Physics - Mechanics (motion, forces, energy, momentum)
The Gifu Entrance Science mechanics section tests whether you can translate a physical situation into equations and then justify the result. Expect kinematics, Newton’s laws, work–energy, and conservation of momentum, often combined in one multi-step problem.
Core rules
- Constant acceleration: v = u + at, s = ut + 1/2 at², v² = u² + 2as. Choose a positive direction and keep signs consistent.
- Newton’s second law is a vector equation: ΣF = ma. On an incline, the normal force is mg cos θ, not mg.
- Work–energy: W_net = ΔK. Friction does negative work; gravity does work mgh only when height changes.
- Momentum is conserved in a closed system: m1u1 + m2u2 = m1v1 + m2v2. Kinetic energy is conserved only in elastic collisions.
Worked example
A 2.0 kg block starts from rest and slides 5.0 m down a frictionless 30° incline. Find its speed at the bottom using g = 9.8 m s⁻². The vertical drop is h = 5.0 sin 30° = 2.5 m. Energy conservation gives mgh = 1/2 mv², so v = √(2gh) = √(2 × 9.8 × 2.5) = 7.0 m s⁻¹. The mass cancels-do not substitute 2.0 kg into the final step and then forget to cancel it.
Traps
- Mixing g = 9.8 and g = 10 within one calculation.
- Treating momentum as a scalar in two-dimensional collisions.
- Assuming the normal force equals weight on a slope.
- Using v² = u² + 2as with s as vertical height when the acceleration is along the incline.