A 7-kg brick is dragged across the floor by a horizontal force of 42 N. If a frictional force of 18 N also acts on the brick, what is its acceleration?

Fnet = m a

a = Fnet/m = (42 - 18)/7 = 3.4 m/s^2

To find the acceleration of the brick, we need to apply Newton's second law of motion, which states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

The net force acting on the brick is the difference between the applied force and the frictional force:

Net Force = Applied Force - Frictional Force
Net Force = 42 N - 18 N
Net Force = 24 N

Using Newton's second law, we can express the relation between net force, mass, and acceleration:

Net Force = mass * acceleration

Rearranging the equation, we get:

acceleration = Net Force / mass

Plugging in the values, we have:

acceleration = 24 N / 7 kg

Now, we can calculate the acceleration:

acceleration = 3.43 m/s^2

Therefore, the acceleration of the brick is 3.43 m/s^2.