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# Help making an equation for motion?

## Help making an equation for motion? - Physics Forum

### Help making an equation for motion? - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

#1
07-30-2009, 04:19 PM
Pipette Filler

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Help making an equation for motion?

I have trouble understanding the answer of the following question:
A toy rocket of mass 0.2kg is projected vertically upwards from rest by means of a force which decreases uniformly in 2 seconds from 2kg wt to zero and thereafter ceases.
Write equations of motion for the rocket for the case t < 2.

dV/dt = (9 - 5t)g.

Why is that so? Thanks!
#2
07-30-2009, 04:19 PM
Pipette Filler

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F = ma = m dv/dt

You are told the propelling force decreases linearly in time, so it is of the form F = a + bt, and b is negative because it is decreasing.

At t = 0, the force is "2 kg wt". I assume that means 2 kg * g. Plug in t = 0 and set it equal to 2g:

F(0) = a + b*0 = a = 2g. So now you know the constant a.

You are told that the force is 0 at t = 2.

F(2) = a + 2b = 0. Solving, b = -a/2 = -2g/2 = -g

Now you know b, so F = a + bt = 2g - gt

One more piece: Don't forget gravity, which acts downward and has magnitude mg. So the total force on the rocket is 2g - gt - mg. This equal to mass times acceleration and that gives you your equation of motion:

2g - gt - mg = m dv/dt

Since you know m = 0.2, you can substitute that to get

2g - gt - 0.2 g = 0.2 dv/dt
1.8g - gt = 0.2 dv/dt

And that is your equation of motion. If you multiply both sides by 5 to just get dv/dt on the right, you'll have the equation you were told is the answer.
#3
07-30-2009, 04:20 PM
Pipette Filler

Join Date: Jun 2009
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Are you making an equation for:
Motion,
Acceleration or
Instantaneous Velocity?

 Tags equation , making , motion , _

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