Summary of Newton's First Law
Newton's first law tells us that changing an object's velocity in any way requires a force acting on the object. Any change in velocity is called an acceleration. An acceleration can be speeding up, slowing down, or changing the direction of the motion. All of these accelerations, or velocity changes, require an outside force.
But, how much force is required?
Newton's Second Law
Newton's first law tells us that a force is required to accelerate an object. Newton's second law answers the question about how much force is required.
If you have ever tried pushing a car, you know that even with the brake off and the transmission in neutral it is very hard to push a car and start it moving from rest. You must push with a great deal of force. Pushing a refrigerator, even one on wheels, across the kitchen floor is easier than the car, but still hard. Finally pushing a toy car is fairly easy. Very little force is needed to accelerate it. Why the difference?
The key is the mass. The mass of the real car is many more kilograms than the mass of the toy car. So it is much harder to accelerate the more massive real car; it takes more force. The refrigerator's mass is between that of the toy and real car, so the force needed to accelerate it is between the that of the toy and real car.
Notice that the difference is mass not size. For example a very large bag of feathers might be as large as a car but relatively easy to push. feathers are not very tightly squeezed, so a large bag of feathers will still have a relatively small mass. It is takes less force to accelerate a bag of feathers the size of a car than the car. Mass and size are different things and the force needed to accelerate something depends on the mass not the size.
Accelerating a more massive real car requires more force than accelerating a less massive toy car. Newton's second law tells us that the more massive an object is, the more force is needed to accelerate it. Newton's second law goes a little further by providing a precise formula for how much force is needed.
Newton's second law can be expressed as a mathematical formula for the amount of force needed to accelerate an object. It is:
Force equals mass times acceleration, or F=ma.
From this formula we can find the force needed to accelerate an object. If the object has a mass of 5 kilograms and the acceleration is 3 meters per second squared, then the force needed to get this acceleration is 15 kilogram meters per second squared, which is also 15 newtons.
Newtons second law extends Newton's first law by giving us a formula for finding how much force is needed to accelerate an object.
Further Reading
Newton's Laws for Kids - Overview
Newton's Laws for Kids - Third Law
Newton's Laws for Kids - Gravity
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
