s12​=21​g(t2−(t−1)2)

\(t^{2} - \left(\right. t - 1 \left.\right)^{2} = t^{2} - \left(\right. t^{2} - 2 t + 1 \left.\right) = 2 t - 1\)

\(14,7 = \frac{1}{2} \cdot 9,8 \left(\right. 2 t - 1 \left.\right)\) \(14,7 = 4,9 \left(\right. 2 t - 1 \left.\right)\)

\(3 = 2 t - 1\) \(2 t = 4\)

\(t=2\text{s}\)

64,8km/h=18m/s

\(54 \textrm{ } \text{km}/\text{h} = 15 \textrm{ } \text{m}/\text{s}\)\(\)

36km/h=10m/s

Ô tô giảm từ \(v_{0} = 18 \textrm{ } \text{m}/\text{s}\) xuống \(v = 15 \textrm{ } \text{m}/\text{s}\) trong \(t = 10 \textrm{ } \text{s}\).

\(a = \frac{v - v_{0}}{t} = \frac{15 - 18}{10} = - 0.3 \textrm{ } \text{m}/\text{s}^{2}\)

a)

v=v0​+at

\(10 = 18 - 0.3 t\) \(0.3 t = 8 \Rightarrow t = \frac{8}{0.3} \approx 26.67 \textrm{ } \text{s}\)

b)

0=18−0.3t \(t = \frac{18}{0.3} = 60 \textrm{ } \text{s}\)

c)

s=v0​t+1/2at2

Thay \(t = 60\), \(a = - 0.3\):

\(s = 18 \cdot 60 + \frac{1}{2} \left(\right. - 0.3 \left.\right) \left(\right. 60^{2} \left.\right)\) \(s = 1080 - 540 = 540 \textrm{ } \text{m}\)