Conceptual Physics problem from the real sat

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Discus: SAT/ACT Tests and Test Preparation: October 2003 Archive: Conceptual Physics problem from the real sat
By Lostgirl (Lostgirl) on Friday, October 10, 2003 - 12:34 am: Edit

A piece of chalk is thrown vertically upward and is caught during its descent at the same height from which it was thrown. Posititon is measured from the location of the chalk when it left the hand. The positive direction for position velocity and acceleration is upward.

What are the signs of the position, velocity and acceleration during the a)ascending part and b) the decending part of the trajectory?

The answers are :
a) Positive, Positive, Negative.
b) Positive, Negative, Negative.

I would be very much obliged if someone would explain the answers..... i am very much confused with the direction conventions.. someone please!!!!!

By Zephyrmaster (Zephyrmaster) on Friday, October 10, 2003 - 02:33 am: Edit

Okie dokie.

a.) It's ascending, going UP. The position will be positive, because it's above the height it was thrown. Velocity would be positive because the chalk is going UP, which is positive according to the problem. Acceleration is negative, because gravity is pulling the chalk DOWN, which is negative.

b.) Same, except that velocity is negative because the chalk is going DOWN, which is negative.

Remember: Velocity is basically speed + a direction. Acceleration of an object thrown into the air is practically ALWAYS -9.8, unless you use some other method.

Hope I helped.

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