PRELIMINARY STAGE 2024
Contest 10
Adonten SHS: 54 points (Winner)
Yaa Asantewaa Girls’ SHS: 36 points
Tolon SHS: 9 points
Contest 11
Amaniampong SHS: 55 points (Winner)
New Juaben SHS: 39 points
St. Mary’s SHS: 22 points
Contest 12
Suhum Presby SHS: 59 points (Winner)
St. Charles Seminary SHS: 28 points
Fiaseman SHS: 21 points
ROUND 5 - RIDDLE
RIDDLE
I am a dynamical physical quantity.
There exists specific physical situations in which I am conserved.
I am a vector quantity.
Changes in my magnitude signal work being done for a given particle.
I am the physical quantity whose rate of change with respect to time equals the resultant force on the particle.
Who am I?
ANSWER: LINEAR MOMENTUM
EXPLANATION:
Linear momentum $\mathbf{p} = m\mathbf{v}$ is a vector, conserved when no external force acts.
Its rate of change equals the resultant force: $\mathbf{F} = \dfrac{d\mathbf{p}}{dt}$.
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PRACTICE QUESTIONS
1. RIDDLE:
I am a vector quantity.
I equal a force multiplied by the time for which it acts.
My SI unit is the newton second.
I equal the change in an object's linear momentum.
Air bags reduce the force by making me act over a longer time.
On a force–time graph I am the area under the curve.
Who am I?
ANSWER: IMPULSE
EXPLANATION:
$J = F\Delta t = \Delta p$.
2. RIDDLE:
I am a property of all matter.
I resist any change in a body's state of rest or motion.
Mass is my measure.
Newton's first law is about me.
Passengers jerk forward when a bus stops suddenly because of me.
A heavier trolley has more of me.
Who am I?
ANSWER: INERTIA
EXPLANATION:
Inertia is the tendency of a body to keep its state of rest or uniform motion.
3. RIDDLE:
I am a scalar quantity.
I belong to a body because it is moving.
I equal half the mass times the square of the speed.
I can be written as $\dfrac{p^2}{2m}$.
My SI unit is the joule.
I am conserved in an elastic collision.
Who am I?
ANSWER: KINETIC ENERGY
EXPLANATION:
$K = \dfrac{1}{2}mv^2 = \dfrac{p^2}{2m}$.
4. RIDDLE:
I am a ratio with no unit.
I compare the speed of separation with the speed of approach.
I equal 1 for a perfectly elastic collision.
I equal 0 when colliding bodies stick together.
For a ball dropped on a floor, I equal the square root of the rebound height over the drop height.
My symbol is e.
Who am I?
ANSWER: COEFFICIENT OF RESTITUTION
EXPLANATION:
$e = \dfrac{\text{speed of separation}}{\text{speed of approach}}$.
5. RIDDLE:
I am a point.
The whole weight of a body can be taken to act through me in a uniform gravitational field.
In an explosion, I keep moving as before.
If no external force acts, my velocity stays constant.
I may lie outside the body, as for a ring.
For a uniform rod I am at the midpoint.
Who am I?
ANSWER: CENTRE OF MASS
EXPLANATION:
With no external force, the centre of mass of a system moves with constant velocity.
6. RIDDLE:
I am a backward motion.
A gun shows me when it fires a bullet.
I happen because momentum is conserved.
The heavier the gun, the slower I am.
A cannon on wheels rolls back because of me.
Rockets depend on the same principle as me.
Who am I?
ANSWER: RECOIL
EXPLANATION:
The gun gains backward momentum equal in size to the bullet's forward momentum.
7. RIDDLE:
I am a vector quantity.
I equal moment of inertia times the rate of rotation.
I am conserved when no external torque acts.
A skater spins faster by pulling in their arms because I am conserved.
My rate of change equals torque.
My SI unit is kg m² s⁻¹.
Who am I?
ANSWER: ANGULAR MOMENTUM
EXPLANATION:
$L = I\omega$ and $\tau = \dfrac{dL}{dt}$.
8. RIDDLE:
I am an SI unit.
I am the unit of force.
I equal one kilogram metre per second squared.
One of me acting for one second gives an impulse of 1 kg m/s.
An apple weighs about one of me.
I am named after the author of the Principia.
Who am I?
ANSWER: NEWTON
EXPLANATION:
1 N = 1 kg m s⁻².
9. RIDDLE:
I am a way of moving forward.
I push hot gases backwards at high speed.
The vehicle gains forward momentum equal to the momentum given to the gases.
I work even in the vacuum of space.
I need nothing outside the vehicle to push against.
I lift satellites into orbit.
Who am I?
ANSWER: ROCKET PROPULSION
EXPLANATION:
A rocket moves forward because the exhaust gases carry momentum backwards.