I was suppose to find the acceleration of the object, the displacement after 4.0

Question

I was suppose to find the acceleration of the object, the displacement after 4.0 seconds, and the speed after falling 100m. I got the right numerical answers but not the right signs. I said that down is the negative direction, therefore the initial velocity is -20m/s and the acceleration is -9.8m/s^2. However, my instructor's answers said that the acceleration is 9.8m/s^2. I don't understand this. If the object is falling down in the negative direction, wouldn't the acceleration be negative?

I also got the the displacement after 4.0 seconds to be -158.4m but the answers said that it is 158.4 meters. Again, wouldn't this be negative if the object is falling down?

For the speed after 100m, I used v^2=v0^2+2a(xf-x0). I said v0 is -20m/s, a is -9.8m/s^2 and xf-x0 is -100m. I got v= +48.6 or -48.6m/s. I know speed is just the magnitude of velocity so the speed is 48.6m/s, but wouldn't the velocity be -48.6m/s? Is this correct?

Could someone please explain to me if my reasoning is correct or if the answers are positve? I am really confused. Thank you for any help.

Explanation / Answer

speed is always a scalar quantity so it has no signs so magnitude of velocity = speed at an instant so sign is decided by the the direction so v = -48.6

Related Questions

Navigate

• Browse (All)
• Browse I
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.