Question

the car travel 72 kilometre per hour in 5 seconds .find acceleration distance travelled .if car takes 2.5 seconds to stop calculate the acceleration and find the distance car takes to stop

Answer 1

Answer

Acceleration = - 8 m/s²

Distance = 25 m

Given

Initial velocity , u = 72 km/h = 20 m/s

Time , t = 5 s

Acceleration , a = ? m/s²

Distance , s = ? m

Final velocity , v = 0 m/s

[ ∵ finally stops ]

Be Careful at

While solving this type of questions see whether unit's are in SI unit's or not . Here Initial velocity is Km/h . So , we need to convert into m/s . To do that we need to multiply with 5/18

Concept Used

We need to apply equation of motion

→ v = u + at

→ s = ut + ¹/₂ at²

→ v² - u² = 2as

Solution

Apply 1st equation of motion ,

⇒ v = u + at

⇒ (0) = (20) + a(2.5)

⇒ 0 = 20 + 2.5a

⇒ 2.5a = -20

⇒ a = -8 m/s²

So , Acceleration , a = -8 m/s²

Note : negative sign of acceleration denotes retardation

Apply 3rd equation of motion ,

$\to\ \rm v^2-u^2=2as\\\\\to\ \rm (0)^2-(20)^2=2(-8)s\\\\\to\ \rm 0-400=-16s\\\\\to\ \rm 16s=400\\\\\to\ \rm 4s=100\\\\\to\ \rm s=25\ m$

So , Distance travelled , s= 25 m

Answer 2

Answer

Acceleration = - 8 m/s²

Distance = 25 m

Given

Initial velocity , u = 72 km/h = 20 m/s

Time , t = 5 s

Acceleration , a = ? m/s²

Distance , s = ? m

Final velocity , v = 0 m/s

[ ∵ finally stops ]

Be Careful at

While solving this type of questions see whether unit's are in SI unit's or not . Here Initial velocity is Km/h . So , we need to convert into m/s . To do that we need to multiply with 5/18

Concept Used

We need to apply equation of motion

→ v = u + at

→ s = ut + ¹/₂ at²

→ v² - u² = 2as

Solution

Apply 1st equation of motion ,

⇒ v = u + at

⇒ (0) = (20) + a(2.5)

⇒ 0 = 20 + 2.5a

⇒ 2.5a = -20

⇒ a = -8 m/s²

So , Acceleration , a = -8 m/s²

Note : negative sign of acceleration denotes retardation

Apply 3rd equation of motion ,

$$\begin{lgathered}\to\ \rm v^2-u^2=2as\\\\\to\ \rm (0)^2-(20)^2=2(-8)s\\\\\to\ \rm 0-400=-16s\\\\\to\ \rm 16s=400\\\\\to\ \rm 4s=100\\\\\to\ \rm s=25\ m\end{lgathered

So , Distance travelled , s= 25 m