有條題目唔係好明‥

仲夏夜．戀人

如有一塊石頭被氣球的線縛住,,慢慢由地面升上空中
如果設向下係positive..
條式就係T-mg=ma 啦!?
但係如果設向上係positive
T-(-gm)=ma
T+mg=ma !?

照道理向上同向下係正條式都冇分別..無理由2條式會1個+mg1個-mg架...我搞錯左d咩? thx

永樂

T+ (-mg)= ma

力係直接加, 正負由方向決定

『帥乂明』

T係咩黎  點解向上向下都係positive

晴兒~

回復 3# 『帥乂明』


    T應該是指張力.

『帥乂明』

回復  『帥乂明』


    T應該是指張力.
晴兒~ 發表於 15-4-2010 20:39

    咁點解兩個情況都係正

maxycy

In this case, we have three force:
1.Tesion is a force where the direction is upward.
2.The weight(mg) is a force where the direction is downward.
3.The acceleration is a force where the direction is downward.

Case1:take downward to be positive
T(negative)-mg(positive)=ma(positive)-(*)

Case2:take upward to be positive
T(positive)-mg(negative)=ma(negative)-(1)
(1)x-1, T(positive)x(-1)-mg(negative)(-1)=ma(negative)(-1)
            T(negative)-mg(positive)=ma(positive)-(2)

Sub up these two case,by the equation(2) and equation(*) are equal, we know that the result of them is same.

[S]【YU】

Downward force is positive when downward is taken positive.
Upward force is positive when upward is taken positive.

T is upwards, mg is downward, T > mg

Upward positive: T-mg = ma                       => a > 0 => upward movement
downward positive: -T + mg = ma              => a < 0 => upward movement