Why Are My Physics Calculations on Force and Acceleration Incorrect?

[buffgilville]

1) A mass 1.3 kilograms is pushed with a horizontal force 17.5 Newtons of a smooth inclined plane which bears an angle 12.5 to the horizontal. Find the acceleration of the mass (g = 9.81 m/s).

I got 2.12, but it was wrong, and I don't know why.

2) A mass 1.9 kilograms is pushed with a horizontal force 20 Newtons of a smooth inclined plane which bears an angle 27 to the horizontal. Find the distance the block moves in 6.5 seconds (g= 9.81 m/s2).

I got 198.13, but it was wrong, and I don't know why.

 

[Sirus]

1) What did you do to find that answer? (free-body diagram?)
2) Same approach as #1, so let's talk about that one first.

 

[buffgilville]

yes, i used a free body diagram.

For the first problem, I used a = (mg) sin12.5 /1.3kg and got 2.12.

For the second problem, what I did was I use vectors to find the x and y components. solve: x components = 1.9a and got a = 9.379. then I plug in a for x=vt + (1-2) (9.379) (6.5 squared) and got 198.13 meters.

What am I doing wrong?

 

[Sirus]

For 1), I'm assuming you meant horizontal force 17.5 N up the incline. If I understand the problem correctly, 17.5 N is the applied force. In order to find acceleration in $F_{net}=ma$, you need to find the net force acting on the mass. Therefore, form your freebody diagram, re-arrange the vectors for normal force and gravitational force to find their sum (opposite direction than the applied force). From there, you should be able to find net force, and thus acceleration. I got 11.3 m/s^2 -- is that right?

 

[marlon]

https://www.physicsforums.com/showthread.php?t=44667

yeah Sirus, that is ok...

marlon

 

[Sirus]

That is why we ask people not to post the same thing in multiple locations. Post once, if it's in the wrong place it will get moved to the right one.