BCECE Physics: Signs, Forces and Fast Formula Use
What BCECE Physics tests
The Physics section tests the 10+2 core syllabus: kinematics and laws of motion, work–energy, rotational motion, gravitation, properties of matter, thermodynamics, oscillations and waves, electrostatics, current electricity, magnetism, EMI and AC, optics, dual nature, and atoms and nuclei. Questions reward fast, clean application of formulae rather than long derivations.
Core rules to keep ready
- Resolve vectors into components before applying F = ma; choose axes along the probable acceleration.
- In circular motion, centripetal force is the net inward radial force, not an extra force.
- Use the first law of thermodynamics as ΔQ = ΔU + ΔW, with W done by the gas.
- For lenses and mirrors, use the Cartesian sign convention consistently: distances measured against the incident light are negative.
- In EMI, use Lenz's law to fix the direction; magnitude comes from e = −dΦ/dt.
- Static friction is not always μN; it takes the value needed to prevent slipping, up to a maximum.
Worked example
A convex lens of focal length 20 cm forms an image of an object placed 30 cm away. Find the image distance and magnification. Using 1/f = 1/v − 1/u with u = −30 cm and f = +20 cm: 1/v = 1/20 + 1/(−30) = (3 − 2)/60 = 1/60, so v = +60 cm. m = v/u = 60/(−30) = −2. The image is real, inverted and twice the object size.
Traps that cost marks
- Mixing sign conventions halfway through a lens or mirror calculation.
- Forgetting to convert cm to m, or grams to kg, before substituting.
- Treating centripetal force as an additional force in a free-body diagram.
- Reporting only the magnitude of an induced e.m.f. when the direction is also asked.
- Using the wrong moment of inertia for a standard body such as a disc or hollow sphere.