Kinetic and Potential Energy

The bobsled is an event in the winter Olympics. In this event, each sled and its riders speed down an icy track. Which factor(s) affect how much kinetic energy the sled and its team have when the sled crosses the finish line?

• mass only

• speed and time

• speed and mass

• speed, mass, and time

Answer explanation

The correct answer is C. Kinetic energy depends on the speed and the mass of the object. Time is not a separate factor affecting kinetic energy. It is included in speed, which is distance per time.

Bonito has potential energy while he is in the starting blocks, getting ready to run his 5-kilometer race. Which of the following statements are true about his potential energy? Choose the three statements that apply.

• It can do work.

• It is stored energy.

• It might be used later.

• It can be changed into kinetic energy.

Answer explanation

The correct answers are B, C, D. Potential energy is stored energy that might be used later, or energy of position. It is not energy of motion, which is kinetic energy. Potential energy can be transformed into kinetic energy when an object at rest starts to move. Potential energy does not do work, because there is no motion.

George rolled four different types of paper into four different cylinders. Identical 10 centimeter rubber bands were placed around the cylinders to hold the paper in place. Each number in the answer choices is the diameter of one of the cylinders. Which cylinder has the rubber band with the greatest elastic potential energy?

• 5 centimeter

• 10 centimeter

• 20 centimeter

• 25 centimeter

Answer explanation

The correct answer is D. Elastic potential energy is based on how much an object is stretched or compressed. For a rubber band, it is based on how much it is stretched. The larger the object is that the rubber band has to go around, the more the rubber band is stretched, and the greater its elastic potential energy.

Two sites on the same river have been surveyed as a place to build a dam. The dam will be part of a hydroelectric power plant. Site A is at the top of a 50-meter waterfall. Site B is at the top of a 35-meter waterfall. Which answer correctly describes where the water will have the greater gravitational potential energy to be converted into electricity?

• Site A

• Neither site has gravitational potential energy to convert to electricity.

• Both sites will produce the same amount of electricity as the water flows over the water fall.

• At both sites the water has the same gravitational potential energy.

Answer explanation

The correct answer is A. Gravitational potential energy is the energy stored in an object due to its position above the Earth’s surface as a result of the force of gravity acting on the object. The higher the object is above the surface of Earth, the greater the stored energy is. The object in this case is the water stored behind the dam. This means, the higher the dam is built above the Earth’s surface the more potential energy the water mass has and the more electricity can be generated.

Which of the following is an example of kinetic energy?

• a roller coaster paused at its highest point

• a bicycle rolling downhill

• a boulder sitting at the top of a mountain

• a ball at the top of its trajectory

Answer explanation

The correct answer is B. Kinetic energy is the energy of motion. The bicycle rolling downhill is using energy of motion. A roller coaster paused at its highest point, a boulder sitting at the top of a mountain, and a ball at the top of its trajectory are describe moments where those objects have potential, or stored, energy.

Marcus is hanging a porch swing on his front porch. The first thing he has to do is attach two springs to the ceiling. Then, he fastens the swing to the springs so that they can stretch and support the weight of the swing. Which two types of potential energy do the springs have? Choose the two statements that apply.

• elastic

• gravitational

• kinetic

• stretched

Answer explanation

The correct answers are A and B. Springs have mass, so they have GPE that depends on their height, and they have elastic energy when they are stretched. The springs have no kinetic energy when they are not moving, so C is not correct. Stretched is a description of the springs, not of their energy, so D is not correct.

Choose the phrase and the number that correctly complete the paragraph.

Jaime has a 20-N bird feeder on a 2-m pole in his yard. The gravitational potential energy (GPE) of the bird feeder is the result of ______________________.

• its speed and mass

• its gravitational pull of the Earth

• its kinetic energy

• its elasticity

Answer explanation

The correct answer is the gravitational pull of Earth. Gravitational potential energy is the product of weight and height.

Chloe is training for her first half-marathon, which requires her to run 21.1 km. Before she runs, she carefully chooses the foods that will give her the energy her body requires for the race. She stands still at the starting line, and then she starts running, her body warming up as she runs.

Which three energy transformations take place in this example?

• chemical energy to kinetic energy

• chemical energy to thermal energy

• mechanical energy to chemical energy

• potential energy to kinetic energy

Answer explanation

The correct answers are A, B, and D. Chloe converts chemical potential energy from food to kinetic energy when she runs, and she also converts chemical energy to thermal energy, because she gets warm. Chloe does not convert her mechanical energy back into the chemical energy of food, so C is not correct.

As of 2017, the tallest roller coaster in the world is the Kingda Ka, located at Six Flags Great Adventure in Jackson, New Jersey. It stands 139 m tall and, at one point, the cars drop 127 m! Roller coasters are a great way to investigate energy. Look at the diagram of a roller coaster, and answer the following questions.

At what point on the track does the car have the greatest potential energy? The greatest kinetic energy?

• At the ending point; at the starting point

• At the starting point; at the ending point

• At the highest point on the track; at the lowest point on the track

• At the lowest point on the track; at the highest point on the track

Answer explanation

The correct answer is C. Potential energy is highest at the highest point on the track, so A, B, and D are not correct.

Her teacher told Fatima that all types of energy are kinetic energy, potential energy, or both. Fatima drew the diagram below to help her organize this information. She then sorted each type of energy into its correct category.

Types of energy: chemical, electrical, electromagnetic, mechanical, nuclear, thermal

• chemical

• electromagnetic

• mechanical

• thermal

Answer explanation

The correct answer is D. Thermal energy is the kinetic energy of atoms and molecules. A, B, and C are all forms of energy may be potential energy, so A, B, and C are not correct.