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Mass, Speed, and Kinetic Energy

In this unit, students analyze the science phenomena of connections between energy, forces, and motion. In this lesson, students use data to construct an explanation about phenomena that occur because of the relationships between an object’s kinetic energy, its mass, and its speed. This page provides an overview of this lesson.

Science Background for Teachers:

Science background gives teachers more detailed information on the phenomena students explore. Here is an excerpt from the science background section on mass, speed, and kinetic energy.

Mass and Speed

There are rules about how much mass a bobsled can have when empty and when the bobsledders are on it. Remember that mass is the amount of matter in an object. An object’s motion will change according to its mass, as well as the sum of the forces acting on it.

This is because of another law of motion, which states that force equals mass times acceleration. Acceleration is an increase in speed over time. It is measured in meters per second squared (m/s2). Speed is the rate at which an object covers distance in a period of time. It is measured in meters per second (m/s). This law says that the amount of force needed to move an object any distance depends on its mass. An object with greater mass needs more force to accelerate than an object with less mass. In other words, a more massive sled will need more force to cause it to accelerate a certain amount than a less massive sled would need.

Once the sled starts moving, a more massive sled has more energy than a less massive sled when moving at the same speed. This is because there is a proportional relationship between the amount of kinetic energy an object has and its mass. The more mass an object has, the more kinetic energy it has. If the mass of an object doubles, its kinetic energy also doubles. If an object’s mass decreases by half, its kinetic energy also decreases by half.

If you were to graph the relationship between an object’s mass and its kinetic energy, you would see a linear proportional relationship, where two quantities vary directly with one another. If one quantity is doubled, the other, related quantity is also doubled.

Mass and Kinetic Energy

Think of a pebble sitting on top of a hill. To get the pebble moving down the hill, you need to apply a force to it, such as by kicking or throwing the pebble. However, because the pebble doesn’t have much mass, it doesn’t require a lot of force to move it.

Now think of a boulder on top of that same hill. You would have to push it with a lot more force to get it to move because it is so much more massive than the pebble.

However, if both the pebble and the boulder move down the hill at the same speed, the boulder will have more kinetic energy than the pebble does because the boulder is more massive. If both the pebble and the boulder were to collide with a trash can at the bottom of the hill, the boulder would transfer more energy to the trash can, either denting it or moving it to another location. In contrast, the pebble would have much less energy to transfer to the trash can, resulting in a much smaller dent (if any) or a smaller movement.

Because of the relationship between mass and energy, in 1952 the Olympics committee established rules for how much a sled could weigh when it was empty and when the bobsledders were on it. Before 1952, bobsledders tended to be big and heavy. After 1952, the emphasis changed. It became more important for bobsledders to be strong enough to push the sled with a lot of force, but light enough to meet the weight requirements.

There is also a relationship between speed and kinetic energy. The faster an object is moving, the more energy it has. This is why collisions are more dangerous at high speeds compared to low speeds. The speed of an object is also proportional to the amount of kinetic energy it has. However, this isn’t a linear relationship. Instead, it is a nonlinear proportional relationship that changes exponentially where the rate of change increases exponentially each period. When the speed of an object doubles, the kinetic energy quadruples.

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Science Lesson: Connecting Mass, Speed, and Kinetic Energy

Once students have figured out how forces transfer energy and the relationship between force and mass, students investigate the relationship between the speed phenomena of a moving object and its kinetic energy and the relationship between the mass of a moving object and its kinetic energy.

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Science Big Ideas

  • All moving objects have kinetic energy. The speed of a moving object determines how much kinetic energy it has.
  • The relationship between the speed of a moving object and the amount of kinetic energy it has is a nonlinear proportional relationship.
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Science Essential Questions

  • What do bobsleds in motion down a hill have in common with race cars moving around a track?
  • Where do race cars get the energy they need to move?
  • What makes a race car an energy system?
  • How do scientists calculate speed?
  • What makes the relationship between the speed of a moving object and its kinetic energy nonlinear?

Common Science Misconceptions

close-circle-red Misconception: Energy is used up, and then it disappears.

checkmark-circle-green Fact: Energy is never created or destroyed. Instead it transforms from one form to another. remain in motion unless acted on by an outside force.

close-circle-red Misconception: Sustaining motion requires a continued force.

checkmark-circle-green Fact: An object in motion will remain in motion unless acted on by an outside force. 

Science Vocabulary

Speed : the rate at which an object covers distance in a period of time; measured in meters per second (m/s)

Velocity : the rate at which an object changes its position in a specific direction

Lexile(R) Certified Non-Fiction Science Reading (Excerpt)

Race Car Driving

When the black and white checkered flag is waved on a race track, it means one thing: the race is over. A plain white flag means drivers have one more lap to go. And green means go.

There are eight flags in stock car racing. Each flag has a specific meaning for the drivers. An easy, visual way to communicate with the drivers is important because they can reach speeds of 322 kilometers (200 miles) per hour. Speed is the rate at which an object covers distance in a period of time. It refers to how fast an object is moving. Speed is a ratio. It is calculated by dividing the distance traveled by the time spent traveling. It is measured in meters per second (m/s). Speed is closely related to velocity, which is the rate at which an object changes its position in a specific direction.

Energy Conversions in a Car

Race cars are built with powerful engines to increase force and acceleration (an increase in speed over time). Engines are important because they convert inputs of chemical potential energy stored in gasoline into outputs of kinetic energy that moves the car and heat. The average racing engine can push a car with the force of 750 horses.

Relationship between Energy, Mass, and Speed

There are many rules in car racing. This is to create an even playing field for all drivers and to keep the drivers safe. Safety is important because the cars move at such high speeds. The faster the car is moving, the more kinetic energy it has. This is why collisions are more dangerous at high speeds compared to low speeds.

The speed of an object is proportional to the amount of kinetic energy it has. It is a nonlinear relationship that changes exponentially. In an exponential relationship, the rate of change increases exponentially each period. When the speed of an object doubles, the kinetic energy quadruples. When the speed of an object halves, the kinetic energy decreases by a factor of four.

To help keep drivers safe, there are rules about how much mass a race car can have, just like in bobsledding. Race car drivers pay close attention to the mass of their cars. They know that more force is needed to cause massive objects to accelerate the same amount as less massive objects. Remember that a force is needed to transfer energy into a system, and the larger the force that is applied, the more energy is transferred into the system.

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Hands-on Science Activity

In this lesson, students design a two-part investigation to explore the relationship between the speed of a moving object and its kinetic energy, and the relationship between the mass of a moving object and its kinetic energy. Students gather data during the investigations to evaluate the phenomena of how the change in relative motion of two cars is influenced by their mass.

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Standards citation: NGSS Lead States. 2013. Next Generation Science Standards: For States, By States. Washington, DC: The National Academies Press. Neither WestEd nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.