force is a push or a pull that is capable of changing the velocity of a mass. Forces are measured in “Newtons” or “N”, in honor of Sir Isaac Newton. According to Mr. Newton, an object will only accelerate if there is “net force” acting upon it. A net force is the sum of all forces acting on an object. A net force is capable of accelerating a mass. For instance, if the wheels of a car push it forward with 5 Newtons and drag is 3 Newtons, the net force is 2 Newtons, forward. Motion to the right is positive. Motion to the left is negative.




As we have said before, a net force is the sum of all forces acting on an object. Look at the picture of the red plane. In this example, the difference between drag and thrust is 15 Newtons, to the left. This net force is capable of accelerating (slowing down) the plane.  Net forces always accelerate masses.



Net force = resultant force = unbalanced force = outside force.


If an object has a net force acting on it, it will accelerate. The object will speed up, slow down or change direction. An unbalanced force (net force) acting on an object changes its speed and/or direction of motion. An unbalanced force is an unopposed force that causes a change in motion. A net force = unbalanced force. If however, the forces are balanced (in equilibrium) and there is no net force, the object will not accelerate and the velocity will remain constant.  


Unbalanced Forces: Placing a box on the seesaw unbalances it. The weight of the box is the unbalanced or net force which causes the seesaw to accelerate downward until it hits the ground.





Let’s assume that the wheels of a car apply 10 N of force. What is the net force if friction and drag are negligible?


The net force would equal 10 Newtons, forward. The mass will accelerate.




What is the net force if the wheels of the car apply 10 Newtonsbut a parachute applies 7 Newtons in the other direction?


The net force would equal 3 Newtons, forward.  The mass will accelerate.




A rocket applies an additional force of 10 Newtons to the 10Newtons that are applied by the wheels. What is the net force if the parachute continues to apply 7 Newtons in the other direction?


The net force would equal 13 Newtons, forward. The mass will accelerate.





Balanced Forces: When forces are in balance, acceleration is zero. Velocity is constant and there is no net or unbalanced force. A plane will fly at constant velocity if the acceleration is zero.





The forces are in balance.



[slide sown a slide]


Consider a person sliding down a slide at constant velocity. In this example, there is no net force when a mass moves at constant velocity. Although friction is acting on the person, there is no change in velocity and friction is not a net force in this case.


Friction is only a net force if it changes the velocity of a mass. If friction slowed the person down, then there is a net force.










A car is moving at 10 m/s. The wheels apply 10 N of force. Drag exerts 10 N in the other direction. There is no net force.


According to Newton’s 2nd Law,


Fnet = MA


The acceleration will be zero, since there is no net force. Will the speed of this car change under the present circumstances?




[fireman sliding down pole]


A fireman slides down a pole at constant velocity. Friction resists his fall. At constant velocity, his weight equals the force of friction. In other words, there is no net force.


If however, he loosens his grip and decreases the friction force, he will accelerate downward. As he accelerates, there is a net force.






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