The first collision because a greater amount of momentum must be taken and used in order to push the cart back, giving it a greater mass and impulse
The weight is 45 N, because the three chains hold the sign, and each contributes 15 N.
Notice that the mass would be the weight/acceleration of gravity, m = 45/9.8 kg. But they ask the weight (force, so Newtons)
The answer is c 1386j
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https://www.omnicalculator.com/physics/specific-heat