An object is launched horizontally from a height of 45 m above the ground with a speed of 10 m/s. The time at which the object will hit the ground is : (Take g = 10 m/s²)
- A.6 s
- B.4 s
- C.3 s
- D.2 s
Correct answer
Explanation
The correct answer is C, 3 s. For an object launched horizontally, the time of fall depends only on the height, because its starting vertical speed is zero.
The horizontal speed of 10 m/s does not change how fast the object falls; it only decides how far it travels sideways. Vertically, the object starts from rest and falls freely, so h = ½gt². Putting h = 45 m and g = 10 m/s² gives 45 = 5t², so t² = 9 and t = 3 s. In those 3 seconds it also moves 10 × 3 = 30 m horizontally, which is its range.
Option A, 6 s, is wrong: it would need a fall of 180 m. Option B, 4 s, is wrong: that fall is 80 m. Option D, 2 s, is wrong: that fall is only 20 m.
Exam tip: horizontal launch, time of flight t = √(2h/g), independent of the horizontal speed; range = speed × t.
Practice Questions
View allWhich one of the following plant tissues has dead cells ?
- A.Epidermis
- B.Parenchyma
- C.Collenchyma
- D.Sclerenchyma
Show answer
Sclerenchyma cells are dead at maturity, with thick walls hardened by lignin. Sclerenchyma is the tissue that makes plants hard and stiff.
Read the full explanation →Imagine a current-carrying straight conductor with magnetic field of lines in anti-clockwise direction. Then the direction of current is determined by
- A.the Right-Hand Thumb rule and it would be in the downward direction.
- B.the Left-Hand Thumb rule and it would be in the downward direction.
- C.the Right-Hand Thumb rule and it would be in the upward direction.
- D.the Left-Hand Thumb rule and it would be in the upward direction.
Show answer
For a straight wire, the right-hand thumb rule links the current to the circular field lines around it. Hold the wire in the right hand with the thumb along the current; the curled fingers then show the direction of the magnetic field lines, which form concentric circles around the wire.
Read the full explanation →Which one of the following statements is true for sound waves propagating in air?
- A.Sound is an electromagnetic wave and transverse in nature
- B.Sound is a mechanical wave and longitudinal in nature
- C.Sound is a mechanical wave and transverse in nature
- D.Sound is an electromagnetic wave and longitudinal in nature
Show answer
Sound needs a material medium to travel, and in air its particles vibrate back and forth along the direction the wave moves.
Read the full explanation →Which one of the following is primarily responsible for conduction of current in a metal?
- A.Bound electrons
- B.Free electrons
- C.Both bound and free electrons
- D.Ions
Show answer
In a metal, the outer electrons of the atoms are not tied to any one atom and drift through the lattice when a voltage is applied, carrying the current.
Read the full explanation →Which one of the following statements is correct about the magnification of an optical microscope?
- A.Magnification increases with the increase in focal length of eyepiece
- B.Magnification increases with the increase in focal length of objective
- C.Magnification does not depend upon the focal length of eyepiece
- D.Magnification decreases with the increase in focal length of eyepiece
Show answer
A compound microscope magnifies more when both its lenses have short focal lengths. In a compound microscope the objective forms a real, enlarged image, and the eyepiece magnifies it further like a simple magnifier.
Read the full explanation →