Scientific Definition: Force
1. Force as a Push or a Pull
Imagine you are playing with a soccer ball. When you kick it, you are applying a force. A force is simply a push (moving the ball away) or a pull (bringing it closer). You cannot see a force directly, but you can always see its effect: it makes things start moving, stop moving, or change how they move.
For example, when you sit on a chair, your body pushes down on the seat (applied force), and the chair pushes up against you (support force). Even if nothing seems to be moving, forces are still acting. This is summed up neatly by Newton's First Law: An object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
2. The Science of Force: Vectors and Units
Because force has a direction, scientists call it a vector. Knowing how hard you push (the magnitude) is not enough; you also need to know which way you are pushing. The standard unit for measuring force is the Newton (N), named after Sir Isaac Newton.
The mathematical relationship between force, mass, and acceleration is given by Newton's Second Law:
3. Everyday Examples of Force at Work
Let's look at two common types of forces in our daily lives. Understanding these examples helps connect the scientific definition to the real world.
| Type of Force | Description | Everyday Example |
|---|---|---|
| Contact Force | Requires physical touch between objects. | Pushing a broken-down car, friction slowing down a bicycle, or holding a book in your hand. |
| Non-Contact Force | Acts at a distance without physical touch. | A magnet pulling a paperclip, gravity keeping the Moon in orbit around Earth, or static electricity making your hair stand up. |
4. Forces in Action: The Elevator Experiment
A great way to feel forces is to ride an elevator. Stand on a bathroom scale inside an elevator and watch the reading change:
- When the elevator starts going up, the scale shows a higher number. You feel heavier because the floor is pushing up on you with a force greater than your weight (acceleration).
- When it moves at a constant speed, the scale shows your normal weight. The forces are balanced (net force is zero).
- When it slows down at the top, the scale shows a lower number. You feel lighter.
This happens because the scale measures the normal force (the support force), which changes as the elevator accelerates and decelerates.
Important Questions
✅ A1: Mass is the amount of matter in an object (measured in kilograms). Weight is the force of gravity acting on that mass (measured in Newtons). Your mass stays the same on Earth and the Moon, but your weight changes because the Moon's gravity is weaker.
✅ A2: Because forces can cancel each other out. This is called equilibrium. For example, the force of gravity pulling you down is balanced by the force of the floor pushing you up. When the net force (the total of all forces) is zero, your motion does not change.
✅ A3: Rockets work because of Newton's Third Law: For every action, there is an equal and opposite reaction. The rocket engine pushes hot gases out the back (action), and those gases push the rocket forward (reaction). They don't need air to push against; they push against the exhaust gases themselves.
Conclusion
Footnote
[1] Newton (N): The SI unit of force. One Newton is the force required to accelerate a mass of one kilogram at a rate of one meter per second squared ($1\ N = 1\ kg \cdot m/s^2$).
[2] Vector: A quantity that has both magnitude (size) and direction. Force, velocity, and acceleration are vectors.
[3] Net Force: The overall force acting on an object after all the individual forces are combined. If the net force is zero, the object's motion doesn't change.