Newton’s First Law

Any object must maintain a uniform linear motion or static state, until the external force to force it to change the state of motion so far.

The skateboard starts from the rest with same speed and direction until the skateboard crushes the book, it stopped. The book changes the motion of the skateboard.

Newton’s Second Law

The magnitude of the object acceleration is proportional to the force of the object, inversely proportional to the mass of the object, and is proportional to the reciprocal of the mass of the object; the direction of the acceleration is the same as the direction of the force.

The same skateboard, when we add two books on it, it changes the mass. So the speed gets lower. We need to put more force on push the skateboard to make the skateboard going the same speed as the first test.

Newton’s Third Law

The interaction between the two objects and the reaction force is always equal in size, the opposite direction, the role of the same line.

When the hand first hit the table, is not hurt because the force effect is small, so the reaction force is small. The second hit is hurt because the force effect is hard, so the reaction force is hard too.