−55a=548∴a=9.6ms−2
Example 4
A body accelerates uniformly from rest at the rate of 3ms−2 for 8s. Calculate the distance covered by the body during acceleration. WAEC 1992
Solution
Using appropriate symbols deduce, the known and unknown from the question.
Acceleration, a=3m/s2; initial velocity, u=0; time, t=8s; distance, s=?
Therefore the most appropriate equation that includes a, u, t and s is the second equation of motion:
s=ut+21at2
Substituting:
s=0×8+21×3×82
=0+21×3×64
=96m
Example 5
Starting from rest, a Formular One car accelerates uniformly at 25m/s2 for 30s. What distance does it cover in the
(i) last one second of motion.
(ii) seventeenth second of motion.
Solution
The distance in the last one second of motion is the distance it covered between time interval of the 29th and 30th second. It is calculated by subtracting the distance in the 29th sec from that of the 30th sec.
Distance, s=S30−S29
But, S30=ut+21at302 and S29=ut+21at292
Starting from rest, initial velocity, u=0. Therefore, ut=0 in each case
∴s=21at302−21at292
=21a(t302−t292)
From the question, acceleration, a=25m/s2; t30=30s; t29=29s. So substituting into above equation:
s=21×25(302−292)
=12.5(900−841)
=12.5(59)
=737.50m.
(ii) From question acceleration, a=25m/s2; initial velocity, u=0; time, t is not equal to 17s because the 17th second is actually the time interval between the 16th and 17th second, therefore time, t=t17−t16.
Substituting into s=21a(t22−t12)
s=21×25(172−162)
=12.5(289−256)
=12.5×33
=412.5m.
Note: In