Net Force Formula | Problems (And Solutions)

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Net Force Formula

When an object is at rest, the value of net force (Fnet) is equal to the sum of normal force (FN) and the force of gravity (Fg). Using the formula of net force: Fnet = FN + Fg, the value of net force acting on an object can be calculated.

Here’s the formula of net force:

  1. When an object is at rest, Fnet = FN + Fg
  1. When an object is in motion, Fnet = Fa + Ff + FN + Fg
  1. When ‘n’ number of forces are acting on an object, Fnet = F1 + F2 + F3 + … + Fn

Let’s solve some problems based on these formulas, so you’ll get a clear idea.

Net Force Practice Problems

Problem 1: Consider one block resting on the floor. If this block is balanced by 6 N of normal force and 6 N of gravity force, then what is the value of net force acting on this block?

Solution:

Given data:
Normal force acting on a block, FN = 6 N
Force of gravity acting on a block, Fg = -6 N (downward)
Net force acting on a block, Fnet = ?

Using the formula of net force, (when an object is at rest)
Fnet = FN + Fg
Fnet = 6 – 6
Fnet = 0 N

Therefore, the net force acting on a block is 0 N.

Problem 2: One book is lying on the desk. When a 15 N of force is applied on this book, a friction force of 5 N acts on it in the opposite direction. If this book is balanced by 20 N of normal force and gravity force, then calculate the value of net force acting on it.

Solution:

Given data:
Applied force acting on a book, Fa = 15 N
Friction force acting on a book, Ff = -5 N (left)
Normal force acting on a book, FN = 20 N
Force of gravity acting on a book, Fg = -20 N (downward)
Net force acting on a book, Fnet = ?

Using the formula of net force, (when an object is in motion)
Fnet = Fa + Ff + FN + Fg
Fnet = 15 – 5 + 20 – 20
Fnet = 10 N

Therefore, the net force acting on a book is 10 N.

Problem 3: Two persons 1 and 2 are pulling the rope from opposite ends. The person 1 (standing on the right side) pulls the rope with a force of 40 N and the person 2 (standing on the left side) pulls the rope with a force of 15 N. Calculate the net force acting on the rope.

Solution:

Given data:
Force acting on the rope, F1 = 40 N
Force acting on the rope, F2 = -15 N (left)
Net force acting on the rope, Fnet = ?

Using the formula of net force, (when ‘n’ number of forces are acting on an object)
Fnet = F1 + F2
Fnet = 40 – 15
Fnet = 25 N

Therefore, the net force acting on the rope is 25 N.

Problem 4: One car gets stuck on the road and 6 persons are pushing this car towards the right direction. All 6 persons are pushing the car with the following forces: 3 N, 4 N, 5 N, 3 N, 7 N and 2 N. Calculate the net force acting on the car.

Solution:

Given data:
Force acting on the car, F1 = 3 N
Force acting on the car, F2 = 4 N
Force acting on the car, F3 = 5 N
Force acting on the car, F4 = 3 N
Force acting on the car, F5 = 7 N
Force acting on the car, F6 = 2 N
Net force acting on the rope, Fnet = ?

Using the formula of net force, (when ‘n’ number of forces are acting on an object)
Fnet = F1 + F2 + F3 + F4 + F5 + F6
Fnet = 3 + 4 + 5 + 3 + 7 + 2
Fnet = 24 N

Therefore, the net force acting on the car is 24 N.

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