Newton's First Law of Motion - Inertia
The law states that: Every object continues in a state of rest or uniform speed in a straight line unless acted on by a nonzero net force. This also means that an object in motion stays in motion and an object at rest stays at rest unless acted on by a net force that’s not equal to zero.
Inertia: It is the property of objects to resist change.
The Equilibrium Rule: When the net force on something is zero, we say that something is in mechanical equilibrium.
Newton's Second Law of Motion
The law states that: The acceleration of an object is directly proportional to the net force acting on the object, is in the direction of the net force, and is inversely proportional to the mass of the object.
Friction: It is a force that is caused by irregularities in the surface when two objects are in contact. It is always opposite to the direction of the motion.
Newton's Third Law of Motion
The law states that: To every action, there is always an equal and opposite reaction.
Applying the Laws in Ballet:
- For a dancer to have balance the first law applies as well as the third law. In the first law we see that when the net force on something is zero, we say that something is in mechanical equilibrium, and for a dancer to have balance the dancer needs to be in mechanical equilibrium. This is achieved thanks to the third law which says that for every action there is always an equal and opposite reaction. So when the dancer is in a specific position trying to gain balance, he/she is exerting a force on the floor (action) and the floor is exerting the same, but opposite force on the dancer (reaction), and as both forces are equal, but opposite, the net force, or the total amount of force is zero which helps her achieve mechanical equilibrium and gain balance. (Figure 1)
Figure 1: The dancer is in mechanical equilibrium because the net force between her foot and the floor is zero.
- Turns: The second law states that when a net force acts on an object, the object will accelerate. The dancer starts in the fourth position, with most of the weight on the leg that’s on the front that will later be the base of the turn. The other leg is directly behind the front leg so when the turn starts that leg pushes on the floor, to give the dancer the necessary torque to start turning and find the right balance. This push on the floor is the force that the dancer exerts in order to accelerate and start the turn. According to the first law, once in motion, the dancer should stay in motion, but because of friction between the dancer's foot and the floor, the turn is slowed down and eventually stops. This is why the dancer (in Figure 2) uses tap shoes, which have less friction which allows her to spin more, and to spin faster. However, the tap shoes have really low friction, which makes it slippery, but the dance floor has more friction, so as the tap shoe has low friction, and the floor has a higher force of friction, it is easier for the dancer to control this force, and spin against it. The dancer also uses her head to keep the focus on a specific spot and turn with more accuracy, to maintain her balance.
Figure 2
- For jumps: The third law states that when you apply a force (action) there is an equal and opposite reaction. So, before a dancer jumps, he or she bends his or her legs and pushes on the floor (action), because of Newton’s law, the floor pushes back with an equal amount of force, but in the opposite direction, so if you are pushing the floor “down”, the floor is pushing back, pushing “up” causing the dancer to gain height and perform the jump. (Figure 3)
Figure 3