Answer :
The combined velocity of the car and the chair is 1.5 m/s.
How fast are they moving?
By the use of the law of the conservation of linear momentum we know that the total momentum before collision is equal to the sum of the final momentum after collision.
We have to first obtain the momentum of the system before and after the collision has occurred as we see below;
Then we have;
Momentum before collision = 2 Kg * 60 m/s = 120 Kgm/s
Momentum after Collison = 80v Kgm/s
Momentum = mv
m = mass of the object
v = velocity of the object
Using the principle of the conservation of momentum;
120 = 90v
v = 120/80
v = 1.5 m/s
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