Calculating Acceleration: Understanding Newton's Second Law

[kitkat]

I am taking a conceptual physics class online and the testbook doesn't seem very clear to me (or maybe its because my kids have all the science genes in my family)

The question is this:
What is the acceleration of a 40-kg block of cement when pulled sideways with a net force of 200N?

Here is what I think...can you tell me if I'm right or at least on the right track?

acceleration = force / mass so wouldn't it be 200/40 = 5 m/s ^2 ?

any help would be greatly appreciated.
Kat

 

[Mindscrape]

Yes, that would be exactly how you do it.

I'm not sure if you dropped your units for the sake of skipping the typing or not, but you should get in the habit of putting them in your calculations.

 

[kitkat]

I see your point...

I think you are right, I see myself forgetting to put the labels in so its good advice..Could I trouble you to look at another problem...this problem and answer was given by the teacher to use as an example but I am having trouble figuring it out still.

Here is the problem
What is the acceleration of a 20-kg pail of cement that is pulled upward (not sideways!) with a force of 300N?

Here is the answer provided
Weight of the pail is mg = 20kg * 10 m/s ^2= 200N
So a = F/m = (300N-200N) / (50kg) = 2 m/s^2

I don't understand why we needed to find the weight, and secondly I don't understand where the 50kg came from (how was it figured?) The only explanation I came up with was to change the 300N into weight = 30kg and then adding the 20kg to it...but why would we?

any help and explanation would be great!
Kat

 

[d_leet]

I think you are right, I see myself forgetting to put the labels in so its good advice..Could I trouble you to look at another problem...this problem and answer was given by the teacher to use as an example but I am having trouble figuring it out still.
Here is the problem
What is the acceleration of a 20-kg pail of cement that is pulled upward (not sideways!) with a force of 300N?
Here is the answer provided
Weight of the pail is mg = 20kg * 10 m/s ^2= 200N
So a = F/m = (300N-200N) / (50kg) = 2 m/s^2
I don't understand why we needed to find the weight, and secondly I don't understand where the 50kg came from (how was it figured?) The only explanation I came up with was to change the 300N into weight = 30kg and then adding the 20kg to it...but why would we?
any help and explanation would be great!
Kat

We need the weight because weight is the force of gravity upon an object so if an upward force is applied, gravity opposes it so you need to find the sum of the forces, as for the 50kg instead of 20 I think that is just a mistake.

 

[kitkat]

strange...

the book uses 50kgs as well... now I am really confused.

 

[Mindscrape]

Okay, so by Newton's second law the sum of all forces equal the mass of the object times the acceleration \sum F = ma or, in this case, F_u +(- F_g) = ma where F_u is the force of the pull upward and F_g is the force of gravity. If you understand up to this point, which is the key, then the rest is just finishing the algebra and plugging in the numbers where the real answer (theirs is wrong) comes out to be 5m/s^2.

 

[kitkat]

thank you

its nice to know that I understood it at least...I kept working the problem and came up with 5m/s^2 as well...again thank you from the help.

Kat