PDA

View Full Version : Identifying Forces


RV-07
Mar 8, 2007, 04:25 PM
A block of dry ice resting on a table is given a brief push. A moment later, what forces act on the block?

manimuth
Mar 8, 2007, 04:37 PM
Friction (opposite direction of movement), acceleration down (massXgravity), normal force (up from the table), and acceleration forward from the force of the push.
I hope I got everything... if not, I'm sure someone else here will catch it.

asterisk_man
Mar 9, 2007, 07:00 PM
first, there is no acceleration down or else the velocity down would change (which it isn't), it's just a force. And there's no acceleration forward from the push because we're talking about "a moment later" therefore the push is done, there is no force, therefore there is no acceleration.

to review (assume push is in +x and that +y is up),
friction in the -x direction
gravity in the -y direction
normal in the +y direction

manimuth
Mar 9, 2007, 07:31 PM
asterisk man,
Isn't gravity more specifically known as acceleration due to gravity? Also, the object has weight and weight is a force defined as: mass X 9.8m/s2

manimuth
Mar 9, 2007, 07:43 PM
Also, I think there is no change in velocity because Normal force is equal and opposite to the force of weight? I think you (asterisk man) are right about no force in the +x direction.

asterisk_man
Mar 9, 2007, 09:07 PM
gravity is a force. In this case the normal force is exactly equal to the force of gravity, therefore the acceleration in the y direction is exactly 0. there is no vertical acceleration. If there was vertical acceleration the vertical velocity would change... and it doesn't.

9.8m/s^2 is the acceleration due to gravity at the surface of the earth. The force of gravity is:
F_{gravity On Earth}=G \frac {m_1 m_{earth}} {{r_{earth}}^2} \\
a_{gravityOnEarth}=\frac {F_{gravityOnEarth}} {m_1} = G \frac {\frac {m_1 m_{earth}} {{r_{earth}}^2}} {m_1}=G \frac {m_{earth}} {{r_{earth}}^2} = 6.6730E-11 m^3 kg^{-1} s^{-2} * \frac { 5.9742E24 kilograms} {{6.37810E6 meters}^2} \approx 9.8 m s^{-2}


acceleration due to gravity does not depend on the mass of the object. Not 100% sure why i just wrote all that out but it's fun at least.

final conclusion:
force of gravity? Yes
acceleration due to gravity? No