A ball of mass 200g is thrown vertically downwards from a height
of 3.2m
(a) Calculate the potential energy of the ball when it is 3.2m
above the grounds
PE =mgh
=200 x 10 x 3.2
= 6.4J
(b) Calculate the speed of the ball just before it reaches the
ground.
(1/2) mv^2 =6.4
0.5 x 0.2 x V^2 =6.4
V^2=64
V=8ms^-1
(c) State the acceleration of the ball when it is falling .
[Student tends to think the acceleration is 8m/s which
is wrong.Because acceleration due to gravity is always constant
]
therefore g =10m/s
(d) Hence ,find the force acting on the ball as it is falling
.
F=mg = 0.2 x 10 = 2N
[Note:when the acceleration of the ball is answered
wrongly, naturally the answer of the Force acting on the ball will
also be wrong ,as wrong value is used]
Source:Ahmad Ibrahim Secondary School ,GCE O level Prelim Exam
2008
