Assumed Values

In physics story problems, there are often assumed values that are hidden in the story. They are a word, phrase, or implied idea that is supposed to give you a number to be used in your equations. 

For motion in one dimension: 

• Starting or ending from rest:  vi or vf = 0m/s
• If something is dropped:  vi=0m/s
• If something gets the the maximum height:  vf = 0m/s (it's not moving up or down at the top of the flight)
• If you are on earth going up and down: a=9.8 or 9.80m/s/s

For 2D motion:

This means ax=0m/s/s while ay=9.8m/s/s

The speed at the top is only the horizontal component

2D Motion Concepts

When thinking about motion that's happening in two dimensions, you need to think about the two components separately. 

This means, you want to divide up your acceleration, velocity, and distance values into x and y. 

Do not put x & y values together in the same kinematic equation. The only value that counts for both x and y is time. 

Because gravity affects vertical, a in the y direction = g = 9.8 or 9.80m/s/s

The acceleration in the x direction is zero! Notice in each of the pictures, the velocity in the x direction is constant. The overall velocity will increase and decrease, but it's the y that is changing not the x.

 

Unit Conversions

 Use equalities or how many ___ fit inside of ___ in order to cancel units. This is called dimensional analysis. Your goal is to get rid of the units that you don't want and get to the units you do want. 

Velocity Graph Notes

 

Motion Graphs - Memorize!