# Question Video: Calculating the Net Force Acting on an Object Physics • 9th Grade

An object has three forces acting on it, as shown in the diagram. What is the magnitude of the net force on the object?

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### Video Transcript

An object has three forces acting on it, as shown in the diagram. What is the magnitude of the net force on the object?

So in this question, we want to find out the magnitude or size of the net force or resultant force on this object here. As we’ve been told in the question and we can see on the diagram, there are three forces acting on it. So in order to find the net force or resultant force, we simply need to add up all of these three forces together, whilst accounting for the direction in which they act on the object.

So we can arbitrarily choose if that this direction towards the right is positive, which means that any force acting towards the right will be positive and any force acting towards the left will be negative. Therefore, we can say that 𝐹 sub net, or the net force on the object, is equal to the positive 80-newton force, which is acting towards the right, minus one of the 30-newton forces, which is acting towards the left, minus another 30 newtons for the other force that’s acting towards the left.

In other words, we’ve got the 80-newton force acting towards the right. So it’s positive. We’ve got the 30-newton force acting towards the left. So it’s negative, and same thing with the other 30-newton force. And this expression is going to give us the net force or resultant force on the object.

Obviously, this expression will also give us the magnitude of the net force because the magnitude simply means the size. And the numerical values calculated in this way will give us the size of the force. Hence, the magnitude of the net force on the object is 20 newtons.