A convex lens is held over a piece of paper outdoors on a sunny day. When the paper is held 26 cm below the lens, the sunlight is focused on the paper and the paper ignites. What is the focal length of the lens?

The focal length of the lens is 26 cm

Explanation:

Focal length (f) of the lens is the distance between the lens and the image. It is also the distance between the lens axis and principal focus.

Applying lens formula;

where;

f is the focal length

di is the image distance

do is the object distance

The object distance in this question is the distance of the sun from the lens, this distance is too large, and its inverse is infinitesimal.

The equation above will reduce to;

given;

di = 26 cm

Therefore, the focal length of the lens is 26 cm

force is mass*acceleration.

we have a very small mass: 11.4*0.001 kg

if we want 0.035n of force, we need an acceleration:

0.035 = 11.4*0.001*a

a = 0.035/(11.4*0.001) m/s^2

one person pulls 90n one way and the other has 100n pulling the other way.

so the net force = 90 - 100 = -10n,

or the ans can be 10n if the picture shows -90n and 100n instead.