የ12ኛ ክፍል ፊዚክስ መግቢያ ፈተና፣ 2017 ዓ.ም. (2025)፣ እትም 2

Relative velocity is the difference of two velocities. Take east as positive, so train Y has $v_Y = +60$ km/h and train X has $v_X = -80$ km/h because it travels west. Then $v_{Y/X} = v_Y - v_X = 60 - (-80) = +140$ km/h, and the plus sign means the direction is east. Option D carries the right number but the wrong direction. It comes from adding the two speeds and then keeping the heading of your own train instead of the heading the subtraction gives.

A Newton's third law pair comes from one single interaction and its two forces act on two different bodies. The tyres push backwards on the road and the road pushes the car forwards: same contact, equal size, opposite directions, different bodies. Option D is the tempting one, because pushing water backwards is a genuine action force, but its partner is the water pushing the swimmer forwards. The pull of gravity on the swimmer belongs to a separate interaction with the Earth.

Momentum is conserved through the collision. Before impact $p = 0.25 \times 4 = 1$ kg m/s, and the two balls move off together as one 1 kg body, so $v = 1/1 = 1$ m/s. Kinetic energy before is $\tfrac{1}{2}(0.25)(4)^2 = 2$ J and after is $\tfrac{1}{2}(1)(1)^2 = 0.5$ J, so $2 - 0.5 = 1.5$ J is lost to heat, sound and deformation. Option B (2 J) is the energy the moving ball started with, not the amount lost, and some of that energy survives as the motion of the pair.

For a frictionless inclined plane the mechanical advantage is the length of the slope divided by the height it lifts through: $MA = \frac{L}{h} = \frac{12}{3} = 4$. The 200 N weight is extra information that the formula does not need. Option B (3.00) is tempting because 3 m is printed in the question, but mechanical advantage is a ratio of two forces or two lengths and carries no unit, so a bare height cannot be the answer.

Levers are classed by what sits in the middle. In a third class lever the effort is applied between the fulcrum and the load, as in the human forearm, a pair of tweezers or a fishing rod. Because the effort arm is then shorter than the load arm, this class always has a mechanical advantage below one and trades force for speed and range of movement. Option B describes the second class lever, where the load is in the middle, as in a wheelbarrow.

Air pressure falls as altitude rises, so each breath of air holds fewer oxygen molecules and the partial pressure of oxygen in the lungs drops. Less oxygen reaches the blood and the working muscles, which brings on breathlessness, headache and quick fatigue. This is why climbers spend days acclimatising before attempting a high summit. Option D reverses the physics: atmospheric pressure decreases with altitude, it does not increase.