1. An engineer at NASA is investigating the impact of meteorites on the planets. He
performs experiments in a vacuum by dropping metal spheres of different sizes and
different heights above a horizontal bed of sand. In one experiment, a metal sphere of
mass 0.1kg is dropped from a height of 1.2 metres. The sphere makes a depression in the
sand of 2cm.
(Gravitational field strength, g = 10 N kg-1)
i)
ii)
iii)
Calculate the potential energy of the sphere.
Calculate the velocity of the sphere just before hitting the surface of the sand.
(Hint: Ep = Ek)
Calculate the average retarding force acting on the sphere as it comes to rest in the
sand. When the object passes through sand it will lose it kinetic energy to
overcome the resisting force by sand.
(Hint : Retarding force or stopping force is simply the force that is applied in the
opposite direction to the direction of motion thus, producing deceleration in a body or
produces a negative acceleration.)
(Hint : Force x displacement = Kinetic Energy, will prove in class)
Thus, if we find the Kinetic energy using 𝑬𝒌 =
using F x d = K.E.
𝒎𝒗𝟐
𝟐
we can find the retarding force
2. Brandon is loading boxes onto the tray of a flatbed truck. He uses ramp of length 8m to
assist him. He pushes a box of mass 120 kg up the ramp with a force of 200N. The box
move through a vertical distance of 0.8m .
(Gravitational field strength, g = 10 N kg-1)
i)
Calculate the change in gravitational potential energy of the box.
ii)
Calculate the work done by the 200N force.
iii)
Calculate the efficiency of the ramp.
Note : N kg-1 is equivalent to ms-2 . Will prove in class
Oberlin High School
Grade 11 Physics