When Motion Changes, Energy Is Transferred
Learn how kinetic energy transfers between objects when motion changes, and identify which object gives and receives energy in real-world collisions and interactions.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on When Motion Changes, Energy Is Transferred, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
When a cue ball hits a pool ball, something remarkable happens: the cue ball slows down while the struck ball speeds up. Where did the moving ball's energy come from? It wasn't created—it came from the cue ball. Energy doesn't appear out of nowhere; it transfers from one object to another whenever motion changes. In this lesson, you will learn to recognize energy transfer in collisions, friction, and impacts, and you'll practice identifying which object gave energy and which one received it.
What Is Energy Transfer?
Energy transfer happens when one object does work on another object, causing a change in motion. When object A pushes or pulls on object B, energy moves from A to B. The kinetic energy of the second object increases (it moves faster), while the kinetic energy of the first object decreases (it moves slower).
The key idea is that kinetic energy doesn't vanish—it moves. A cue ball striking a billiard ball transfers some of its kinetic energy to the stationary ball. The cue ball's speed drops because it has less kinetic energy after the collision. The struck ball's speed increases because it now has the kinetic energy that was transferred to it.
Energy transfer is not the same as energy conservation. Conservation means the total energy in a closed system stays the same. Transfer means energy moves from one object to another. Both are true: energy is conserved overall, but specific objects lose and gain kinetic energy through transfer.
The key idea is that kinetic energy doesn't vanish—it moves. A cue ball striking a billiard ball transfers some of its kinetic energy to the stationary ball. The cue ball's speed drops because it has less kinetic energy after the collision. The struck ball's speed increases because it now has the kinetic energy that was transferred to it.
Energy transfer is not the same as energy conservation. Conservation means the total energy in a closed system stays the same. Transfer means energy moves from one object to another. Both are true: energy is conserved overall, but specific objects lose and gain kinetic energy through transfer.
Identifying the Giver and Receiver
In any energy transfer, two roles are always present: the object that gives energy and the object that receives it. To identify them, look at what happens to each object's motion.
The giver is the object that slows down or loses kinetic energy. Its speed decreases, which means its kinetic energy decreases. Examples: a cue ball after it strikes another ball; a bicycle when its brakes are applied; a ball after being caught.
The receiver is the object that speeds up or gains kinetic energy. Its speed increases, which means its kinetic energy increases. Examples: a struck pool ball; a bicycle's brake pad (which heats up); a stationary ball that is kicked.
You can identify givers and receivers by comparing the motion before and after the interaction. If an object's speed went down, it gave energy away. If an object's speed went up (or from zero to some speed), it received energy. Be careful: sometimes an object is both—for example, when two moving balls collide, one may slow down (giver) while the other speeds up (receiver), but the one that slows down has given energy to the other.
The giver is the object that slows down or loses kinetic energy. Its speed decreases, which means its kinetic energy decreases. Examples: a cue ball after it strikes another ball; a bicycle when its brakes are applied; a ball after being caught.
The receiver is the object that speeds up or gains kinetic energy. Its speed increases, which means its kinetic energy increases. Examples: a struck pool ball; a bicycle's brake pad (which heats up); a stationary ball that is kicked.
You can identify givers and receivers by comparing the motion before and after the interaction. If an object's speed went down, it gave energy away. If an object's speed went up (or from zero to some speed), it received energy. Be careful: sometimes an object is both—for example, when two moving balls collide, one may slow down (giver) while the other speeds up (receiver), but the one that slows down has given energy to the other.
Energy Transfer Through Contact Forces
Energy transfer happens through contact forces—when two objects actually touch or press against each other. When the cue stick pushes the cue ball, it exerts a force that does work on the ball, transferring energy to it. When the cue ball strikes another ball, it does the same thing. These are not magic or invisible; they are physical pushes.
Brakes demonstrate energy transfer clearly. When you apply bicycle brakes, brake pads press against the wheel rim. Friction between the pads and rim exerts a force that slows the wheel down. The kinetic energy of the wheel doesn't disappear—it transfers into thermal energy (heat) in the brakes. That is why bicycle brakes get hot. The faster you were going and the harder you brake, the more kinetic energy transfers to heat, and the hotter the brakes become.
Another example: a bat hits a baseball. The bat exerts a large force on the ball for a short time. This force does work on the ball, transferring energy to it. The ball leaves the bat moving much faster than it was before (often from zero speed to 90+ miles per hour). The bat slows down slightly because it gave energy to the ball. The force of contact is what makes the transfer possible.
Brakes demonstrate energy transfer clearly. When you apply bicycle brakes, brake pads press against the wheel rim. Friction between the pads and rim exerts a force that slows the wheel down. The kinetic energy of the wheel doesn't disappear—it transfers into thermal energy (heat) in the brakes. That is why bicycle brakes get hot. The faster you were going and the harder you brake, the more kinetic energy transfers to heat, and the hotter the brakes become.
Another example: a bat hits a baseball. The bat exerts a large force on the ball for a short time. This force does work on the ball, transferring energy to it. The ball leaves the bat moving much faster than it was before (often from zero speed to 90+ miles per hour). The bat slows down slightly because it gave energy to the ball. The force of contact is what makes the transfer possible.
Evidence That Energy Was Transferred
You can recognize energy transfer by observing changes in motion. If an object that was at rest starts moving, or an object that was moving speeds up, energy was transferred to it. If an object slows down or stops, energy was transferred away from it.
Common evidence of energy transfer includes: a struck object suddenly moving; an object speeding up after being pushed; an object slowing down or stopping; and heat production (like hot brakes). Heat is especially important because it shows that kinetic energy has been transformed into thermal energy.
When constructing an argument that energy was transferred, cite the specific observations: the cue ball's speed decreased (it gave energy), the struck ball's speed increased (it received energy), and the force of contact between them was the mechanism. You should always name both objects and explain what happened to each one's motion. Vague statements like "energy moved" are not enough; you need to point to the motion changes that prove it.
Remember: if you see a change in speed, energy transfer has occurred. The faster or more massive the object, the more kinetic energy it has to give or receive.
Common evidence of energy transfer includes: a struck object suddenly moving; an object speeding up after being pushed; an object slowing down or stopping; and heat production (like hot brakes). Heat is especially important because it shows that kinetic energy has been transformed into thermal energy.
When constructing an argument that energy was transferred, cite the specific observations: the cue ball's speed decreased (it gave energy), the struck ball's speed increased (it received energy), and the force of contact between them was the mechanism. You should always name both objects and explain what happened to each one's motion. Vague statements like "energy moved" are not enough; you need to point to the motion changes that prove it.
Remember: if you see a change in speed, energy transfer has occurred. The faster or more massive the object, the more kinetic energy it has to give or receive.
Common Misconceptions
One common wrong idea is that energy disappears when an object stops. It does not. When a bicycle brakes, the kinetic energy does not vanish—it converts to heat in the brakes and surrounding air. When a ball is caught, the kinetic energy does not disappear—it converts to thermal energy, sound, and motion of the person catching the ball.
Another misconception is that the object that receives energy must start from rest. It does not have to. Two moving objects can collide, and one can transfer additional energy to the other, speeding it up even further. Conversely, a moving object can transfer energy away and slow down, even if it does not stop completely.
Students sometimes think that if one object speeds up, the other must slow down by the same amount. This is not always true in real collisions because energy can be lost to heat, sound, and deformation. A tennis ball hit by a racket speeds up a lot while the racket slows down only a little. Not all the racket's energy goes into the ball—some becomes heat and sound.
Finally, do not assume that the object that was moving first is always the giver. In a collision between a fast-moving object and a slow-moving object, the slow object can still gain energy (become the receiver) from the faster one.
Another misconception is that the object that receives energy must start from rest. It does not have to. Two moving objects can collide, and one can transfer additional energy to the other, speeding it up even further. Conversely, a moving object can transfer energy away and slow down, even if it does not stop completely.
Students sometimes think that if one object speeds up, the other must slow down by the same amount. This is not always true in real collisions because energy can be lost to heat, sound, and deformation. A tennis ball hit by a racket speeds up a lot while the racket slows down only a little. Not all the racket's energy goes into the ball—some becomes heat and sound.
Finally, do not assume that the object that was moving first is always the giver. In a collision between a fast-moving object and a slow-moving object, the slow object can still gain energy (become the receiver) from the faster one.
Key terms
- Kinetic energy.
- The energy an object has because of its motion; depends on both the object's mass and its speed, calculated as .
- Energy transfer.
- The process by which energy moves from one object to another, usually through a contact force that does work on an object.
- Work.
- The energy transferred to or from an object by a force acting over a distance; when a force pushes or pulls an object and causes it to move, work is being done.
- Contact force.
- A force that occurs when two objects physically touch, such as friction, normal force, or the force of a bat striking a ball.
- Thermal energy.
- The energy associated with the random motion of atoms and molecules in a substance; often produced when kinetic energy is dissipated through friction or collision.
- Energy giver.
- An object that loses kinetic energy during an interaction and transfers that energy to another object.
- Energy receiver.
- An object that gains kinetic energy during an interaction, usually becoming faster or starting to move.
Worked example
A bowling ball traveling at 8 meters per second rolls into a stationary pin. After the collision, the bowling ball slows to 6 meters per second and the pin flies off at 4 meters per second. Construct an argument that energy was transferred from the ball to the pin, identifying which object gave energy and which received it.
Step 1: Identify the motion before and after. Before collision, the bowling ball moves at 8 m/s and the pin is stationary (0 m/s). After collision, the ball moves at 6 m/s and the pin moves at 4 m/s.
Step 2: Determine which object slowed down. The bowling ball went from 8 m/s to 6 m/s, so it slowed down. This means the ball's kinetic energy decreased. Therefore, the bowling ball is the energy giver.
Step 3: Determine which object sped up. The pin went from 0 m/s to 4 m/s. It started moving and increased its speed. This means the pin's kinetic energy increased. Therefore, the pin is the energy receiver.
Step 4: Identify the mechanism of transfer. The bowling ball and pin were in contact during the collision. The ball exerted a contact force on the pin, doing work on it and transferring kinetic energy to it.
Step 5: Construct the argument. "The bowling ball transferred energy to the pin. The evidence is that the ball's speed decreased from 8 m/s to 6 m/s, showing it lost kinetic energy. At the same time, the pin's speed increased from 0 m/s to 4 m/s, showing it gained kinetic energy. The contact force between them during the collision was the mechanism that allowed this energy to transfer from the ball (the giver) to the pin (the receiver)."
Step 2: Determine which object slowed down. The bowling ball went from 8 m/s to 6 m/s, so it slowed down. This means the ball's kinetic energy decreased. Therefore, the bowling ball is the energy giver.
Step 3: Determine which object sped up. The pin went from 0 m/s to 4 m/s. It started moving and increased its speed. This means the pin's kinetic energy increased. Therefore, the pin is the energy receiver.
Step 4: Identify the mechanism of transfer. The bowling ball and pin were in contact during the collision. The ball exerted a contact force on the pin, doing work on it and transferring kinetic energy to it.
Step 5: Construct the argument. "The bowling ball transferred energy to the pin. The evidence is that the ball's speed decreased from 8 m/s to 6 m/s, showing it lost kinetic energy. At the same time, the pin's speed increased from 0 m/s to 4 m/s, showing it gained kinetic energy. The contact force between them during the collision was the mechanism that allowed this energy to transfer from the ball (the giver) to the pin (the receiver)."
Practice questions
A soccer ball at rest on the ground is kicked and flies through the air at 15 meters per second. Which of the following best describes the energy transfer that occurred?
- Energy was transferred from the ground to the ball because the ball started moving.
- Energy was transferred from the kicker's foot to the ball because the foot exerted a contact force that increased the ball's speed.
- Energy was transferred from the air to the ball because the ball moves through the air.
- Energy was transferred from the ball to the ground because the ball is no longer touching the ground.
Answer: Energy was transferred from the kicker's foot to the ball because the foot exerted a contact force that increased the ball's speed.
The soccer ball gained kinetic energy, which means it received energy from another object. The contact force came from the kicker's foot, which pushed the ball and did work on it. The ground did not transfer energy (the ball was already resting on the ground with zero kinetic energy, so contact with the ground is not what caused the change). The air does not push the ball faster—it actually resists motion through air resistance. The ball does not transfer energy to the ground by leaving it; instead, the kicker's foot transferred energy to the ball.
A bicycle is moving down a hill at 10 meters per second. The rider applies the brakes, and the bicycle slows to 4 meters per second. The brake pads become very hot. Identify the energy giver and energy receiver in this situation, and explain where the kinetic energy went.
Answer: The bicycle (or the rider and bicycle system) is the energy giver because its speed decreased from 10 m/s to 4 m/s, so its kinetic energy decreased. The brake pads are the energy receiver because they absorbed the kinetic energy and converted it to thermal energy (heat). The kinetic energy that the bicycle lost was transferred to the brake pads and surrounding materials, where it was transformed into heat. This is why the brakes become hot.
When the brakes are applied, friction between the brake pads and the wheel rim exerts a force that opposes the bicycle's motion. This force does negative work on the bicycle, removing its kinetic energy. That energy does not disappear—it is transferred to the brake pads, which heat up. You know energy was transferred because the bicycle clearly slowed down (lost kinetic energy) and the brakes clearly became hot (gained thermal energy). The amount of heat produced depends on how much kinetic energy the bicycle had and how much it slowed down. This is why braking from a higher speed produces more heat than braking from a lower speed.
Two ice skaters push off each other. Skater A (heavier) was moving at 3 meters per second before the push, and slows to 1 meter per second after. Skater B (lighter) was moving at 2 meters per second before the push, and speeds up to 4 meters per second after. Which skater gave energy and which received energy during the push?
Answer: Skater A gave energy because his speed decreased from 3 m/s to 1 m/s. Skater B received energy because her speed increased from 2 m/s to 4 m/s. The contact force between them during the push transferred kinetic energy from Skater A to Skater B.
In any collision or contact interaction, look at the change in speed for each object. If speed went down, that object is a giver (lost kinetic energy). If speed went up, that object is a receiver (gained kinetic energy). Here, Skater A slowed down, so Skater A gave energy away. Skater B sped up, so Skater B received energy. The fact that they pushed against each other with a contact force is what made the energy transfer possible. Note that the lighter skater sped up more than the heavier skater slowed down—this is normal and does not violate conservation of energy because kinetic energy depends on both mass and speed, and the lighter skater's small mass means she can gain a lot of speed from less energy.
FAQ
- Why does a cue ball stop moving after it hits another ball?
- The cue ball doesn't always stop completely, but it does slow down because it transfers kinetic energy to the struck ball through the contact force of the collision. The more energy it transfers, the more it slows down. If the collision is direct and the cue ball transfers enough energy, it can nearly stop while the struck ball moves away with most of that energy.
- Where does the energy go when brakes make a bicycle stop?
- The kinetic energy does not disappear. Friction between the brake pads and the wheel rim converts the kinetic energy into thermal energy (heat). That heat is released into the brake pads, the wheel, and the surrounding air. The hotter the brakes become, the more kinetic energy was transferred and converted to heat. You can feel the heat by carefully touching the brake area after hard braking.
- Can two objects both gain kinetic energy from each other?
- No, not from each other directly. In an interaction between two objects, one must lose kinetic energy (the giver) and one must gain it (the receiver). However, both objects can gain kinetic energy from a third source—for example, if you push two blocks that were at rest, both gain kinetic energy from your pushing force, but they are both receivers in that interaction.
- Is energy transfer the same as energy conservation?
- No, they are related but different. Energy transfer means kinetic energy moves from one object to another—one loses it, one gains it. Energy conservation means the total energy in a closed system stays the same (although it may change form, like kinetic energy becoming heat). When a bicycle brakes, kinetic energy is transferred from the bicycle to the brake pads, where it becomes thermal energy. The energy is conserved overall because the kinetic energy became heat, but the energy was definitely transferred from the bike to the brakes.
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The Crimsora tutor teaches When Motion Changes, Energy Is Transferred live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.