[Grade 7 Science] Force and Its Effects: Uncovering the Secrets of Invisible "Force"
Hello! Let's work together to learn about "Force and Its Effects" in science.
We are surrounded by many different "forces." But, since force itself is invisible, it can be a bit mysterious, right?
In this chapter, we will clearly explain from the basics what these invisible forces actually do and how to represent them using numbers.
It might feel a bit challenging at first, but it will make perfect sense if you think about everyday examples!
1. What are the "effects" of force?
Force generally has three main roles. Whenever a force acts on something, one of the following is guaranteed to happen:
① Changing the shape of an object
If you squeeze a ball, it dents. This is the force "changing its shape."
Examples: Kneading clay, sitting on a cushion
② Changing the movement of an object
If you kick a ball that is at rest, it starts moving; if you catch a flying ball, it stops. Also, if you hit a flying ball back, its direction changes. These are all instances of force "changing movement."
Examples: Applying bicycle brakes, hitting a baseball with a bat
③ Supporting an object
Even if you put a book on a desk, it doesn't fall to the floor. This is because the desk is "supporting" the book.
Examples: Holding a bag with your hand, sitting on a chair
【Pro Tip!】
On tests, you might be asked, "List the three effects of force." Remember the trio: "deformation," "change in movement," and "supporting!"
2. Types of forces
There are many forces around us with specific names. Here are the typical ones you'll learn in the 7th grade.
① Elastic force
The force that tries to return an object to its original shape when a spring or rubber band is stretched.
Examples: Drawing a bow in archery, a trampoline
② Frictional force
The force that acts at the contact surface between objects to resist motion. The rougher the surface, the greater this force becomes.
Examples: Dragging a heavy box across the floor, the treads on the soles of shoes
③ Magnetic force
The "attractive force" or "repulsive force" between magnets.
Examples: Compasses, refrigerator magnets
④ Electric force (Static electricity)
The force that makes your hair stand up when you rub it with a plastic ruler.
⑤ Gravity
The force with which the Earth pulls objects toward its center. It acts on everything on Earth.
【Did you know?】
Forces can be divided into two groups: "contact forces" (like friction) and "non-contact forces" (like gravity and magnetic force). Magnets start pulling toward each other before they even touch because the force reaches across the distance!
3. How to represent force and units
In science, the magnitude of force is expressed in a unit called Newtons (\(N\)).
Unit of Force: Newton (\(N\))
As a rule of thumb, the gravitational force acting on an object of about \(100g\) is approximately \(1N\).
・\(1\) apple (\(100g\)) ≒ \(1N\)
・\(1\) milk carton (\(1kg\) or \(1000g\)) ≒ \(10N\)
The Three Elements of Force
When drawing force using a diagram (an arrow), you need these three pieces of information:
1. Point of application: The point where the force begins to act (the base of the arrow).
2. Direction of force: The direction in which the force is acting (the direction of the arrow).
3. Magnitude of force: The strength of the force (the length of the arrow).
【Common Mistake】
When drawing an arrow, the point of application should be placed inside the "object receiving the force." Also, the rule is to start drawing the arrow for gravity from the center of the object (center of gravity)!
4. The difference between "Mass" and "Weight" (Super Important!)
This is where many people get confused, so let’s be sure to distinguish them!
● Mass
・The amount of matter in the object itself, which does not change regardless of location.
・Units are \(g\) (grams) or \(kg\) (kilograms).
・Measured with a balance scale.
● Weight
・The "magnitude of gravity" acting on an object.
・Changes depending on the location (Earth, Moon, etc.)!
・Unit is \(N\) (Newtons).
・Measured with a spring scale.
【Example: Going to the Moon】
The moon's gravity is about one-sixth that of Earth.
If a person with a mass of \(60kg\) goes to the moon:
・Their mass remains \(60kg\) (the body itself doesn't shrink).
・Their weight decreases from \(600N\) on Earth to \(100N\) (they feel floaty).
5. Spring extension and force (Hooke's Law)
When you pull a spring, it stretches, and there is a neat rule to this.
Hooke's Law
This is the law that "the extension of a spring is proportional to the magnitude of the force applied."
In other words, if you double the force, the "extension" of the spring also doubles.
【Caution!】
A common test trick is the difference between "total spring length" and "spring extension"!
・\(Total\ length\ of\ spring = Original\ length + Extension\)
In calculation problems, always focus only on the "extension" part.
6. Pressure
We learn that how a force is transmitted depends on the "area" over which it acts.
Pressure Formula
\(Pressure [Pa] = \frac{Force\ pressing\ vertically\ on\ a\ surface [N]}{Area\ where\ the\ force\ acts [m^2]}\)
The unit used is Pascals (\(Pa\)). \(1Pa = 1N/m^2\).
【Visualize it!】
When walking on snow, you sink in normal shoes, but if you wear "snowshoes" (which have a large surface area), you don't sink. This is because by increasing the area, you are reducing the pressure.
【Calculation Tip】
The hardest part is converting the area unit from \(cm^2\) to \(m^2\).
\(1m^2 = 10000cm^2\). If you're ever stuck, just imagine a square that is \(100cm \times 100cm\)!
Summary: Key points of this chapter
・Force has three effects: "deformation," "change in movement," and "supporting."
・The magnitude of force is expressed in "Newtons (\(N\))" (\(100g \approx 1N\)).
・"Mass" is constant, while "weight" is the magnitude of gravity and changes depending on location.
・Hooke's Law: The "extension" of a spring is proportional to the force.
・Pressure increases as the area decreases.
Great job! Although "force" is invisible, using formulas and diagrams makes it clear and understandable. Start by getting a good feel for the \(100g = 1N\) rule. I'm cheering for you!