Which one of the following holds true for freely falling object ?
- A.It moves with a uniform velocity.
- B.It moves with a uniform speed.
- C.It moves with a non-uniform acceleration.
- D.It moves with a uniform acceleration.
Correct answer
Explanation
The correct answer is D. A freely falling object moves with a uniform acceleration, the acceleration due to gravity, g, about 9.8 m/s² near the Earth's surface.
Free fall means the only force acting on the body is its weight; air resistance is ignored. Since the force stays the same, the acceleration stays the same, and the velocity increases by about 9.8 m/s every second. Galileo showed that, without air, all bodies fall with the same acceleration whatever their mass, which is why a feather and a hammer dropped on the Moon by the Apollo 15 astronaut David Scott landed together. The equations of uniformly accelerated motion, such as v = u + gt and h = ut + ½gt², apply to free fall.
A is wrong: the velocity keeps increasing, so it is not uniform.
B is wrong: the speed also increases every second.
C is wrong: near the Earth the acceleration g stays constant, not changing.
Exam tip: Free fall — constant acceleration g = 9.8 m/s², independent of mass.
Practice Questions
View allConsider the following animals :
1. Hedgehog
2. Marmot
3. Pangolin
To reduce the chance of being captured by predators, which of the above organisms rolls up/roll up and protects/protect its/their vulnerable parts?
- A.1 and 2
- B.2 only
- C.3 only
- D.1 and 3
Show answer
Hedgehogs and pangolins curl into a tight ball when threatened, covering the soft belly and face with spines or scales; marmots do not.
Read the full explanation →Which one of the following is used in preparing a natural mosquito repellent?
- A.Congress grass
- B.Elephant grass
- C.Lemongrass
- D.Nut grass
Show answer
Lemongrass and its close relative citronella grass yield aromatic oils that keep mosquitoes away, so they are the base of many natural repellents.
Read the full explanation →Which one of the following devices is non-ohmic ?
- A.Conducting copper coil
- B.Electric heating coil
- C.Semi conductor diode
- D.Rheostat
Show answer
A diode does not obey Ohm's law: the current through it is not proportional to the voltage across it. An ohmic conductor gives a straight-line graph of current against voltage, so its resistance stays constant at a fixed temperature.
Read the full explanation →What is the formula mass of anhydrous sodium carbonate? (Given that the atomic masses of sodium, carbon and oxygen are 23 u, 12 u and 16 u respectively)
- A.286 u
- B.106 u
- C.83 u
- D.53 u
Show answer
Anhydrous sodium carbonate is Na₂CO₃, and 2 × 23 + 12 + 3 × 16 = 106 u. Formula mass is the sum of the atomic masses of all the atoms in one formula unit of an ionic compound.
Read the full explanation →Which one of the following minerals is used as a fuel in nuclear power stations ?
- A.Bauxite
- B.Quartz
- C.Feldspar
- D.Pitchblende
Show answer
Pitchblende, or uraninite, is the chief ore of uranium, the fuel used in most nuclear power stations. Pitchblende is a heavy, black mineral made mostly of uranium oxide.
Read the full explanation →