Relative motion between two objects is apparent a. even at everyday speeds, such as \(10 \mathrm{km} / \mathrm{h}\). b. only at very large speeds, such as \(0.8 c\). c. only near very large masses. d. only when both objects are in the same reference frame.

A reference frame is a perspective from which motion is observed and measured. Think of it as your personal viewpoint or 'camera angle' on the world. Everyone and everything has its own reference frame. For example, if you're sitting in a car, your reference frame is the car itself.
Reference frames can be:

• Inertial: Moving at a constant speed or being stationary.
• Non-inertial: Accelerating or decelerating.

The key point is that the laws of physics work the same, no matter your reference frame. What changes is how you observe motion. An object's speed and direction may look different, depending on whether you're standing still or moving yourself.

Relative motion is not just a concept for scientists or space travelers. It's something you experience every day. Even at speeds as low as 10 km/h, you can notice the effects of relative motion. For instance, consider two cars driving side by side. Even a slight difference in their speeds can be easily noticed. You might feel like one car is moving forward or backward relative to yours, even if both cars are moving.
This concept is crucial for activities such as driving, walking, or even swimming. Whenever two objects move relative to each other, you observe relative motion. It helps you make sense of the world in a very practical manner, helping you navigate safely and accurately.

Observing motion involves comparing the movement of one object relative to another. This is different from absolute motion, which would require a fixed point in space (something that doesn't practically exist). When we observe motion, we use another object or point as a frame of reference.
Here are some everyday examples:

• Watching trees move past you when you're on a train.
• Seeing the ground move beneath your feet while walking.
• Noticing that a plane flies by rapidly when you're on the ground, but seems slower if you're flying next to it.

All these observations depend on your frame of reference. This is why motion can look different from different viewpoints. Understanding these differences can help you grasp how relative motion works in real-life scenarios and appreciate its significance.