Newton's Laws of Motion Explained

Newton's three laws of motion describe the relationship between an object and the forces that act upon it. Today, we will explore the laws of motion using the Yamashita vault.

Newton's First Law: Inertia

Newton's First Law states that an object at rest stays at rest, and an object moving at a constant speed stays moving at a constant speed unless either are enacted upon by an unbalanced force. This can be demonstrated by looking at the motion of the beat board.

Before the gymnast hits the board, it is at rest.

Because no force is applied to the board, and the force of gravity is equal to normal force (the force pushing up on the board from the ground), the object stays at rest.

However, after the gymnast hits the board, the board leaps into the air.

This happens because the force applied by the gymnast creates an imbalanced force. This force imbalance causes the board to accelerate upward, which will be explained when we explore the third law.

Newton's Second Law: Net Force

Newton's Second Law states that the sum of all forces is equal to the mass of the object multiplied by the acceleration of said object. We will explain this with the gymnast's run. The gymnast's mass is measured to be 49 kg, so all we need to figure out is her acceleration. On the kinematics page, you can see that we determined that the gymnast's maximum velocity was 6.64 m/s forward. When we know this, it is fairly simple to figure out the acceleration of her run.

Now that we know the mass and acceleration, we can figure out the net force of the run.

Newton's Third Law: Action/Reaction Forces

The third law states that for every action there is an equal and opposite reaction. This is demonstrated in a few ways. First, the run the gymnast takes. Each step she takes pushes her up and forward, giving her greater speed. Second, the leap she takes gives her extra force with gravity pushing her down on the beat board, which is the third example; the force that she pushes down on the board with in turn will push her up to block off the vault, our final example.

Interesting Fact:

An ideal block for a gymnast includes reaching far on the vault horse (maximising her distance), hitting the vault as fast as she can (the faster she blocks, the more action force hits the horse and therefore more reaction force pushes her up and out), and having an open body (lifts her heels faster and helps her flip faster and higher). These three techniques will help the gymnast execute vaults better.