In this lesson, we will define net force, and explore the magnitude of a net force, the relevant equations and examples that will help make the concept of net force clearer.
Definition of a Net Force
When you kick a soccer ball, the ball takes off and moves through the air. At that moment, there is a net force acting on the ball. When the ball starts to come back to the ground and eventually stops, there is a net force acting on the ball as well. Newton's Second Law says that when a net force is acting on an object, then that object must be accelerating, that is, its speed changes from second to second. When you first kick the soccer ball, it accelerates, and when the soccer ball begins to slow down to a stop, it is also accelerating.
A net force is defined as the sum of all the forces acting on an object. The equation below is the sum of N forces acting on an object.
There may be several forces acting on an object, and when you add up all of those forces, the result is what we call the net force acting on the object. If the net force adds up to zero, then the object is not accelerating, therefore it moves with a constant speed. If the net force adds up to a non-zero value, then the object is accelerating.
Magnitude and Equation
According to Newton's Second Law, when an object accelerates, then there must be a net force acting on it. Conversely, if a net force acts on an object, that object will accelerate. The magnitude of the net force acting on an object is equal to the mass of the object multiplied by the acceleration of the object as shown in the formula below.
If the net force acting on an object is zero, then the object is not accelerating and is in a state that we call equilibrium. When an object is in equilibrium, then two things can be true: either the object is not moving at all, or the object is moving with a constant speed. The formula for equilibrium is shown below.
Consider a hypothetical situation in space. You are doing a spacewalk and are fixing something on your shuttle. While working on the issue with a wrench, you get angry. You throw the wrench away from you. What happens? Well, once the wrench leaves your hand, it will continue moving with the same speed that you gave it when you threw it. This is an example of a zero net force situation. The wrench will move with the same speed and will not accelerate in space. If you throw the same wrench on Earth, the wrench will fall to the ground and eventually stop. Why did it stop? There is a net force acting on the wrench causing it to slow down and stop.
In another example, let's say you are on an ice rink. You grab a hockey puck and slide it across the ice. Eventually, the hockey puck will slow down and stop, even on the smooth and slippery ice. This is another example of a situation with a non-zero net force.
Another example would be taking a long drive. When you start your car and begin to drive, you are accelerating, and, therefore, there is a non-zero net force acting on you. But now, you are driving on a long flat stretch of a freeway and decide to use your cruise control. Cruise control will keep the speed of your car constant until you either step on the brakes or the accelerator. When the cruise control is on, you are not accelerating; therefore, this is an example of a zero net force situation. When your trip comes to an end, and you eventually stop, you are again accelerating. Therefore, the end of your trip is another situation where the magnitude of the net force is not zero.
Every object has a bunch of forces acting on it. The net force is just the sum of all of these forces acting on an object. When the magnitude of this net force is non-zero, the object is speeding up or slowing down. When the magnitude of this net force is zero, the object is either not moving or moving with a constant speed. We call this equilibrium.
This equation is the sum of n forces acting on an object. The magnitude of the net force acting on an object is equal to the mass of the object multiplied by the acceleration of the object, as shown in this formula.
Definitions and Equations
An object accelerating must have a net force acting on it
The sum of all forces acting on an object
When an object is not accelerating: either moving at a constant speed or at rest
Once you are finished, you should be able to:
Define net force and equilibrium
Recall the equations for net force and equilibrium
Explain the relationship between net force and equilibrium
In the following practice problems, students will use Newton's Second Law to examine the net force and/or acceleration of objects.
1. If the net force on a car is 3000 Newtons, and the mass of a car is 1000 kg, what is the car's acceleration?
2. Find the net force on a car with mass 1200 kg and acceleration 4 m/s^2.
3. If an object is in equilibrium and the object is moving, what can be said about the net force on the object? What can be said about the object's acceleration? What can be said about the rate at which the object is moving?
1. Using the formula
3000 = 1000a
3 = a
So the acceleration of the car is 3 m/s^2.
2. Using the formula
with m = 1200 and a = 4, we have
So the net force on the car is 4800 Newtons.
3. If an object is in equilibrium, then its net force is zero by definition. Since the object is in equilibrium, it is not accelerating and so a = 0. Since this object is moving, but has acceleration 0, the object must be moving at a constant rate.
Register to view this lesson
Are you a student or a teacher?
Unlock Your Education
See for yourself why 30 million people use Study.com
Did you know… We have over 220 college
courses that prepare you to earn
credit by exam that is accepted by over 1,500 colleges and universities. You can test out of the
first two years of college and save thousands off your degree. Anyone can earn
credit-by-exam regardless of age or education level.