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Newton's first law of motion says something like "An object remains
in constant, uniform motion until acted on by an external force".
Constant uniform motion means no change in speed or direction.
If an object changes from rest to motion, that's definitely a change
of speed. So it doesn't remain in the state of constant uniform
motion (none) that it had when it was at rest, and that tells us
that an external force must have acted on it.
Answer:
V = 0.39 m/s
Explanation:
Given that,
Mass of hockey puck, m = 0.2 kg
Mass of goalie = 40 kg
Speed of hockey puck, v = 80 m/s
We need to find the speed with which the goalie slide on the slide. Let V be the speed. Using the conservation of momentum as follows :

So, the required speed is 0.39 m/s.
They first change into chemical, then to radiant, then finally to thermal energy.
Answer:
The required mass of ice is 12.5 kg.
Explanation:
Mass of hot tea,
= 1.8 kg
Initial temperature of hot tea = 80°C = 353 K
Initial temperature of ice = 0.00°C = 273 K
Final temperature of mixture = 10°C = 283 K
Heat of fusion of ice, L = 334 kJ/kg
Specific heat capacity of tea,
= Specific heat capacity of water = 4190 J/kg/K
Heat lost tea = Heat gained by ice

Δ
=
L + 
Δ

Δ
=
(L +
Δ
)
1.8 x 4190 x (353 - 283) =
(334 + (4190 x (283 - 2730))
1.8 x 4190 x 70 =
(334 + (4190 x 10))
527940 = 42234
= 
= 12.5004
= 12.5 kg
The required mass of ice is 12.5 kg.