Mechanics
How do i know if initial and final velocity is positive or negative and if distance is positive or negative in these type of questions like I don't get when to make it positive or negative. Let's say we take up as positive what would the values be and if down is taken as positive what would be values for u, v and s.
Many thanks
Many thanks
Original post by Bird1234
How do i know if initial and final velocity is positive or negative and if distance is positive or negative in these type of questions like I don't get when to make it positive or negative. Let's say we take up as positive what would the values be and if down is taken as positive what would be values for u, v and s.
Many thanks
Many thanks
the method varies but for me I always took the upward motion as negative as it is against the direction of gravity which is downward. so in the case of the photo, the intial will be negative and the downward motion so the final velocity will be postive. "s" is displacement so it is not distnace between the start point and end point not the distance travelled. so by using suvat, s = the distnace between where the ball went upward and when the ball drops to the floor. u = -39.2 m/s v= ?? a = -9.81 t = ??? I hoped this helped. Oh and one more point, the g value which is 9.81 I alwyas set as -9.81 when the initial motion goes upward and poisitive for downward motion
Original post by ysl060804
the method varies but for me I always took the upward motion as negative as it is against the direction of gravity which is downward. so in the case of the photo, the intial will be negative and the downward motion so the final velocity will be postive. "s" is displacement so it is not distnace between the start point and end point not the distance travelled. so by using suvat, s = the distnace between where the ball went upward and when the ball drops to the floor. u = -39.2 m/s v= ?? a = -9.81 t = ??? I hoped this helped. Oh and one more point, the g value which is 9.81 I alwyas set as -9.81 when the initial motion goes upward and poisitive for downward motion
Thank you, could u pls clarify the s part for me as well I'm still confused. Like when is it positive and negative.
Original post by Bird1234
Thank you, could u pls clarify the s part for me as well I'm still confused. Like when is it positive and negative.
um i never really ever had to make the value of s negative. It is always postive.
Original post by ysl060804
um i never really ever had to make the value of s negative. It is always postive.
Ok thank you
Original post by Bird1234
Ok thank you
You can take either direction (up / down in this case) as positive and the opposing direction is negative. You generally choose the direcction which makes the (suvat) callculations easiest and so the values "make sense", so the least number of negative values. For your example, up positive would be the most sensible choice and youd have
u = 39.2
a = -g = -9.8
s being positive would denote a displacement above the origin/initial point and s being negative would denote a displacement below the origin/initial point. So up positive would mean that the max height was represented by positive displacement, for instance.
However, as long as it applied consistently, the choice of positive direction should not affect the final answer, as long as you interpret it correctly.
(edited 3 weeks ago)
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