Haro

尤其是work and energy 同 circular motion

JasonFung0310

你可以做吓past paper 同 radian嘅exercise~
要學好mechanics ,我覺得起碼以下嘅都要識嘅。

第一課嘅equation of motion 一定要識先啦，(area / slope of s-t graph , v-t graph有咩meaning 同埋點將s-t graph 轉v-t graph , v-t graph 轉 a-t graph 等等)(未必一定出呢d graph，要留意佢比咩嘢你，有時佢會比咗v^2你)

當你equation of motion好熟嘅時侯，你就可以學force and motion ，因為好多時侯都要用equation of motion揾咗acceleration先，之後先可以用F=ma (Newton's second law of motion)
newton's laws of motion 一定要好熟 ，
first law : 冇net force 嘅話，就會remain its state of motion (原本係undergo uniform motion嘅之後都仲係uniform motion ,原本at rest嘅之後仲係at rest)。
second law : F=ma
(F係net force)(same direction嘅force加，opposite direction就減，自己set 邊個direction係正負)(要拆x,y component)
third law : action 同reaction force 係唔會act on same object ，佢地係same magnitude 同opposite direction嘅(e.g. gravitational force acting on A by Earth ，咁reaction嘅direction就係gravitational force acting on the Earth by A)
做親題目都拆x-direction同y direction咁睇就啱。

work and energy (scalar) 其實係考緊你law of conservation of energy ，
你只要明白energy loss = energy gain 呢個concept 其實都已經夠，
例如a ball is released from rest ，咁loss of gravitational potential energy = gain of kinetic energy + work done against air resistance.
不過有d嘢要注意，如果個direction of force 係perpendicular to the direction of motion 嘅話(e.g. uniform circular motion ) ，work done by centripetal force = 0 J 。
同埋你要識拆component , work done 咪= Fs ,你要將個force拆到有個component 係同s same direction嘅。
要學吓睇P.E.-t graph , P.E-s graph , K.E.-t graph , K.E.- s graph 等等。

momentum，就係考緊你law of conservation of momentum ，而呢度會教你三種collision ，
perfectly elastic collision ，partially inelastic collision 同perfectly inelastic collision。
無論係邊種collision，total momentum before collision = total momentum after collision
(記住,momentum 係vector ，same direction就+，opposite direction就減)
(有時d題目會問A collide with B , 咁momentum of A 係咪conserved ，咁就唔係conserved喇，因為佢唔係問緊你total momentum，同埋collision之後佢 direction / magnitude實有change，所以要小心字眼)
perfectly elastic collision : sum of K.E. is conserved + total momentum is conserved (冇癡埋一齊)
partially inelastic collision : total momentum is conserved , but total K.E. is not conserved (冇咗嘅可能變咗sound / heat ) (冇癡埋一齊)
perfectly inelastic collision (癡埋一齊!!!) : total momentum is conserved + total K.E. is not conserved (lost 嘅K.E.係最多嘅)
要識睇F-t graph ， Ft= impulse = change in momentum = mv-mu
(計數一樣要拆component ，記住，rate of change of momentum = net force ，小心係唔係用緊net force嚟計數)
而circular motion呢，你就要明白一樣嘢perform circular motion 嘅話centripetal force一定存在，
而centripetal force 係provided by 其他force 嘅，例如magnetic force , tension , gravitational force 等等，
之後其實都只係就咁用返d equation 同埋sub數。
學electromagnetism嗰陣，咪有一條equation係F=BQv ，或者學gravitational force嗰陣，咪有條equation係F=GMm/r^2，有時侯就會+埋circular motion一齊考。

記住~學mechanics 嗰陣，唔同嘅graph都要識睇先學得好。
可能表達得有d差，唔明再問~

[ 本帖最後由 JasonFung0310 於 2017-5-25 05:00 PM 編輯 ]

Haro

Thx😊

556556

學下點樣推書入面比你既式
例如1/2mv^2=E
就去研究下點解要x 1/2 x v^2
當你識推一條式表示你有一定的Concept 。

Haro