum of forces would then be equal to the force you pulled up with minus the Weight (in Newtons) of the object, so 100 - (10*9.8), and that would be equal to teh mass of the rock times its acceleration. Then you divide by the mass to get the acceleration
 
  • #7
anderrod said:
Thanks for the help, but I still am not getting it. I thought the answer was 10 m/s but when I submitted that answer, it was marked wrong. When I use your equations, I get .2 m/s and that just seems all wrong.

What am I not getting here?

I appreciate the help.

Thanks

Check your units. Acceleration is not measured in m/s.
 
  • #8
Tier said:
When something is being pulled (assuming the rope is massless) the force that it is being pulled with is not the only force acting on it, teh mass and gravity does work on the rock as well, making it harder to pull up with a faster acceleration. The sum of forces would then be equal to the force you pulled up with minus the Weight (in Newtons) of the object, so 100 - (10*9.8), and that would be equal to teh mass of the rock times its acceleration. Then you divide by the mass to get the acceleration

Except that the question asks for the applied force, not the net force.
 
  • #9
geoffjb said:
Except that the question asks for the applied force, not the net force.

it has to be the net force though, becasue if you only factored in the applied force, you would get 10, which he said was wrong when he submitted it. In order to get the acceleration, Newton's Second Law has to be used, which implies net force
 
  • #10
Ok...I have worked through the problem and I still am getting that same lame answer of .2 m/s^2. I get 2kgm/s^2=10kg*a
solving for acceleration, I get 2 kgm/s^2/10kg
which equals .2 m/s^2

So what am I still doing wrong?

Thanks. I see that before I neglected to account for the conversion to get the units right, but I still am not getting the right answer, so ?

Thanks for the assistance, I actually feel like I might make it through this problem.

C.
 
  • #11
anderrod said:
Ok...I have worked through the problem and I still am getting that same lame answer of .2 m/s^2. I get 2kgm/s^2=10kg*a
solving for acceleration, I get 2 kgm/s^2/10kg
which equals .2 m/s^2

So what am I still doing wrong?

Thanks. I see that before I neglected to account for the conversion to get the units right, but I still am not getting the right answer, so ?

Thanks for the assistance, I actually feel like I might make it through this problem.

C.

An answer of [itex]10.0 m/s^{2}[/itex] (watch your sig figs and units) should not be wrong.
 
  • #12
Yeah, I guess that would be right, didn't read the question close enough
 
  • #13
Hmmm...well now I am confused. I just checked with another student and they entered 10.0 m/s^2 and they got it wrong as well. I was pretty confident in my answer until I got it wrong. So now I don't know what to do. Thanks for the help anyway.

C.
 
  • #14
anderrod said:
Hmmm...well now I am confused. I just checked with another student and they entered 10.0 m/s^2 and they got it wrong as well. I was pretty confident in my answer until I got it wrong. So now I don't know what to do. Thanks for the help anyway.

C.
Might acceleration be negative upwards?
 

1. What is the relationship between force and acceleration?

The relationship between force and acceleration is described by Newton's Second Law of Motion, which states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. In other words, the greater the force applied to an object, the greater its acceleration will be, and the more massive an object is, the less its acceleration will be for a given force.

2. What is the unit of force and acceleration?

The unit of force is the Newton (N), which is equivalent to kg*m/s^2. The unit of acceleration is meters per second squared (m/s^2).

3. How does mass affect the acceleration of an object?

As mentioned in the first question, mass is inversely proportional to acceleration. This means that for a given force, an object with a larger mass will have a lower acceleration compared to an object with a smaller mass. In other words, a heavier object will require more force to achieve the same acceleration as a lighter object.

4. Can an object have acceleration without any force acting on it?

No, according to Newton's First Law of Motion, an object will remain at rest or continue to move at a constant velocity unless acted upon by an external force. Therefore, acceleration cannot occur without some kind of force acting on the object.

5. How does the direction of the force affect the acceleration of an object?

The direction of the force applied to an object will determine the direction of its acceleration. If the force is applied in the same direction as the object's motion, it will increase the speed of the object. If the force is applied in the opposite direction, it will decrease the speed of the object. If the force is applied at an angle to the direction of motion, it will result in both a change in speed and direction of the object's motion.

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