One very useful definition if energy is "the ability to do work".
Relative density is defined as:
dr = ds / dw
where:
dr: relative density
ds: density of the substance.
dw: density of water.
In this case we have the relative density of oak wood:
dr = 0.64.
We want to find the density of the substance: ds
Therefore we need to know the density of water in the cgs unit system:
dw = 1g / c ^ 3
Finally:
ds = dr * dw
ds = 0.64 * 1 g / cm ^ 3
ds = 0.64 g / c ^ 3
The density of the oak wood in cgs is 0.64 g / cm ^ 3
A resultante das duas forças será zero, já q os sentidos são opostos e sãos iguais em módulo, elas se anulam. Logo, se a força resultante é zero, e F=ma, aceleração também será igual a zero.
Frictional forces act in the direction opposite to the MOTION. That direction could be the same OR opposite to applied force.
-- If you push a loaded heavy wagon from behind, trying to get it going faster, friction is acting against you, opposite to your force.
-- If you push a loaded rolling heavy wagon from in front, trying to make it slow down, friction is acting with you, in the same direction as your force.
-- Opposite to the motion both times.