የ12ኛ ክፍል ፊዚክስ መግቢያ ፈተና፣ 2018 ዓ.ም. (2026)፣ እትም 1

Uniform motion means the velocity never changes. If the velocity never changes then the acceleration is zero at every moment, so describing the motion as having an acceleration that stays uniform does not fit. Options A and B are two ways of saying the speed and the direction stay the same, and D is the straight line a constant speed gives on a distance time graph. Option D is the one that tempts, because a sloping line looks like something is speeding up, but on a distance time graph a constant slope is what constant speed looks like.

Displacement is the straight line from start to finish. Both students begin at the school and end at the same home, so their displacements are the same. Distance is the length of the path actually walked, and the road curving around the hill is longer than the straight short cut, so student B covers more distance. Option A says the right thing with the names swapped: A is the one who took the short cut, so A cannot be the one with the greater distance.

Instantaneous velocity is the velocity at one moment. Average velocity is the total displacement divided by the total time for a whole trip. Statement D hands the description of instantaneous velocity to average velocity, so D is the wrong one. Option A can look suspicious, but it is true: the average needs the whole displacement, while the instantaneous value needs only what is happening at that instant.

Newton's law of gravitation gives $F = \frac{Gm_1m_2}{r^2}$, so the force depends on $1/r^2$. When the separation drops to a quarter, $r$ becomes $r/4$, so $r^2$ becomes $r^2/16$ and the force becomes 16 times larger: $F' = 16 \times 250 = 4000\ \text{N}$. The value of $G$ is not needed, because the masses do not change and only the ratio matters. Option B, 8000 N, would need the force to grow 32 times, and no reading of the change in distance gives that factor.

Young's modulus is stress divided by strain. Strain is one length divided by another, so it carries no unit, which leaves Young's modulus with the unit of stress, $\text{N/m}^2$. The other three are all ratios of two like quantities: specific gravity compares two densities, strain compares two lengths, and the index of refraction compares two speeds. Option C, strain, is the tempting pick, but it is the very quantity whose lack of a unit is what gives Young's modulus its unit.

In uniformly accelerated motion the acceleration holds the same value at every moment, so the value at any instant equals the value averaged over any interval. That makes D true. A fails at the top of a thrown ball's flight, where the velocity is zero while gravity still pulls. B fails for a car at steady speed, where the acceleration is zero but the velocity is not. Option C is the tempting one: deceleration means the speed is falling, and for a body moving in the negative direction a positive acceleration is what slows it, so a deceleration is not always a negative acceleration.