Help How Do I solve Problems Like These- NOT HW- KINEMATICS

[dd353165]

Help! How Do I solve Problems Like These- NOT HW- KINEMATICS

KINEMATICS

Hey guys grade 11 physics student here,
just need a little help!

how do i solve questions that involve two missin variables?

For Example:

the uss enterprise suddently accelerates for 6.0s reaching a final velocity of 350m/s[e] the displacement for this interval was 9000m [e]

what was its intial velocity
what was its acceleration

I know the formulas but i can't use any because I don't have the variables

ALL THESE QUESTIONS ARE CONSTANT ACCELERATION

please don't post the answer but can you please show me how to go about solving them?

Thank You Very Much

 

[Doc Al]

Hint: Use two kinematic formulas and solve them simultaneously.

 

[JDHalfrack]

Along with what Doc Al said:

As you know, there are 5 different variables one can use within the 4 kinematic equations:
x, v_i, v_f, a, and t

Each kinematic has one variable not included. The only variable "required" for each kinematic is v_i. So, you want to find one of the kinematics that does not include both v_i AND a. In that case, you would be left with two unknown variables. Instead, find the kinematic that only has v_i as the unknown, and solve for v_i. Then you will have enough information to solve for a.

Hope this helps, and I hope I didn't say too much!

 

[mikelepore]

I think "solve them simultaneously" is misleading. You solve two equations, one at a time. In each one, there is only one unknown.

 

[Doc Al]

I think "solve them simultaneously" is misleading. You solve two equations, one at a time. In each one, there is only one unknown.

That entirely depends on what equations you use. Using the usual suspects--the standard kinematic formulas listed https://www.physicsforums.com/showpost.php?p=905663&postcount=2" for example--you would need to solve two equations simultaneously. None of them has only one unknown. (At least for this particular problem.)

Otherwise the problem would be too easy.

 

[JDHalfrack]

Oh, do you not use \Deltax = \frac{1}{2}(v_i + v_f)\Deltat ?

Basically, since v_avg = \Deltax / \Deltat
we know that \Deltax = (v_avg)\Deltat

In cases where the acceleration is constant, v_avg is equal to (v_i + v_f) / 2

Since, in the OP's example, acceleration is constant, we know that \Deltax = \frac{1}{2}(v_i + v_f)\Deltat

We can re-write that equation to solve for v_i as follows: vi = ( 2\Deltax / \Deltat ) - v_f

That's why I said there were four equations each with one variable "missing."

 

[Doc Al]

Oh, do you not use \Deltax = \frac{1}{2}(v_i + v_f)\Deltat ?

Hey, you're right. Forgot about that one.