4th Grade Physical Science - Lesson 2.1-2.3

A force is a push or pull that can cause an object to start moving. When a force is applied to an object, it can overcome any resistance or inertia and initiate motion. Therefore, a force is the reason behind the initiation of movement in an object.

Fire is classified as a chemical reaction because it involves the rapid combination of oxygen with a fuel source, resulting in the release of heat, light, and various combustion products. This reaction is known as combustion, which is a chemical process that involves the breaking and forming of chemical bonds. Fire cannot be classified as a physical reaction because it does not involve a change in state or composition of matter without the involvement of a chemical reaction.

When a substance changes states, such as from solid to liquid or liquid to gas, it undergoes a physical change. This means that the substance's physical properties, such as its shape, size, and state of matter, are altered, but its chemical composition remains the same. In a physical change, the molecules or atoms of the substance rearrange themselves, but no new substances are formed. This is different from a chemical change, where new substances with different chemical compositions are produced. Therefore, the correct answer is physical.

According to Newton's laws of motion, an object in motion will continue to move in the same direction at the same speed unless acted upon by an external force. Therefore, the statement "If you kick a moving ball, it will keep moving in the same direction at the same speed" is true according to Newton's laws.

Molecules are too small to be seen with the naked eye. The size of water molecules is extremely tiny, making them impossible to observe directly. Even with the aid of a microscope, the size of water molecules is still beyond the resolution capabilities. Therefore, the inability to see molecules moving in water is primarily due to their minuscule size.

The yellowish-orange color of a flame is caused by glowing dust particles. When a substance burns, it releases energy in the form of heat and light. The heat causes nearby particles, such as dust or soot, to become hot and emit light. These glowing particles give the flame its characteristic color. Fire, burnt gases, and oxygen do play a role in the combustion process, but they are not directly responsible for the color of the flame.

The three states of matter are liquid, solid, and gas. These states are based on the arrangement and movement of particles. In a solid, particles are tightly packed and have a fixed shape and volume. In a liquid, particles are close together but can move and flow, taking the shape of their container. In a gas, particles are spread out and move freely, filling the entire space available to them. These states of matter are fundamental and can be observed in various substances and conditions.