NLM flash cards

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

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

   Hint: Law of inertia.

2. 2.What is inertia?

   The inherent tendency of a body to resist any change in its state of rest or of uniform motion. It is measured by the body's mass.

   Hint: Resistance to change in motion.

3. 3.Name the three types of inertia.

   Inertia of rest, inertia of motion, and inertia of direction.

   Hint: Rest, motion, direction.

4. 4.On what physical quantity does the inertia of a body depend?

   Only on its mass. Greater mass means greater inertia. It does not depend on shape, velocity, or the surroundings.

   Hint: More mass, more inertia.

5. 5.State Newton's second law of motion.

   The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction of the force: $\vec{F}=\dfrac{d\vec{p}}{dt}$.

   Hint: $F=dp/dt$.

6. 6.Derive $\vec{F}=m\vec{a}$ from Newton's second law (for constant mass).

   $\vec{F}=\dfrac{d\vec{p}}{dt}=\dfrac{d(m\vec{v})}{dt}=m\dfrac{d\vec{v}}{dt}=m\vec{a}$, valid when $m$ is constant.

   Hint: $p=mv$, differentiate.

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

   Linear momentum $\vec{p}=m\vec{v}$, a vector in the direction of velocity. SI unit: $\text{kg m s}^{-1}$ (equivalently $\text{N s}$).

   Hint: $p=mv$.

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

   To every action there is an equal and opposite reaction. Forces always occur in pairs: $\vec{F}_{12}=-\vec{F}_{21}$.

   Hint: Action–reaction pair.

9. 9.Why do action and reaction not cancel each other?

   Because they act on two different bodies. Cancellation requires forces on the same body.

   Hint: Different bodies.

10. 10.List three key properties of an action–reaction pair.

    They are equal in magnitude, opposite in direction, act on different bodies, and are of the same type (both gravitational, both contact, etc.). They act simultaneously.

    Hint: Equal, opposite, different bodies.

11. 11.Define impulse of a force.

    Impulse $\vec{J}=\vec{F}\,\Delta t$ (for constant force) $=\displaystyle\int \vec{F}\,dt$. It equals the change in momentum: $\vec{J}=\Delta\vec{p}$.

    Hint: $J=F\Delta t=\Delta p$.

12. 12.State the impulse–momentum theorem.

    The impulse of the net force on a body equals its change in linear momentum: $\vec{F}\,\Delta t=\Delta\vec{p}=m\vec{v}-m\vec{u}$.

    Hint: Impulse = change in momentum.

13. 13.What is SI unit of impulse?

    $\text{N s}$, which is dimensionally the same as $\text{kg m s}^{-1}$ (unit of momentum).

    Hint: Same as momentum unit.

14. 14.Why does a cricketer draw his hands back while catching a ball?

    To increase the time $\Delta t$ of catching. Since $F=\Delta p/\Delta t$, a larger time reduces the force on the hands for the same momentum change.

    Hint: Increase time, reduce force.

15. 15.State the law of conservation of linear momentum.

    If the net external force on a system is zero, its total linear momentum remains constant: $\vec{p}_{\text{total}}=\text{constant}$.

    Hint: No external force → p constant.

16. 16.How does conservation of momentum follow from Newton's laws?

    If $\vec{F}_{\text{ext}}=\dfrac{d\vec{p}}{dt}=0$, then $\vec{p}=\text{constant}$. Internal forces cancel in pairs by the third law.

    Hint: $F_{ext}=0 \Rightarrow dp/dt=0$.

17. 17.Explain recoil of a gun using momentum conservation.

    Initial momentum is zero. After firing, bullet momentum $m\vec{v}$ and gun momentum $M\vec{V}$ sum to zero: $M\vec{V}=-m\vec{v}$. Recoil velocity $V=-\dfrac{m}{M}v$ (opposite to bullet).

    Hint: $MV=-mv$.

18. 18.What is the principle behind rocket propulsion?

    Conservation of momentum: hot gases ejected backward at high speed give the rocket forward momentum. It is a variable-mass system.

    Hint: Eject gas back, rocket goes forward.

19. 19.Define a force.

    A force is an external agency that changes or tends to change the state of rest or motion of a body, or its shape. It is a vector; SI unit newton (N).

    Hint: Push or pull.

20. 20.Define one newton of force.

    One newton is the force that gives a mass of $1\text{ kg}$ an acceleration of $1\text{ m s}^{-2}$: $1\text{ N}=1\text{ kg m s}^{-2}$.

    Hint: $1\,kg \times 1\,m/s^2$.

21. 21.Distinguish contact and non-contact (field) forces with examples.

    Contact forces need physical touch (friction, normal, tension). Non-contact/field forces act at a distance (gravitational, electrostatic, magnetic).

    Hint: Touch vs at a distance.

22. 22.What is a free-body diagram (FBD)?

    A diagram showing a single body isolated from its surroundings with all external forces acting on it drawn as vectors. It is the key tool for applying $\vec{F}=m\vec{a}$.

    Hint: Isolate body, draw all forces.

23. 23.What is the normal reaction force?

    The contact force exerted by a surface on a body, directed perpendicular to the surface, away from it. It adjusts itself to prevent interpenetration.

    Hint: Perpendicular to surface.

24. 24.Is the normal force always equal to $mg$?

    No. $N=mg$ only for a body on a horizontal surface with no vertical acceleration and no other vertical forces. On inclines, in lifts, or with applied vertical forces it differs.

    Hint: Only in special horizontal case.

25. 25.What is tension in a string?

    The pulling force transmitted along a string, rope or cable. For an ideal (massless, inextensible) string it is the same throughout and always pulls along the string, away from the body.

    Hint: Pull along the string.

26. 26.For an ideal massless string over a frictionless pulley, what is true about tension?

    The tension is the same on both sides and throughout the string. A frictionless massless pulley only changes the direction of tension.

    Hint: Same tension both sides.

27. 27.Define equilibrium of a particle.

    A particle is in equilibrium when the net (vector) external force on it is zero: $\sum\vec{F}=0$, i.e. $\sum F_x=0$ and $\sum F_y=0$.

    Hint: Net force zero.

28. 28.Distinguish static and dynamic equilibrium.

    Static equilibrium: body at rest with $\sum\vec{F}=0$. Dynamic equilibrium: body moving with constant velocity ($a=0$) and $\sum\vec{F}=0$.

    Hint: Rest vs constant velocity.

29. 29.State the condition for three concurrent forces in equilibrium (triangle law / Lami's theorem).

    Lami's theorem: if three concurrent forces keep a body in equilibrium, $\dfrac{P}{\sin\alpha}=\dfrac{Q}{\sin\beta}=\dfrac{R}{\sin\gamma}$, where each angle is opposite its force.

    Hint: Each force / sine of opposite angle.

30. 30.What is apparent weight?

    The reading of a weighing scale, equal to the normal reaction $N$ on the body. It equals true weight only when vertical acceleration is zero.

    Hint: Equals normal force N.

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