Suppose you are faced with a problem such as:

*A motorcycle stunt rider needs to reach a final velocity of 799 m/s in order to make the jump needed for the scene in the movie. If she has an initial velocity of 10 m/s and the total mass of her and the motorcycle is 125 kg and if she can create a force of 100 N, how much time will be needed to achieve the needed final velocity?*

On the surface, it looks (exciting, but also) like this could be hard to do.

IT'S NOT!

The velocity formula is very easy:

*vf = vi + (a)(t)*

In the problem above, you are looking for t and vf and vi are given. And then there's that mass and force...

You need an acceleration(a), but you have mass and force. Thankfully Newton did that thing:

*F = ma*

So, we'll be doing two steps.

Step 1:

Use

*F=ma*to find a

Step 2:

Then use THAT in the velocity formula.

*vf = vi + (a)(t)*

EXAMPLES

Example 1

Let's take a look at another example (even easier!) and work it out:

*A force of 50 N acts on a object with a mass of 10 kg. If it has an initial velocity of 20 m/s, what is its final velocity after an elapsed time of 8 seconds?*

The question tells us that we need to find final velocity (vf). But there is no acceleration given. So, do step 1 (above):

Find a where:

F = 50 N

m = 10 kg

*F = ma*

*50 N = (10 kg)(a)*

*50 N/10 kg = a*

*5 m/s/s = a*

Next, use THAT calculated a to find final velocity (step 2 above):

Find vf where

vi = 20 m/s

t = 8 s

a = 5 m/s/s

vf = vi + (a)(t)

vf = 20 m/s + (5 m/s/s)(8 s)

vf = 20 m/s + 40 m/s

vf = 60 m/s

Example 2

How about

*seeing*it worked out?
SUMMARY:

You gotta do it in steps!

This process requires doing the work in steps. Depending on what is given, you use the two formulas below:

*F = ma*

*vf = vi + (a)(t)*

So, read the problem, write down what is being looked for, write down what is given, THEN... use the two formulas. Two steps!

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