For speed you can differentiate the equation, for acceleration you can again differentiate the equation .
at t=0 the particle is slowing down , when you get equation for velocity put t=0 then only -1 is left
As the plastic sphere is charged, therefore it experience an electric force when placed in an electric fields and also experiences gravitational force acts downward so the electric force must act upward.
Let
is electric force and
is gravitational force.
If these forces are balanced, therefore
or 
Given,
and
.
Substituting these values in above equation we get,

Thus, the magnitude of electric field is
.
As the charge is negative, the electric field at the location of the plastic sphere must be pointing downward.
Answer:
3.73 * 10^16 photons/sec
Explanation:
power supply = 3.0 V
Emits 440 nm blue light
current in LED = 11 mA
efficiency of LED = 51%
<u>Calculate the number of photons per second the LED will emit </u>
first step : calculate the energy of the Photon
E = hc / λ
=( 6.62 * 10^-34 * 3 * 10^8 ) / 440 * 10^-9
= 0.0451 * 10^-17 J
Next :
Number of Photon =( power supply * efficiency * current ) / energy of photon
= ( 3 * 0.51 * 11 * 10^-3 ) / 0.0451 * 10^-17
= 3.73 * 10^16 photons/sec
882 divided by 9.81 (this is acceleration due to gravity) it equals 89.91