What are the standard traps in kinematics and Newton's laws that keep costing me marks, and how do I avoid them?

4 answers & replies

• [+][–]CPCommons Physics DeskTeam· 14d ago

  Here is the list we would put on a card, with fixes.

  Kinematics

  • Equations of motion hold only for constant acceleration. If acceleration changes, use graphs or calculus ideas.
  • Choose a positive direction and keep it. A ball thrown up with $u = +20$ m/s has $a = -g$, not $+g$.
  • At the top of the flight, $v = 0$ but $a = g$. Zero velocity does not mean zero acceleration.
  • Area under a $v$-$t$ graph is displacement. Area below the axis counts negative. Distance needs absolute values.
  • Average speed is not average velocity, and $\dfrac{u+v}{2}$ is average velocity only for constant acceleration.
  • Distance in the $n$-th second: $s_n = u + \dfrac{a}{2}(2n-1)$.

  Newton's laws

  • Lift accelerating upward: apparent weight $N = m(g+a)$. Downward: $N = m(g-a)$. Free fall: $N = 0$.
  • Action and reaction act on different bodies, so they never cancel inside one free-body diagram.
  • Static friction is not always $\mu N$. It equals the applied force until the limit $\mu_s N$.

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• [+][–]CMCommons Mock & PYQ DeskTeam· 13d ago

  Turn this list into a personal trap log after each mock. A row looks like this:

  Question	What I did	What the question wanted	Trap name
  Block in a lift	Used $N = mg$	$N = m(g+a)$, lift going up	Lift
  $v$-$t$ graph area	Added all areas	Subtract area below axis for displacement	Sign of area
  Friction on incline	Used $\mu N$	Block at rest, so friction $= mg\sin\theta$	Static friction

  After two or three mocks, you will probably notice that three or four trap names keep reappearing. Those are the ones worth a focused session.

  Also read the verbs in each question before solving: "displacement" or "distance", "at rest" or "about to slide", "constant" or "uniformly changing". A lot of marks go away in the first four seconds of reading.

  Do not rush the free-body diagram in the exam. A quick sketch with arrows takes a few seconds and prevents the most common wrong equations.

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• [+][–]CSCommons Strategy DeskTeam· 13d ago

  Build a free-body-diagram habit before building speed. A routine that works for many students:

  1. Draw each body separately.
  2. Mark every force with an arrow: weight, normal, tension, friction, applied force.
  3. Choose axes (along the incline, if there is one).
  4. Write $\Sigma F = ma$ for each axis for each body.
  5. If bodies are connected, write the constraint (same acceleration, or related by a pulley).

  For practice, do a session where you only do steps 1 to 4 for ten problems and do not solve. This trains the setup, which is where the marks are lost.

  Then do a second session where you solve a smaller set fully. Useful results to recognise rather than re-derive:

  • Atwood machine: $a = \dfrac{(m_1 - m_2)g}{m_1 + m_2}$
  • Incline with friction, sliding down: $a = g(\sin\theta - \mu\cos\theta)$
  • Projectile: range is $\dfrac{u^2\sin 2\theta}{g}$, with $\theta$ and $90^\circ - \theta$ giving the same range.

  If you have a mechanics routine that works, add it below.

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• [+][–]CDCommons Dropper DeskTeam· 13d ago

  In a repeat year, mechanics is where discipline pays most visibly. If you fall for the same trap in the same type of question twice, treat it as a missing habit, not bad luck.

  A simple rule: when you make a mechanics mistake, write the trap on a sticky note under the heading "Before I solve". Over time you build a short checklist, such as "sign convention", "constant acceleration?", "what is accelerating?" and "free-body diagram done?". Read it before every practice set until it becomes automatic.

  Try this honest test: pick five old mechanics questions you got wrong earlier in the year, and redo them cold. If you still miss the same ones, the issue is not the chapter. It is that the habit has not been installed yet.

  There is no shame in going slow on the base chapters. Almost everything in the rest of Physics sits on them.

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