Leo and david are always competing to see who is the strongest. in one contest, leo lifts 15 boxes, each with a mass of 30 kg, onto a shelf that is 2.3 m high in a time of 5.1 s. david lifts 18 boxes, each having a mass of 27.3 kg, onto a shelf that is 2.5 m high in a time of 5.7 s. which one produces the most power lifting all their boxes?

To compare the strength of the two we need to find the power deliver by each person.

If a person deliver more power then his strength will be more

Now in order to find the power we can say

in order to define work to move mass to certain height above the ground we can say it is

now we will have

Power of Leo is

Power of David is

So here we can say that power of David is more so David is more strong than Leo.

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.