# Problems Linear motion.

In this post and  in few of my posts to come, I would like to solve problems on linear motion,freely falling bodies,vertically projected up bodies and projectiles .

1.An object accelerates from rest to a velocity 20m/sec in 4seconds.If  the   object  has uniform acceleration, find its acceleration and displacement in this time.

Soln: From the data given in the problem we have,

Initial velocity = u =0,

final velocity v=20 m/sec,

Time of journey t=4sec,

Acceleration a = $\frac{(v-u)}{t}$ = $\frac{(20-0)}{4}$= 5 $msec^{-2}$

Displacement S = ut + $\frac{1}{2} at^2$ = 0 (4)+$\frac{1}{2}\times5\times4^2$

S = 40m.

2.An object starting from rest moves with uniform acceleration of 3$msec^{-2}$ for 6sec.Find its velocity and displacement after 6seconds.

Soln: From the data given in the problem we have,

Initial velocity of the object u = 0,

Acceleration of the object a= 3$msec^{-2}$,

Time of journey t =6sec.

Final velocity of the object v=u+at =0 + 3(6) = 18 m/sec.

Displacement S= ut + $\frac{1}{2} at^2$ = 0 (6)+$\frac{1}{2}\times3\times6^2$

S= 54m.

3.An object starting from rest moves with uniform acceleration of 4$msec^{-2}$.Find its displacement i) 5seconds   ii) in 5th second iii) 8th second.

Soln : From data given in the problem

Initial velocity of the object u=0,

Acceleration of the object a=4$msec^{-2}$,

i) time t=5 seconds,

Displacement of the object S=ut + $\frac{1}{2} at^2$ = 0 (5)+$\frac{1}{2}\times4\times5^2$

Displacement of the object in 5seconds  S= 50m.

ii) Displacement of the body in 5th second =?

Let us substitute n=5 in the formula $S_n$=u+a(n-1/2)

$S_5$=0+4(5-1/2) = 4(4.5) =18m.

iii)Displacement of the body in 8th second =?

Let us substitute n=8 in the formula $S_n$=u+a(n-1/2)

$S_8$=0+4(8-1/2) = 4(7.5) =30m.

4.An object started moving with an initial velocity of 10m/sec, after traveling a distance of 5m  gets a velocity 20m/sec.Find its i) acceleration ii) time taken for 5m displacement.

soln: From the data given in the problem,

Initial velocity of the object u=10m/sec,

Final velocity of the object    v= 20m/sec,

Displacement S=5m,

i) acceleration a=?

Substitute the values of u,v and S in the equation $V^2-U^2$ =2as,

we get $(20)^2-(10)^2$=2a(5)

300 = 10a  or a = 300/10=30$msec^{-2}$.

ii)Time t=?

Substitute the values of u,v and a in the equation v=u+at,

we get  20=10+(30)t ;  10=30t

t =10/30 = 1/3 = 0.333 sec.

5.When an observer started observing a car it’s velocity was x m/sec , if it travels for 10sec with uniform acceleration 2.5$msec^{-2}$ and its velocity increases to 75m/sec.Find i) Initial velocity of the car ii) Displacement of the car in 10sec iii) Displacement of the car in first 5sec and last 5sec, what is your inference.

Soln: From the data given in the problem,

Initial velocity of the car u = x (say),

Final velocity = 75m/sec,

Acceleration a = 2.5$msec^{-2}$,

Time of journey t=10sec.

i) Substitute the values of u,v,a and t in the equation v=u+at

we get   75 = x+2.5(10) ; x=50 m/sec.

ii) Let the displacement of the car in 10sec  be S

Substitute the values of u,a and t in the equation S=ut +$\frac{1}{2} at^2$

We get    S = 50(10)+$\frac{1}{2} (2.5)(10)^2$ ;

S = 500+125 ; s=625 m

ii) Let the displacement in first 5sec be $S_1$

Substitute t=5 sec and thve values of u and a in the equation $S_1$=ut +$\frac{1}{2} at^2$

we get $S_1$=50(5) +$\frac{1}{2} (2.5)(5)^2$

$S_1$ = 250 + 31.25 = 281.25 m.

Let the displacement in  next  5sec be $S_2$

$S_2$ = S –$S_1$

$S_2$ = 675 – 281.25 = 393.75 m

We can observe that, even though the time of journey is same , $S_2$>$S_1$

Displacement of the body in second half  $S_2$ is greater than in the first half time $S_1$  of its journey.

6 .A cheetah can accelerate from rest to 24.0 m/s in 6.70 s.

Assuming constant acceleration, how far has the cheetah run in this time?(Question by suzi in yahoo answers).

Soln: From the data given in the problem,

Initial velocity of the cheetah  u=0,

Final velocity of cheetah   v=24 m/sec,

Time t = 6.7sec

Distance traveled by cheetah  be S=?

The equations of motion are $v^2-u^2$ = 2aS- – – – –   (1)

and  a = $\frac{(v-u)}{t}$  – – –  –  –  (2)

From equations (1) and (2) we get $v^2-u^2$=2$\frac{(v-u)}{t}$S

simplifying it we get   S = $\frac{(v^2-u^2)}{2}$$\frac{t}{(v-u)}$ = $\frac{(v+u)}{2}\times t$

Substitute the values of u,v and t    we get S = $\frac{(24+0)}{2}\times6.7$ =12(6.7)=80.4 m

7) A car covers first halt of the distance between two places at a speed of 30 km/hr and the second half at a 90km/hr.What will be the average speed of the car?

Soln: Method I: Let the total distance between the places be S.

Time taken to cover First half  $t_1$= $\frac{S}{2\times30}$ = $\frac{S}{60}$ hours.

Time taken to cover Second  half  $t_1$= $\frac{S}{2\times90}$ = $\frac{S}{180}$ hours.

Total time   t = $t_1$+ $t_2$ =$\frac{S}{60}$+$\frac{S}{180}$=$\frac{4S}{180}$ = $\frac{S}{45}$.

Average speed = Total distance /total time.

= S/t = $\frac{(S)(45)}{S}$

= 45km/hr.

Method II (Short cut):  If the body covers 1st half of distance with a speed x and the second half with a speed y,then the average speed = $\frac{2xy}{x+y}$.

Average speed  = $\frac{2(30)(90)}{30+90} =$latex \frac{2700}{60}\$ = 45km/hr.

8)  A body starts from rest and acqires a velocity of 400m/sec  in 10seconds.Calculate the acceleration and distance traveled.

Soln: From the data given in the problem

Initial velocity of the body u=0,

Time of journey   t=10sec,

Final velocity v=400m/sec,

Acceleration  a=?  and distance traveled in 10sec   s=?

Substitute the values of u,v and t in the equation   v=u+at,

we get     400= (0)(10) + a (10) ;  10a=400

a= 40$m/sec^2$

Substitute the  values of u,a and t in the equation S= ut +$\frac{1}{2} at^2$

S=(0)(10)+$\frac{1}{2} (40)(10)^2$ = 0+2000 = 2000m.

9) A body moving with uniform acceleration covers 6m in $2^{nd}$ second and 16m in $4^{th}$ second.Calculate the initial velocity,acceleration and distance moved in  $6^{ th }$ second .

Soln: From the data given in the problem

Distance moved in $2^{nd}$ second $s_2$ = 6m,

Distance moved in $4^{th}$ second $s_4$ = 16m,

$s_4$$s_2$ = a(4-2) =2a

therefore 2a = $s_4$$s_2$=16-6 =10

a = 5 m/$s^2$

Substitute the values of  $s_2$,a  and n=2 in the equation $s_2$ =u+a(n -1/2)

we get    6=u+5(2-1/2) ; 6=u+7.5

u=-7.5 +6 = -1.5 m/sec.

#### Distance moved in the $6^{th}$ second = u+a(6-1/2),

Substitute the values of   u,a  in the above equation we get $6^{th}$ second=-1.5+5(5.5)

$6^{th}$ second =-1.5+27.5 =26m.

10) An object started traveling with a velocity 2m/sec moves with an uniform acceleration of 3 m/$s^2$.

i) Find the ratio of displacements in   a) $1^{st}$,$3^{rd}$,$5^{th}$  seconds  b) $2^{nd}$,$4^{th}$,  and $6^{th}$  seconds .

ii)Find the ratio of velocities   a) $1^{st}$,$3^{rd}$,$5^{th}$  seconds  b) $2^{2d}$,$^{th}$,$6^{th}$  seconds .

Soln: From the data given in the problem

Initial velocity  u=2 m/sec,

Acceleration   a =3 m/$sec^2$,

i- a) The ratio of displacements in   $1^{st}$,$3^{rd}$,$5^{th}$  seconds

From the formula $s_1$:$s_3$:$s_5$ = (2u+a) : (2u+5a) : (2u+9a)

$s_1$:$s_3$:$s_5$ = (4+3) : (4+15) : (4+27)

$s_1$:$s_3$:$s_5$ = 7:19:31 .

i-b) The ratio of displacements in   $2^{nd}$,$4^{th}$,$6^{th}$  seconds

From the formula $s_2$:$s_4$:$s_6$ = (2u+3a) : (2u+7a) : (2u+11a)

$s_2$:$s_4$:$s_6$ = (4+9) : (4+21) : (4+33)

$s_2$:$s_4$:$s_6$ = 13 : 25 : 37 .

ii -a) From the formula $v_1$:$v_2$:$v_3$: .  .  .  .  .  .  .  . :$v_n$= (u+a) : (u+2a) : (u+3a) : .   .   .  .  .  .  .  .  . : (u+na).

The ratio of velocities   a) $1^{st}$,$3^{rd}$,$5^{th}$  seconds

$v_1$:$v_3$:$v_5$ = (u+a) : (u+3a) : (u+5a) = 5 :11 : 17 .

ii- b) The ratio of velocities   a) $2^{nd}$,$4^{th}$,$6^{th}$  seconds

$v_2$:$v_4$:$v_6$ = (u+2a) : (u+4a) : (u+6a) =8 :1 4 :20 .

11) A body travels 200cm in the first two seconds and 220cm in the next four seconds.What will be the velocity at the end of the seventh second from the start?

Soln: The displacement of the body in first 2 sec $S_1$ =200cm,

Let the initial velocity = u(say) ,

Acceleration = a(say), time $t_1$ =2sec

$S_1$ = $ut_1$+$\frac{1}{2} at_1^2$,

Substitute the value of  $t_1$  in above equation, we get

$S_1$ = 2u+$\frac{1}{2} a(2)^2$,

$S_1$ = 2u +2a ; 2(u+a) =200

Therefore          u+a = 100 – – – – – – – – – – – – –  – – – – –  – (1)

Given that the body travels 220cm in next 4sec.That is from the start it displaces  200+220 = 420 cm in  6sec.

Displacement $S_2$ =420 cm,

time $t_2$=6 sec,

substitute theses values in the equation $S_2$ = u$t_2$+$\frac{1}{2} at_1^2$,

$S_2$ = 6u+$\frac{1}{2} a(6)^2$=6u+18a,

6(u+3a) = 420 ; u+3a = 70 – – – – – – – – – – – – –  – – – – –  – (2)

Solving equations (1) and (2)     or Eq (2) – Eq(1)

we get  2a= -30 ;     a=-15 cm/$sec^2$,

Substitute value of  a in Eq(1) we get  u-15 = 100,

u = 115 cm/sec.

The velocity at the end of $6{th}$ second  v=u+a$t_2$

we get    v= 115+(-15)(6) =115-90 =25cm/sec.

Therefore  final velocity v=25cm/sec.

12.A subway train starts from rest at a station and accelerates at a rate of 16.5$m/sec^2$  for  13.1 sec.It runs at constant speed for 69.7s and slows down at a rate of 3.45 $m/sec^2$until it stops at the next station.What is the total distance covered?

Soln: From the data given in the problem,

Initial velocity of the train u = 0 m/sec,

Acceleration a = 16.5 $m/sec^2$,

time t = 13.1 sec,

Velocity after 13.1 sec v=?

and the distance traveled $S_1$ =?

Substitute  the values in the equation v =u+at

we get v= 0+(16.5)(13.1) = 216.15 m/sec.

Substitute in equation $S_1$ = ut + $\frac{1}{2} at^2$

we get $S_1$ = (0)(13.1)+$\frac{1}{2} (16.5)(13.1)^2$

$S_1$ = 0+1415.78 =1415.78 m

After that the train travels with velocity v=216.15 m/sec for  69.7sec. calculate distance traveled $S_2$ during this time.

v=216.15 m/sec, t = 69.7 sec $S_2$=?

substitute the values in equation $S_2$= vt = (216.15)(69.7)=15065.66 m

Finally the trains decelerates  at the rate of 3.45 $m/sec^2$ and comes to rest.find distance$S_3$ traveled before coming to rest.

Acceleration a = -3.45 $m/sec^2$ ,

Initial velocity u = 216.15m/sec,

Final velocity v=0,

distance traveled $S_3$ =?

substitute the values in $v^2 - u^2$ = 2as,

we get $0^2 - 216.15^2$ = 2(-3.45) $S_3$

-6.90 $S_3$ = -46720.82

$S_3$ = 6771.13.

Total distance traveled by train S = $S_1$ +$S_2$+ $S_3$ = 1415.78+15065.66+6771.13=23252.57 m  or 23.252 km.

Advertisements