There's no reason why the center of gravity must be in a place where there is a any mass.
The center of gravity is simply a LOCATION ... the place where the amount of mass in any direction from it is the same amount.
For that matter, whenever you know the location of the center of gravity for ANY object, you can always go in there and scoop our a tiny spherical hole at that place. Then the center of gravity won't move, but it will be in an empty space, 'outside the body' of the object.
Here are a few more points to ponder:
-- The center of gravity of a basketball, beach ball, tennis ball, or any other inflated ball is the center of the ball, where there is no part of the skin.
-- The center of gravity of a party balloon is somewhere inside the balloon, where there is no rubber. If the balloon is spherical, then its center of gravity is the center of the sphere.
-- The center of gravity of a square is the center of the square, not on any of its sides.
-- The center of gravity of a triangle is the centroid of the triangle, not on any of its sides.
The football players collide in a completely inelastic collision, in other words they have the same velocity after the collision, this velocity has a magnitude V=1.6m/s.
We need to use the conservation of momentum Law, the total momentum is the same before and after the collision, at the initial point the receiver does not have any speed
Total time will be the sum of the partial times between stations plus the time stopped at the stations.
Due to the distance between stations is the same, and the time between stations must be the same (Because the train starts from rest in each station) we can find total time, finding the time for any of the distance between two stations, and then multiply it times the number of distances.
At any station, the train starts from rest, and then accelerates at 1.1m/s2 till it reaches to a speed of 95 km/h.
In order to simplify things, let's first to convert this speed from km/h to m/s, as follows:
Applying the definition of acceleration, we can find the time traveled by the train before reaching to this speed, as follows:
Next, we can find the distance traveled during this time, assuming that the acceleration is constant, using the following kinematic equation:
In the same way, we can find the time needed to reach to a complete stop at the next station, applying the definition of acceleration, as follows:
We can find the distance traveled while the train was decelerating as follows:
Finally, we need to know the time traveled at constant speed.
So, we need to find first the distance traveled at the constant speed of 26.4m/s.
This distance is just the total distance between stations (3.0 km) minus the distance used for acceleration (x₁) and the distance for deceleration (x₃), as follows:
x₂ = L - (x₁+x₃) = 3000 m - (316.8 m + 158.4 m) = 2525 m (6)
The time traveled at constant speed (t₂), can be found from the definition of average velocity, as follows:
Total time between two stations is simply the sum of the three times we have just found:
t = t₁ +t₂+t₃ = 24 s + 95.6 s + 12 s = 131.6 s (8)
Due to we have six stations (including those at the ends) the total time traveled while the train was moving, is just t times 5, as follows:
tm = t*5 = 131.6 * 5 = 658.2 s (9)
Since we know that the train was stopped at each intermediate station for 22s, and we have 4 intermediate stops, we need to add to total time 22s * 4 = 88 s, as follows:
Ttotal = tm + 88 s = 658.2 s + 88 s = 746 s (10)
b)
Using all the same premises that for a) we know that the only difference, in order to find the time between stations, will be due to the time traveled at constant speed, because the distance traveled at a constant speed will be different.
Since t₁ and t₃ will be the same, x₁ and x₃, will be the same too.
We can find the distance traveled at constant speed, rewriting (6) as follows:
x₂ = L - (x₁+x₃) = 5000 m - (316.8 m + 158.4 m) = 4525 m (11)
The time traveled at constant speed (t₂), can be found from the definition of average velocity, as follows:
Total time between two stations is simply the sum of the three times we have just found:
t = t₁ +t₂+t₃ = 24 s + 171.4 s + 12 s = 207.4 s (13)
Due to we have four stations (including those at the ends) the total time traveled while the train was moving, is just t times 3, as follows:
tm = t*3 = 207.4 * 3 = 622.2 s (14)
Since we know that the train was stopped at each intermediate station for 22s, and we have 2 intermediate stops, we need to add to total time 22s * 2 = 44 s, as follows:
