Phuong trinh dinh luat II Newton:

P+F=0 (1)
Chieu(1) len huong P
=>
P=F <=> mg=k.delta l <=>delta l=mg
__
k
=0,5.10
______
100
=0,05(m)

=>chieu dai lo xo l1=l + delta l=40 + 5=45(cm)
b/
l2=l+delta l=48(cm) <=> delta l= 8(cm)=0,08(m)

khi do m=k.delta l
_______
g
=100.0,08
_______
=0,8 (kg)

vay khoi luong vat can treo :0,8 kg

m1=60 kg
v1=4m/s

m2=90 kg
v2=3m/s

v=?

a/ Cung chieu
pt=ps <=>m1v1=m2v2
m1v1+m2v2= v (m1+m2)
v=m1v1+m2v2
_________
m1+m2
v=60.4+3.90
_________
60+90
v=3,4 (m/s)
b/ Nguoc chieu
v= -60.4+3.90
_________
60+90
v=0.2