The presence of mass makes gravity. Doesn't matter whether it's a planet, a black hole, a puppy, or a speck of dust.
21) Acceleration from D to E: 
22) The acceleration of the bus from D to E is 
Explanation:
21)
The acceleration of an object is equal to the rate of change of velocity of the object. Mathematically:

where
u is the initial velocity
v is the final velocity
t is the time elapsed
In this problem, we want to measure the acceleration of the bus from point D to point E. We have:
- Initial velocity at point D: u = 0
- Final velocity at point E: v = 5 m/s
- Time elapsed from D to E: t = 21 - 16 = 5 s
Therefore, the acceleration between D and E is

22) This question is the same as 21), so the result is the same.
Learn more about acceleration:
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Distance = (30+40+50) = 120 km
It's back where it started, so displacement = zero
No, since their gravity is powerful enough to keep them together even while the universe expands as a whole. Space is not expanding within clusters of galaxies.
<h3>What is a galaxy?</h3>
A galaxy is a massive clump of gas, dust, and billions of stars and their solar systems bound together by gravity.
No, since their gravity is powerful enough to keep them together even while the universe expands as an entire.
Hence,space is not expanding within clusters of galaxies.
To learn more about the galaxy, refer to the link;
brainly.com/question/2905713
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