Newton's 2nd Law

Sometimes to understand physics subject, we need to use some common sense. Let me give u one example, assuming that a coconut and a tennis ball  is falling from the same height onto your head..which one will hurt u the most?? I can bet that a 13 year school boy who didn't learn physics can tell us  that the coconut will exert greater force. All he needs is to use some common sense.

Now lets look at  physics point of view. We learned that gravity will pull both coconut & the ball at the same acceleration if the air resistance is negligible. Then curiosity is telling us there must be other factor that causes a painful force produced by the coconut. Finally we can conclude that the different mass explained different forces produce by the ball & coconut. This situation describes how the concept of Newton's 2nd law can be applied.

According to this law,the force of a moving object is equal to product of its mass & acceleration. These two factors i.e the mass & acceleration will determine how big the force is produced. In other word, Newton 2nd law is concerned in relating acceleration, mass & the net force.

Fnet= ma

If u notice, i use the term net force instead of force only. This is because, in SPM most of the question we deal with a case that involves more than one force. The net force means that we need to add or minus all the forces involved. For example,

A car with a mass of 100 kg moves with a driving force of 500N. Each tyre of the car causes a frictional force of 5N. Determine the car's aceleration??

Before u solve this problem, u need to identify all the information given. There are two type of forces involved i.e driving force & frictional force. Both forces are in different direction...seems each tyre causes friction, thus 4 tyres causes (4 x 5N) = 20 N. This 20 N  frictional force will oppose motion of the car. 

Thus the NET FORCE will be (500 - 20)N. The final calculation should look like this;

      Fnet = m x a

500 - 20 = 100 x a

           a = 4.8 ms-2

To understand better about Newton's 2nd law of motion, u should watch the following videos.