Question

某人用100N的力将一质量为50g的小球以10m/s的速度从某一高处竖下向下抛出，经1s小球刚好落地，不考虑空气阻力，选地面为零势能点，g=10m/s²．                                                                            

求：（1）小球刚抛出时的动能和势能各多大？                                                      

（2）小球着地时的动能和势能各多大？                                                               

                                                                                                                            

Answer 1

（1）小球下落的高度：h=

小球的重力势能：E_P1=mgh=0.5×15=7.5J

  E_K1===2.5J．

（2）小球落地时的重力势能E_P2=0

根据机械能守恒定律得：E_P1+E_K1=E_P2+E_K2

代入数据解得：E_K2=10J

答：（1）小球刚抛出时的动能和势能各为2.5J，7.5J．

（2）小球着地时的动能和势能分别为10J，0J．