Newton's Second Law (Forces)
Newton's second law states that force is equivalent to to mass times acceleration. In other words, F=ma
That's it! However, the applications for this is practically endless. One important thing you do need to understand, however, is this: Forces depend on motion and mass. Without mass, no matter how fast something is moving, nothing is really moving, so there is no force at all. And without acceleration, nothing is actually moving, so it's just an object sitting there and there is no force at all.
Newton's second law also shows why bullets and crushers can pack a lot of force:
bullets, although having a small mass, moves very quickly, so it can easily puncture skin and organ tissues
crushers, although moves quite slowly, is extremely heavy, so they can exert large forces that can crush cars and other metal. Exercise
A block of mass 2kg has a constant velocity when pushed on a table by a force of 5N. Find its acceleration when pushing force is increased to 9N.
Let's start with the formula:
F=ma
What do we have? We have the force and the mass, the only thing that's missing is the acceleration. The first thing here is to find its resultant force - because the resultant force is the only true force that is acting on the object. How do we know? We know the block moves at a constant velocity with no acceleration at 5N, so we know friction and any opposing motion is 5N against the block.
Resultant force: 9N - 5N = 4N
Next is a piece of cake. Plug the values in and ta-da! 4=2a
4/2=a
Acceleration = 2m/s^2.
