Question

A car starts from rest and moves with a constant
acceleration of 5 m/s2 for 10 seconds before the
dreiver applies the brake. If then decelerates for
5 seconds coming to rest. The average speed of
the car over the entire journey of the car is
1) 23 m/s
2) 30 m/s
3) 33 m/s
4) 25 m/s​

Answer 1

u = 0 m/s

a = 5 m/s²

t = 10 s

t' = 5 s

Average Speed = Total distance travelled / Total time taken

total time = t + t'

total time = 15 s ------(i)

s = ut + 1/2 at²

s = 0×10 + 1/2 × 5 × 10 × 10

s = 0 + 250

s = 250 m ---------(ii)

v = u + at

v = 0 + 10×5

v = 50 m/s

Decelerates for 5 s since driver applies break and the car comes to rest.

a = -5 m/s²

t = 5s

2as' = v² - u²

2×-5×s' = 0² - 50²

-10s' = -2500

s' = 2500/10

s' = 250 m

Total distance = s + s'

d = 250 + 250

d = 500 m

Average speed = Total distance / time taken

avg. speed = 500/15

avg. speed = 33.33 m/s

So, The answer is (3)

Answer 2

$\huge{\mathfrak{Bonjour!♡}}$•°⭑

$\bullet\underline\bold{\sf{\color{grey}{Question:-}}}$

•A car starts from rest and moves with a constant acceleration of 5 m/s2 for 10 seconds before the dreiver applies the brake. If then decelerates for 5 seconds coming to rest. The average speed of the car over the entire journey of the car is?

$\bullet\underline{\sf{\color{grey}{AnsWer:-}}}$

1) 23 m/s

2) 30 m/s

3) 33 m/s

4) 25 m/s

$\bullet\underline{\textsf{\color{grey}{SteP By SteP ExplainaTion:-}}}$

$\bf{Given}\begin{cases}\sf{u=0}\\ \ \sf{a=5ms^{-2}}\\ \ \sf{t=10\:s}\\ \ \sf{v=?}\end{cases}$

Using 1st equation of motion;

$\rightarrow{ \bf{v = u + at}} \\ \\ \rightarrow \: { \sf{v = 0 + 5 \times 10}} \\ \\ \rightarrow \: { \bf{v = 50 \: ms {}^{ - 1} }}$

Now, distance = velocity× time

$\rightarrow \:{ \tt{ 50 \times 10 }} \\ \\ \rightarrow { \bf{500 \: m}}$

Total distance :

= 500 m

Total time:

=10 + 5 = 15 s

Hence, average speed=

$\leadsto \:{ \sf{ \frac{total \: distance}{total \: time} }} \\ \\ \rightarrow \: { \tt{\frac{500}{15} }} \\ \\ \rightarrow \:{ \boxed{ \bf{ \: 33 \: ms^{-1}\:approx }}}$