Here it is: A small mailbag is released from a helicopter that is descending steadily at 1.50 m/s. After 2.00 s, (a) what is the speed of the mailbag, and how far is it below the helicopter? (c) What are your answers to part (a) and (b) if the helicopter is rising steadily at 1.50 m/s?
My textbook shows the answers are: (a) 21.1 m/s (b) 19.6 m (c) 18.1 m/s; 19.6 m. I don't think that that is correct because my teacher says the first answer should be -21.1 m/s. I didn't think speed or velocity could be negative, either. If you could, please, show me your work and reasoning I'd, greatly appreciate it.
My textbook shows the answers are: (a) 21.1 m/s (b) 19.6 m (c) 18.1 m/s; 19.6 m. I don't think that that is correct because my teacher says the first answer should be -21.1 m/s. I didn't think speed or velocity could be negative, either. If you could, please, show me your work and reasoning I'd, greatly appreciate it.
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Your teacher is wrong about part (a), because the question specifically asks for the SPEED of the mailbag. If the question had asked for the VELOCITY, your teacher would be correct.
Maybe you already know this, but speed is a scalar quantity while velocity is a vector quantity. A scalar quantity has only a magnitude, or a measure of something. If you say that a car is traveling at 45 miles per hour, you have given the car's speed. 45 is the magnitude and miles per hour is the unit that gives the measurement meaning.
A vector quantity has both magnitude and direction. To give the car's velocity, you would have to say that it is traveling at 45 miles per hour NORTH (or whatever direction it's going). Adding a direction transforms a scalar quantity into a vector quantity.
In many cases, the direction of a velocity can be listed as + or -. This assumes that + and - have been defined in the context of the problem. For problems that involve falling bodies, it's common to make "up" positive and "down" negative.
To say that the bag is moving at 21.1 m/s, therefore, is to give its speed. To say that the bag is moving at -21.1 m/s is to give its velocity, since you SPECIFICALLY mentioned the direction of the bag's motion by putting the negative sign in front of it. Your teacher is wrong in that regard.
Maybe you already know this, but speed is a scalar quantity while velocity is a vector quantity. A scalar quantity has only a magnitude, or a measure of something. If you say that a car is traveling at 45 miles per hour, you have given the car's speed. 45 is the magnitude and miles per hour is the unit that gives the measurement meaning.
A vector quantity has both magnitude and direction. To give the car's velocity, you would have to say that it is traveling at 45 miles per hour NORTH (or whatever direction it's going). Adding a direction transforms a scalar quantity into a vector quantity.
In many cases, the direction of a velocity can be listed as + or -. This assumes that + and - have been defined in the context of the problem. For problems that involve falling bodies, it's common to make "up" positive and "down" negative.
To say that the bag is moving at 21.1 m/s, therefore, is to give its speed. To say that the bag is moving at -21.1 m/s is to give its velocity, since you SPECIFICALLY mentioned the direction of the bag's motion by putting the negative sign in front of it. Your teacher is wrong in that regard.
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