Forces in Motion as the Dancer Leaps
The above picture shows a dancer doing a split leap. The arrows in the picture show the forces acting upon the dancer just prior to her leap. The front leg would have been off the ground and the back leg would have been applying friction, pushing down and in a backwards motion against the ground. At that point in time, the forces would have been unbalanced.
The dancer is able to leap into the air because of Isaac Newton's third law, for every action there is an equal and opposite reaction. The action is the dancer pushing down and backwards against the ground with her foot, and the reaction is the dancer being pushed upwards and forwards into the air. This happens because the dancer has MUCH less mass than the earth, so when force from the dancer is applied to the earth, the earth doesn't move, instead the dancer is pushed up and forwards into the air. The force of the dancer’s foot pushing against the ground is an example of friction.
In the moments immediately proir to the dancer leaving the ground, her weight remains the same, but her support force increases. Because these forces are unbalanced, there is a net force acting on the dancer in a upwards direction, which is why she goes up at the start of the leap. At this point in the dancer’s leap she is not only pushing down against the ground in order to accelerate upwards, but she is also using friction with her back foot against the floor. The dancer pushes her back foot in a backwards motion against the floor and this friction between her foot and the floor have an equal and opposite reaction (also Newton's third law). This action increases the dancer’s thrust so that air resistance (friction) is less that the thrust, creating a net force in a forwards direction, allowing the dancer to accelerate forwards into her leap.
Note: Thrust is also working to lift the dancer, but this is not included in the diagram for simplicity.
The dancer is able to leap into the air because of Isaac Newton's third law, for every action there is an equal and opposite reaction. The action is the dancer pushing down and backwards against the ground with her foot, and the reaction is the dancer being pushed upwards and forwards into the air. This happens because the dancer has MUCH less mass than the earth, so when force from the dancer is applied to the earth, the earth doesn't move, instead the dancer is pushed up and forwards into the air. The force of the dancer’s foot pushing against the ground is an example of friction.
In the moments immediately proir to the dancer leaving the ground, her weight remains the same, but her support force increases. Because these forces are unbalanced, there is a net force acting on the dancer in a upwards direction, which is why she goes up at the start of the leap. At this point in the dancer’s leap she is not only pushing down against the ground in order to accelerate upwards, but she is also using friction with her back foot against the floor. The dancer pushes her back foot in a backwards motion against the floor and this friction between her foot and the floor have an equal and opposite reaction (also Newton's third law). This action increases the dancer’s thrust so that air resistance (friction) is less that the thrust, creating a net force in a forwards direction, allowing the dancer to accelerate forwards into her leap.
Note: Thrust is also working to lift the dancer, but this is not included in the diagram for simplicity.