Mass and Motion

Students use the phenomenon of the mass of a baseball and its motion to analyze how objects transfer energy in a collision, tracing how energy changes from one form to another in an energy system and then exploring the relationship between an object’s mass and its speed.

• Forces are acting everywhere in the universe all the time. A force is any push or pull that acts on an object, changing its speed, direction, or shape.
• A stationary object still has forces acting on it. Because the object’s motion isn’t changing, the forces acting on it must be balanced.
• The motion of an object will only change when the forces acting on it become unbalanced.
• Whenever two objects come into contact with one another, they exert a force on each other (action-reaction forces), and that force transfers energy from one object to the other.
• Energy is the ability to do work. Work is any change in position, speed, or state of matter due to force.
• Forces transfer energy into or out of objects or systems. A system is a set of connected, interacting parts that form a more complex whole.

• What forces are acting on you right now?
• When have you exerted a force on an object?
• What unbalanced forces would have to happen to cause an object to move?
• What are some examples from everyday life that provide evidence for the idea that a change in motion only occurs with an unbalanced force?
• How would a baseball in motion move if there were no forces acting on it?
• What energy transfer occurs when a pitcher throws the ball?
• What evidence is there that energy has transferred from the pitcher to the ball?
• How do we know that energy is transferred when two objects come into contact with one another, and that each object exerts an equal but opposite force on the other?

Misconception: Energy can be used up. 

Fact: Just like mass, energy is always conserved. Energy transforms from one form to another, and can transfer into or out of objects or systems, but the total amount of energy is conserved.

Energy : the ability to do work

Force : a push or pull that acts on an object, changing its speed, direction, or shape

Kinetic energy : the energy of motion

Mass : the measure of the amount of matter that makes up an object or substance; measured in grams (g)

Potential energy : energy that is stored

Proportion :the relationship between things, as to size, quantity, or number

Speed: the rate at which an object covers distance in a period of time

System : a set of connected, interacting parts that form a more complex whole

Work : any change in position, speed, or state of matter due to force

Forces and Energy

Forces are directly related to energy. Energy is the ability to do work. Work is any change in position, speed, or state of matter due to force. Examples of work include heating up an object or moving an object.

In other words, forces transfer energy into or out of objects or systems. A system is a set of connected, interacting parts that form a more complex whole.

To understand this relationship between energy, work, and force, it is first important to know that energy can be stored or in motion. Potential energy is energy that is stored. Kinetic energy is the energy of motion.

Forms of Energy

Energy of one kind can transform (change) into energy of another form in an energy system. Gravitational potential energy is the energy stored in an object as a result of its vertical position or height above the ground. For example, the higher a baseball is hit, the more gravitational energy it has. This is a cause-and-effect relationship. The height of the ball causes the amount of energy stored in the ball to change (the effect).

Transferring Energy

Any object that is moving has kinetic energy. A moving car and a running dog both have kinetic energy because they are in motion.

When a pitcher throws the ball, they apply a force to the ball that transfers kinetic energy to the ball. This transfer of kinetic energy is what powers the ball’s movement from the pitcher’s hand to the batter’s bat. When the batter swings, they generate their own kinetic energy. At the moment of contact between the ball and the bat, energy is transferred between the two objects.

Action-Reaction Forces

When the bat and the ball come into contact with one another, the ball exerts a force on the bat. The bat exerts the same amount of force on the ball.

We see evidence of this action- reaction pair in the change in motion of both the bat and the ball. Depending on how the batter hits the ball, the ball will change direction, moving away from the batter. The transfer of energy from the bat to the ball caused this change.

At the same time, the ball exerted a force on the bat that transferred energy. Evidence for this comes from the motion of the bat after contact. The bat might swing backwards a bit. If the force of contact is great enough, wooden bats have been known to splinter. This is because of the force exerted by the ball. All of these interactions occur because of Newton’s action-reaction law.

Energy Conservation

In a perfect system, the total amount of energy is conserved as it transforms from one form to another. This means that in a perfect system, the total amount of kinetic energy transferred from the pitcher to the ball would be the same amount of kinetic energy that the ball transferred to the baseball bat.

However, the real world is not a perfect system. Energy transfers out of the system at various points. For example, as soon as the ball leaves the pitcher’s hand, a force called drag occurs between the ball and the air. Drag, also called air resistance, is caused by air pushing against the baseball in motion. Drag occurs between a solid substance and a fluid such as air, and it slows motion because it causes some of the energy of the moving substances to change into heat. Drag transfers energy out of the system, causing the ball to slow down.

At the moment of contact between the bat and the ball, friction causes more energy to transfer out of the system. Friction is another force that slows motion when two objects rub against each other. Friction slows motion because it causes some of the energy of the moving object to change into heat.

Friction explains why a ball rolling on the ground slows down and eventually stops. According to Newton’s first law, the ball will continue moving unless the forces acting on it become unbalanced. In this case, friction is an unbalanced force that changes the ball’s motion.

At that same moment of contact, some of the energy is transferred out of the ball as it transforms to sound energy, which is energy produced by sound vibrations moving through a substance in waves. We hear evidence of this energy transfer in the loud crack made by the bat and the ball making contact.

Remember how the force of the contact distorts the baseball to half its original diameter? This is because the ball’s kinetic energy transforms to a form of potential energy called elastic energy, which is energy stored in objects when stretched. As the baseball’s shape is restored, the elastic potential energy transforms back to kinetic energy and the ball moves through the air.

In this lesson students investigate the phenomena of how the mass of a marble affects its average speed after a collision. They collect data from the experiment and look for patterns that might indicate a relationship between the mass of the marble and its resulting speed.