Bagrut Physics 5 Units common mistakes
Bagrut Physics 5 Units · Understand the traps and how to avoid them.
Using the scalar equation v = u + at or the energy relation without assigning a sign convention, then adding a downward acceleration to an upward initial velocity as if both were positive, producing a wrong time of flight or maximum height.
Candidates memorize the kinematic formulas as pure magnitudes and treat the choice of positive direction as an afterthought. Because the formulas are taught first with positive numbers, the sign of g is applied inconsistently, especially when the motion reverses direction at the top of a trajectory.
What to do instead
Before substituting, write one line at the top of the solution: 'positive direction = upward, so a = -g'. Then substitute every quantity with its signed value and keep the sign through the algebra. Check the result physically: a time of flight must be positive and the peak must occur where v = 0. Practise a projectile thrown upward from a cliff so the same convention must survive a negative displacement.
Drawing a free-body diagram that omits a force or invents one, typically leaving out the normal force on an incline, adding a 'centripetal force' arrow alongside tension, or drawing the reaction to the weight on the same body as the weight itself.
Newton's third law pairs are confused with balanced forces on one body, and centripetal force is treated as an extra agent rather than the net radial component of real forces. Textbooks often draw the net force separately, which encourages double counting.
What to do instead
List the agents touching the body one by one (Earth, surface, string, hand) and draw exactly one arrow per agent, labelled by agent. Never label an arrow 'centripetal force'; instead resolve the real forces along the radius and write sum of radial components = mv^2/r. For third-law pairs, draw the two bodies separately and verify each force appears once on each diagram with opposite direction.
Applying conservation of momentum to a collision without checking whether external impulse is negligible, or treating an inelastic collision as if kinetic energy were conserved and solving for a final speed from energy instead of momentum.
The two conservation laws look interchangeable because both are 'conservation' statements, and candidates default to energy because it is scalar and easier. The distinction between elastic and inelastic is memorized as vocabulary rather than as a statement about which quantity survives.
What to do instead
Before choosing a law, ask two questions: is the collision time short enough that external impulse is negligible (then momentum is conserved), and is kinetic energy recovered (only then is KE conserved)? Use momentum for all collisions and energy only for elastic ones or to find the energy lost. Compute the KE before and after an inelastic collision to see the deficit explicitly.
Mixing up the formulas for a resistor and a capacitor in a DC circuit, for example writing the voltage across a capacitor as V = IR during charging, or treating the capacitor as an open circuit at the instant a switch closes.
Ohm's law is over-learned and applied to every component. Candidates do not internalize that a capacitor's voltage is set by charge, V = Q/C, and that its current depends on the rate of change of that charge, so the initial and final states of an RC circuit are reversed in their intuition.
What to do instead
Write the component laws side by side: resistor V = IR (instantaneous, no memory), capacitor Q = CV and I = C dV/dt (voltage cannot jump, current can). For any switching problem, state the initial condition (capacitor voltage unchanged at t = 0+) and the final condition (capacitor acts as open circuit, current zero). Verify with the time constant tau = RC.
Adding resistor values in parallel, or adding potential differences across parallel branches, instead of recognizing that parallel branches share the same voltage and series elements share the same current.
The reciprocal formula for parallel resistance is memorized as a symbol pattern without the underlying conservation of charge and Kirchhoff loop reasoning, so it is easily inverted or applied to the wrong configuration, particularly in mixed networks drawn diagonally.
What to do instead
Redraw the network so parallel branches are visibly side by side and series elements are in a line, then label nodes: elements between the same two nodes are in parallel and carry the same voltage. Compute the equivalent resistance step by step, checking that the total is smaller than the smallest parallel resistor and larger than the largest series resistor.
Using the thin-lens or mirror equation with object and image distances inserted as unsigned magnitudes, then misclassifying a real image as virtual, or placing the image on the wrong side of the lens.
The sign convention (real-is-positive for object distance in the standard Cartesian convention, negative focal length for diverging elements) is taught as a rule to memorize, and candidates drop it under exam pressure because the magnitudes alone often give a plausible number.
What to do instead
Adopt one convention and write it at the top of every optics solution: distances measured in the direction of incoming light are positive, focal length negative for diverging lenses and convex mirrors. After solving, interpret the sign of the image distance and the magnification: negative magnification means inverted and real for a single lens. Sketch the three principal rays to confirm the algebra.
Describing double-slit interference by saying 'the waves add up' without linking the path difference to the wavelength, and using the single-slit or grating formula with the wrong order number or the wrong spacing.
The condition for constructive interference (path difference = m times wavelength) is memorized as a formula but not connected to the geometry of the two paths, so candidates cannot tell whether they need the double-slit, grating, or single-slit expression, and they confuse the fringe spacing with the position of a fringe.
What to do instead
Always draw the two paths from the slits to the point on the screen and write the path difference explicitly before substituting. State the condition in words: bright fringe when the path difference is a whole number of wavelengths, dark when it is a half-integer. For a grating, use d sin(theta) = m times wavelength with d the slit spacing, and check that the maximum order satisfies sin(theta) <= 1.
Calculating the photoelectric threshold or maximum kinetic energy with the wrong quantity, for example using the work function as the photon energy, or ignoring that no emission occurs when the photon energy is below the work function.
The Einstein equation is learned as a single line and the roles of the terms blur. Candidates also treat the stopping potential as a property of the light rather than of the metal, and they forget that intensity affects the current, not the maximum kinetic energy.
What to do instead
Write the equation with named terms: photon energy hf = work function W plus maximum kinetic energy, and stopping potential satisfies eV_stop = maximum kinetic energy. For each problem, first compare hf with W to decide whether emission happens at all. Separate the two independent knobs: frequency sets the maximum kinetic energy, intensity sets the number of electrons per second.
Reporting an experimental result with more significant figures than the data support, or adding percentage uncertainties arithmetically when quantities are multiplied or divided.
Calculators return many digits and candidates copy them, and the rules for combining uncertainties are memorized as 'add the percentages' without distinguishing sums from products, powers, and repeated measurements of the same quantity.
What to do instead
Keep the uncertainty to one significant figure and match the value to it. For sums and differences add absolute uncertainties; for products, quotients, and powers add fractional uncertainties (multiply the fractional uncertainty by the power). For repeated readings of one quantity, use the standard deviation of the mean. State the final result as value plus or minus uncertainty with units.
Treating the first law of thermodynamics with an inconsistent sign convention, so that work done by the gas and heat added are given the same sign, and then computing an efficiency or a temperature change that violates energy conservation.
Two sign conventions circulate (work done by the system versus work done on the system), and candidates switch between them mid-problem. The internal energy of an ideal gas is also confused with heat, although internal energy depends only on temperature for an ideal gas.
What to do instead
Fix the convention at the start: change in internal energy = heat added minus work done by the gas. Then use the ideal-gas internal energy dependence on temperature alone to evaluate changes, and reserve Q and W for path-dependent transfers. Sanity-check with an adiabatic process (Q = 0, so work done by the gas equals the drop in internal energy) and an isothermal one (internal energy change zero, so heat added equals work done).