Homework Help: Particle motion

1. Jun 5, 2010

thereddevils

1. The problem statement, all variables and given/known data

At time , t s , a particle with a mass 1 kg moves under the action of force F=6(i+t j) N. If the particles starts to move at time t=0 ,find the velocity of the particle at t=5 .

2. Relevant equations

3. The attempt at a solution

By F=ma

the acceleration vector , a = 6i+6t j

integrate this gives , v= (6t+c)i+(3t^2+c)j

and when t=0 , v=0

therefore , v=(6t)i+(3t^2)j

and at t=5 s, v=(30)i+(75)j and my next job is to find its magnitude

Is everything allright ?

2. Jun 5, 2010

rock.freak667

Yes that looks to be correct, although you can leave your answer in the vectro form v=30i+75j. If you just put the magnitude, you'd need to put the angle as well since velocity is a vector quantity in that it has both magnitude and direction.

3. Jun 5, 2010

CompuChip

Just a tiny comment... you write F = ma and then write down a at once. You have used that the mass is equal to 1 here, you might have mentioned that more explicitly, perhaps.

Also the integrations are independent, so the "+ c" that goes with the i-component is not necessarily the same as the "+ c" in the j-component. You would better use different letters for those, for example

v= (6t+c1)i+(3t^2+c2)j
Then from v(t = 0) = 0 (= 0 i + 0 j) it indeed follows that c1 = c2 = 0 (but in general, you can get different constants).

Just some nitpicking from my side though, because you did correctly solve the problem.

4. Jun 5, 2010

thank you !