NLM flash cards

1. 1.State Newton's first law of motion.

   A body continues in its state of rest or of uniform motion in a straight line unless acted upon by a net external force. It defines inertia and identifies inertial frames.

   Hint: Law of inertia.

2. 2.What physical quantity does the first law introduce, and what does the second law quantify?

   The first law introduces the concept of inertia (resistance to change in state of motion); the second law quantifies force as the rate of change of momentum.

   Hint: Qualitative vs quantitative force.

3. 3.Define inertia. On what does it depend?

   Inertia is the tendency of a body to resist any change in its state of rest or uniform motion. It depends only on mass — more mass means more inertia. Independent of shape, speed, or location.

   Hint: Measure of mass.

4. 4.Name the three types of inertia with an everyday example each.

   Inertia of rest (dust flies off when a carpet is beaten), inertia of motion (a passenger lurches forward when a bus brakes), inertia of direction (mud flies off tangentially from a spinning wheel).

   Hint: Rest, motion, direction.

5. 5.State Newton's second law in its most general form.

   $\vec{F}_{net}=\dfrac{d\vec{p}}{dt}$, the net external force equals the rate of change of linear momentum. For constant mass this reduces to $\vec{F}_{net}=m\vec{a}$.

   Hint: Rate of change of momentum.

6. 6.Why is $\vec{F}=\dfrac{d\vec{p}}{dt}$ more fundamental than $\vec{F}=m\vec{a}$?

   The momentum form also handles variable-mass systems (rockets, falling chains, conveyor belts) where $m$ changes with time; $\vec{F}=m\vec{a}$ is valid only when mass is constant.

   Hint: Rockets and chains.

7. 7.Define linear momentum and give its SI unit and dimensions.

   $\vec{p}=m\vec{v}$, a vector along the velocity. SI unit: $\text{kg m s}^{-1}$ (or $\text{N s}$). Dimensions: $[MLT^{-1}]$.

   Hint: Mass times velocity.

8. 8.State Newton's third law of motion.

   For every action there is an equal and opposite reaction: if body A exerts force $\vec{F}$ on B, then B exerts $-\vec{F}$ on A. The forces act on different bodies, simultaneously, and are of the same type.

   Hint: Action–reaction pair.

9. 9.Why do action–reaction pairs never cancel each other?

   They act on two different bodies. Only forces acting on the same body can be added to give a net force, so an action–reaction pair can never balance out.

   Hint: Different bodies.

10. 10.List the four key properties of an action–reaction pair.

    (1) Equal in magnitude, opposite in direction; (2) act on different bodies; (3) same type of force (both gravitational, both contact, etc.); (4) exist simultaneously — no time lag.

    Hint: Magnitude, bodies, type, timing.

11. 11.A book rests on a table. Is the book's weight and the table's normal force on the book an action–reaction pair?

    No. Both act on the same body (the book) and are different force types (gravity vs contact). The reaction to the book's weight is the book's gravitational pull on Earth; the reaction to $N$ is the book pushing down on the table.

    Hint: Same body → not a pair.

12. 12.What is a free-body diagram (FBD) and why is it drawn?

    A diagram of a single chosen body showing all external forces acting on it (as vectors from the body), isolated from surroundings. It organises force analysis before applying $\vec{F}=m\vec{a}$.

    Hint: Isolate one body, show its forces.

13. 13.Which forces do you include in an FBD, and which do you exclude?

    Include only external forces the body experiences: weight, normal, tension, friction, applied, spring. Exclude forces the body exerts on others, and never draw $m\vec{a}$ as a force.

    Hint: Forces ON the body, not BY it.

14. 14.Define the normal force. Along which direction does it act?

    The normal force $N$ is the contact (push) force a surface exerts perpendicular to itself on the body. It is a self-adjusting reaction and can never pull.

    Hint: Perpendicular, push only.

15. 15.Is the normal force on a block always equal to its weight?

    No. $N=mg$ only for a horizontal surface with no vertical acceleration or extra vertical force. On an incline $N=mg\cos\theta$; in a lift or with applied vertical forces $N$ differs.

    Hint: Depends on other vertical forces.

16. 16.What is tension in a string, and what is assumed for an ideal string?

    Tension is the pulling force transmitted along a string, acting away from the body along the string. An ideal string is massless and inextensible, so tension is uniform throughout and connected bodies share the same acceleration magnitude.

    Hint: Massless, inextensible → uniform T.

17. 17.For an ideal (massless, frictionless) pulley, what happens to the tension in a string passing over it?

    The tension is the same on both sides; the pulley only changes the direction of the tension, not its magnitude.

    Hint: Same T on both sides.

18. 18.If a pulley has mass (moment of inertia), is the tension the same on both sides?

    No. A net torque is needed to angularly accelerate the pulley, so tensions differ: $(T_1-T_2)R=I\alpha$. Equal tensions hold only for a massless pulley.

    Hint: Torque needs unequal T.

19. 19.State Hooke's law for a spring and define its spring constant.

    $F=-kx$, the restoring force is proportional to and opposite the deformation $x$. The spring constant $k$ (N/m) measures stiffness; it is a property of the spring.

    Hint: Restoring force $\propto$ stretch.

20. 20.For an ideal (massless) spring, is the tension the same at both ends?

    Yes. A massless spring has the same force $kx$ at both ends. Its force depends on extension/compression, not on acceleration, and cannot change instantaneously.

    Hint: Same force at both ends.

21. 21.How do the effective spring constants combine in series and in parallel?

    Series: $\dfrac{1}{k_{eff}}=\dfrac{1}{k_1}+\dfrac{1}{k_2}$ (softer). Parallel: $k_{eff}=k_1+k_2$ (stiffer).

    Hint: Series like resistors' reciprocal.

22. 22.Two springs of the same material and thickness: how does cutting a spring into two equal halves change $k$?

    $k\propto 1/L$, so each half has double the spring constant $2k$. Shorter springs are stiffer.

    Hint: $k\propto 1/L$.

23. 23.Why can a string's tension change instantly but a spring's force cannot?

    An ideal string is inextensible, so tension adjusts instantly to whatever the constraint demands. A spring's force is $kx$; changing it requires changing length $x$, which cannot happen in zero time. Hence a spring force is continuous.

    Hint: Spring needs time to change length.

24. 24.Distinguish contact forces from field (non-contact) forces with examples.

    Contact forces need physical touching: normal, friction, tension, spring, applied push. Field forces act at a distance: gravitational, electrostatic, magnetic.

    Hint: Touch vs at-a-distance.

25. 25.What is the origin of contact forces like normal and friction at the microscopic level?

    They arise from electromagnetic interactions between atoms/molecules of the surfaces in contact (repulsion of electron clouds). They are macroscopic manifestations of electrostatic forces.

    Hint: Electromagnetic in origin.

26. 26.Define impulse and state the impulse–momentum theorem.

    Impulse $\vec{J}=\int \vec{F}\,dt=\vec{F}_{avg}\,\Delta t$. The impulse–momentum theorem: $\vec{J}=\Delta\vec{p}=\vec{p}_f-\vec{p}_i$. Impulse equals the change in momentum.

    Hint: Area under F–t = $\Delta p$.

27. 27.On an $F$–$t$ graph, what does the area under the curve represent?

    The impulse delivered, $\int F\,dt$, which equals the change in momentum of the body.

    Hint: Area = impulse.

28. 28.Why do airbags, sand pits, and bending your knees on landing reduce injury?

    They increase the contact time $\Delta t$ over which momentum changes. Since $\Delta p$ is fixed, a larger $\Delta t$ means a smaller average force $F_{avg}=\Delta p/\Delta t$.

    Hint: Increase $\Delta t$ → decrease F.

29. 29.State the principle of conservation of linear momentum and its condition.

    If the net external force on a system is zero, its total linear momentum is constant: $\vec{p}_i=\vec{p}_f$. It follows directly from Newton's second and third laws; internal forces cannot change total momentum.

    Hint: No external force → $\vec p$ conserved.

30. 30.How does momentum conservation explain recoil of a gun?

    Initial momentum is zero. After firing, bullet momentum $m_bv_b$ forward is balanced by gun momentum $M_gv_g$ backward: $M_gv_g=m_bv_b$, so the gun recoils with $v_g=\dfrac{m_bv_b}{M_g}$.

    Hint: Total $\vec p$ stays zero.

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