Physics Help Forum Simple Force problem

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Oct 26th 2008, 02:18 PM #1 Member   Join Date: Sep 2008 Posts: 42 Simple Force problem A lobster is in a tug of war against some smaller lobsters. Its mass = 20.0 kg The small lobsters drag the big lobster 1.55 m at 0.550 m/s. Lobster is initally at rest. What is the magnitude of the force applied to the lobster? Ignore friction and water.
Oct 28th 2008, 09:11 PM   #2
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 Originally Posted by Morgan82 A lobster is in a tug of war against some smaller lobsters. Its mass = 20.0 kg The small lobsters drag the big lobster 1.55 m at 0.550 m/s. Lobster is initally at rest. What is the magnitude of the force applied to the lobster? Ignore friction and water.
use the equation of motion
$\displaystyle v^2-u^2=2as$
u is the initial velocity which would be zero.
s is the displacement , a is the acceleration

You find the acceleration and then you use
F = ma
to find the force.

Oct 29th 2008, 02:14 AM   #3
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 Originally Posted by Morgan82 A lobster is in a tug of war against some smaller lobsters. Its mass = 20.0 kg The small lobsters drag the big lobster 1.55 m at 0.550 m/s. Lobster is initally at rest. What is the magnitude of the force applied to the lobster? Ignore friction and water.
Lets assume constant force and hence acceleration, that the average speed over the $\displaystyle 1.55$m is $\displaystyle 0.550$m/s, and the initial speed is $\displaystyle 0$, and the initial position is $\displaystyle s=0$. Also assume movement is in the positive direction so our speed and velocity are the same as long as we are moving to the right.

Then the equation of motions give:

$\displaystyle v=at$

$\displaystyle s=\frac{at^2}{2}$

and if $\displaystyle t_1$ is the time over which the displacement takes place, the average speed (velocity) is:

$\displaystyle \bar{v}=0.55=\frac{at_1}{2}$

and

$\displaystyle s_1=1.55=\frac{at_1^2}{2}$

Now solve these for $\displaystyle t_1$ and a, and then use $\displaystyle F=ma$ to find the force.

CB

Last edited by CaptainBlack; Oct 29th 2008 at 02:17 AM.

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