Newton's Second Law of Motion

Newton's Second Law of Motion : 

Newton's Second Law of Motion

Balanced and Unbalanced Forces : 

Balanced and Unbalanced Forces Whenever the forces acting on an object are equal in magnitude and opposite in direction, they are referred to as balanced forces .  Look at the picture on the left.  The two forces are equal in magnitude (10 N) but opposite in direction.  The forces cancel each other out; therefore, the forces are balanced. Whenever there are forces acting on an object that are unequal in magnitude and/or direction, they are referred to as unbalanced forces .  Unbalanced forces cause objects to accelerate.  In the picture on the right, a push force has been applied that pushes the box from left to right.  Although the force of gravity and the force of the table are still balanced, the force of the push (5 N) is larger than the force of friction (1 N); therefore, the forces are unbalanced and the box accelerates to the right. The pictures below are known as free-body diagrams .  Free-body diagrams show all the forces acting on an object in a given situation.

Net Force : 

Net Force The net force ( F net ) is the sum of all vector forces acting on an object.  Let's look at the free body diagram from the previous slide as an example. The push force is 5 N rightward, while the friction force is 1 N leftward.  These two forces are opposite in direction and the difference between these forces is 4 N (5 N - 1 N = 4 N).  Because the push force is the larger of the two forces, the net force is in the direction of the push (rightward). The difference between the force of gravity and the force of the table is 0 N because those two forces are balanced (10 N - 10 N = 0 N). By adding the differences between the two pairs of opposite forces (4 N + 0 N) we can determine that the net force on the box is 4 N in the rightward direction.

Newton's Second Law of Motion : 

Newton's Second Law of Motion Newton's second law of motion states that an object's acceleration is dependent upon an object's mass and the net force acting on the object.  The law is expressed in the equations shown below. Newton's second law is concerned with unbalanced forces.  Remember, unbalanced forces occur when the different forces acting on an object are unequal in either direction and/or magnitude.

Using Newton's Second Law : 

Using Newton's Second Law If two of the three variables are known, then Newton's second law can be used to determine an object's mass, acceleration, or the net force on the object by doing some basic algebra.  Look at the example below.

Using Newton's Second Law (cont.) : 

Using Newton's Second Law (cont.) The previous slide was an example of calculating an object's acceleration.  The example below shows how to calculate an object's mass.

Using Newton's Second Law (cont.) : 

Using Newton's Second Law (cont.) If a free body diagram is not available, the net force may be calculated by multiplying the object's mass by its acceleration.  The example below demonstrates how to calculate the net force on an object.

What You Have Learned : 

What You Have Learned You learned about balanced and unbalanced forces. - Balanced forces are those that are equal in magnitude and opposite in direction. - Unbalanced forces are those that are unequal in either magnitude and/or direction. You learned how to calculate net force. - The net force ( F net ) is the sum of all vector forces acting on an object. You learned about Newton's second law of motion. - Newton's second law of motion states that an object's acceleration is dependent upon the net force acting on the object and on the object's mass. - The fundamental equation of Newton's second law of motion is F net = ma. - You learned how to use Newton's equation to solve for an object's mass, acceleration, or net force.