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## Recent Comments

## Giancoli 6th Edition, Chapter 9, Problem 43

the answer has an extra 9

## Giancoli 7th Edition, Chapter 4, Problem 18

Hi joseotilio25, why $\textrm{m/s}^2$? Well, two answers: 1) acceleration always has units of $\textrm{m/s}^2$, so that's the short answer. A more detailed answer as to how that follows from the units in this solution would be 2) $\textrm{N}$ has units of $\textrm{kg m/s}^2$ since $F = ma$, and in the last line of this solution the fraction has units of $\textrm{N}$ in both terms on top, whereas there are units of $\textrm{kg}$ on the bottom. The units on top can be written differently as $\textrm{kg m/s}^2$, which means the $\textrm{kg}$ cancel, leaving you with units of $\textrm{m/s}^2$.

Cheers,

Mr. Dychko

## Giancoli 7th Edition, Chapter 4, Problem 6

Hi joseotilio25, there is no rule along the lines of "velocity is always positive" or "acceleration is always negative". Instead, you have to choose your coordinate system (it really is a choice you get to make, so you'll see different people making different choices). When you "choose your coordinate system", that's a fancy way of saying you have to choose which direction is positive. I chose the initial direction of motion of the car to be positive, and typically I choose to make the direction of initial velocity the positive direction, but it's not a "rule" to do so. Since the car is stopping, it's acceleration is in the opposite direction to the initial velocity. If the acceleration was in the same direction as the initial velocity, the car would be speeding up. However, the car is slowing down and quickly coming to a stop. This means the acceleration is in the opposite direction to the initial velocity. Since the initial velocity direction was chosen to be positive, this means the acceleration is in the negative direction.

Cheers,

Mr. Dychko

## Giancoli 7th Edition, Chapter 4, Problem 2

Hi joeotilio25, part D) asks for the weight in outer space, so the two terms that need explaining are

weightandouter space.weightmeans the force exerted by gravity (which is different from 'mass', by the way).outer spacemeans being in a location very far away from anything else. Sure, as you say, you can think of outer space as being a vacuum. The important feature of outer space is that there is nothing there to exert any gravitational force on the astronaut. With no gravitational force, the astronaut will not have any weight.Hope this helps,

Mr. Dychko

## Giancoli 7th Edition, Chapter 4, Problem 18

How do you got m/s^2