Question

A man weighing 80 kg is standing in a trolley weighing 320 kg. The trolley is resting on frictionless horizontal rails. If the man starts walking on the trolley with a speed of 1 m / s, then after 4 sec his displacement relative to the ground will be________________.

Answer 1

Final Answer: 16/5m

Steps:
1) Let the initial x coordinate of COM be at origin.
Since, there is no external Force along x direction, so there will not be change in x co ordinate of COM.

2) With respect to trolley, separation between man and trolley is 4 * 1 = 4m.
Let the displacement of man be 'x'm then displacement of trolley becomes (4-x) m.

3) Now,
We can say by step 1 ,
80 * x = 320(4-x)
=> x= 16-4x
=> x = 16/5 .

So, displacement of man after 4s is 16/5 m.

Answer 2

Answer:3.2m.

Explanation:

Since, no external force is acting we can apply law of conservation of momentum.

Here, initial momentum mv,

80*1

Final momentum -(M+m) V,

-(400) *V

Therefore, V= -80/400= -0.2m/s.

Distance covered by the man with respect to trolley is 4*1=4m,

Distance covered by the trolley with respect to ground is -0.2*4= -0.8.

So the distance travelled by man with respect to ground is 4-0.8= 3.2m.

Thanku...