Student Portal

Our Solar System – Investigation 5 CAP


Our Solar System: Investigation 5 – CAP


In this CAP we will begin by discussing the forces involved in the lab experiment for Investigation 5. We will then continue on by discussing forces in a more general sense and include the concept of balanced and unbalanced forces. Newton’s First Law of Motion states that an object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force. In this CAP, you will see how this is true by performing a couple of simple experiments right at your desk or at home.


This slide can be used to refresh your memory of the flour and ping pong ball experiment performed in the lab. The question “What happens when the pull of gravity is removed” (that is when the string is let go), is that the ball leaves its orbit in a straight line. We will look at this closer in the next two slides.


Here the moon orbiting the Earth is used in place of the experimental lab setup with flour and a ping pong ball. Notice that while the orbit is circular around the Earth, it is made up of repeated positions in which the Moon would continue in a straight line and into space if not for the opposing force of the Earth’s gravity acting (pulling) on it.


This slide shows the forces acting on the Moon at each of its positions around the Earth. Notice that at each point in its orbit, the forward forces that would cause the Moon to go straight off into space, leaving the Earth behind, are exactly balanced by the force of gravity pulling it toward the Earth. This continuous balance of forces has kept the Moon orbiting the Earth for over 4 billion years.

This slide also shows the impact of a situation in which the forces causing the Moon’s orbit around Earth become unbalanced. When the balancing force of the Earth’s gravitational field is removed, the Moon immediately heads off into space in a perfectly straight line. With only the single unbalanced force of the Moon’s momentum, the Moon will continue moving away from Earth in a straight line unless it encounters other forces to act upon it. In the absence of any such force, the Moon would continue moving in a straight line and at the same speed forever.


In this slide, we begin to look at balanced and unbalanced forces in other examples. This is the first of three similar slides. One can easily picture the invisible forces at work in this rugby match. Forces from the left confront forces coming from the right. The pile of players will not move one way or another very much as long as the forces are balanced.

What could happen next in this picture to cause the pile to move to the left or to the right? For example, one of the players in the pile could get tired and begin to be pushed back. Alternatively, another player from one side or the other could rush into the pile and expert more force from his side and get the pile to move in the opposite direction. Athletes train so that they are faster to get to such a pile-up and so that they are stronger and can exert more force in situations such as these.


Here the forces acting on the rope toy are approximately balanced and the two dogs are at an impasse. The rope will not move unless one of the two dogs is able to exert a greater force. Notice that the balanced forces in this example (pulling away from each other) are in the exact opposite direction as the forces in the rugby example (pushing towards each other). Nonetheless, whether pulling or pushing, if the forces are balanced, no movement of the object on which the forces are acting will occur.


Finally, this slide shows yet another example of balanced forces competing with each other. In what direction are the forces being exerted in this situation? Are the forces in this arm wrestle more similar to the rugby situation or the tug-o-war between the two dogs? Hopefully, given these simple examples, you will learn to look for forces operating in other situations around you.

Don’t just look for the forces involved in moving objects like cars, basketballs, or the Moon. Just because an object isn’t moving doesn’t mean that no forces are acting on it. It only indicates that the combined forces acting on it are balanced. When sitting quietly on a chair there are two opposing and equal forces. One force is your mass pushing down toward the floor because of gravity, while the other force is the Earth pushing up at you and the chair. There may be a great deal of force, but if they are equal and balanced, no movement will occur.


The last several slides depict some very simple demonstrations of balanced and unbalanced forces that you can perform alone or with a partner. This slide depicts a situation in which a coin is pressed, from the top, hard onto a table. Despite all the effort and force that you can exert downward on the coin, it does not move downward because the table itself is exerting an equal force in the opposite direction. The harder you press down on the coin, the harder the table presses back.


If however, the opposing force of the table is circumvented by pushing the coin over the edge of it, the table can no longer provide an opposing force and the penny is no longer stuck between two balanced forces and can move. It moves down.

This is an interesting situation since the force of gravity is the only force acting on the penny once it escapes the surface of the table. The finger is no longer needed to exert a force and the penny will move (fall) because no force is available to balance the force of gravity. When the penny lands, it will once again encounter an opposite and equal force – the floor – and will remain stationary as long as the forces remain balanced.


This slide shows another simple model for you to experience balanced and unbalanced forces. In this form, you can perform the demonstration on your own.


Finally, a demonstration of balanced and unbalanced forces can be performed with partners. This form of the demonstration is more interesting (and fun) since each student controls only one of the two opposing forces. As force is applied, the key is to watch the direction the penny moves. Under conditions where both students exert the exact same force, regardless of how strong that force may be, the penny will remain stationary.