Question

we were asked to find the time taken for a ball to reach a specific point B. The abscissa of point B is 2m. the acceleration is -4, the initial velocity is 5, and the velocity at B is 3. I used the formula from the time equations that says that x=at^2/2+V initial * t + x initial. i got two answers for time, one which is 2 s and the other is 0.5 s. My teacher used another way and got only 0.5 seconds. Why is 2 seconds rejected?

Answer 1

found time values must satisfy the equation for speed

$v=v_0+a \cdot t$

By condition

$3=5-4 \cdot t$

equality will be fulfilled only when $t_2=0.5s$

$3=5-4 \cdot t_2$

$3=5-4 \cdot 0.5$

$3=3$