Question

For each of the motions described below, determine the algebraic sign (+, -, or 0) of the velocity and acceleration of the object at the time specified. For all of the motions, the positive y axis is upward.
Part A
An elevator is moving downward when someone presses the emergency stop button. The elevator comes to rest a short time later. Give the signs for the velocity and the acceleration of the elevator after the button has been pressed but before the elevator has stopped.
Enter the correct sign for the elevator's velocity and the correct sign for the elevator's acceleration, separated by a comma. For example, if you think that the velocity is positive and the acceleration is negative, then you would enter +,- . If you think that both are zero, then you would enter 0,0 .


Part B
A child throws a baseball directly upward. What are the signs of the velocity and acceleration of the ball immediately after the ball leaves the child's hand?
Enter the correct sign for the baseball's velocity and the correct sign for the baseball's acceleration, separated by a comma. For example, if you think that the velocity is positive and the acceleration is negative, then you would enter +,- . If you think that both are zero, then you would enter 0,0 .

Answer 1

Answer: A) velocity is negative and acceleration is positive (-, +)

B) velocity is positive and acceleration is positive (+, +)

Explanation: A) It has been stated in the question that upward motion is to the positive y axis which implies that downward motion is to the negative axis.

The elevator is moving downward which implies that all quantities such as velocity, acceleration and height has negative value (time is always positive).

Before the button was pressed, the body is accelerating and moving with a velocity but since we are along the negative axis they are both negative.

Once the button was pressed the elevator starts decelerating making it have a negative acceleration (-a). This negative acceleration is on the negative y axis this making us have - (-a) =+a.

The elevator still moves with the same value of velocity thus making it retain it negative sign due to the negative axis

B) This motion is along the positive y axis since it is an upward motion thus making all physical quantities such as velocity, acceleration and height positive.

The boy throws the baseball up meaning that an initial velocity is needed to start up the motion.

This velocity is positive (+). As the baseball starts it motion it accelerates which is also positive (+), thus making both velocity and acceleration positive.