Câu $14 : B$
$v_{tb}=\dfrac{s}{t_1+t_2}=\dfrac{s}{\dfrac{s}{2.v_1}+\dfrac{s}{2.v_2}}=\dfrac{1}{\dfrac{1}{2.v_1}+\dfrac{1}{v_2}}=\dfrac{1}{\dfrac{1}{2.20}+\dfrac{1}{2.5}}=8m/s$
Câu $15 : A$
$t=\dfrac{s}{v}=\dfrac{780}{1,2}=650s=10phút50giây$
Câu $16 : B$
$\dfrac{s}{1}-\dfrac{s}{1,2}=330$
$\Rightarrow s=1980m$
Câu $17 : A$
$v_{tb}=\dfrac{v_1+v_2}{2}=\dfrac{50+60}{2}=55km/h$
Câu $18 : C$
PTCĐ
$40t=120-20t$
$\Rightarrow t=2h$
$\Rightarrow x_A=40t=40.2=80km$
Câu $19 : C$
$a=\dfrac{8-2}{2}=3m/s^2$
Câu $20 : C$
Câu $21 : B$
Gia tốc
$a=\dfrac{0-15}{10}=-1,5m/s^2$
$v'=15+(-1,5).6=6m/s$
