Lesson 25: Understanding Force
A force is a push or pull with both size and direction. This lesson covers contact and non-contact forces, measurement, and how balanced and unbalanced forces affect motion and structures.
Key Ideas
- A force is a push or a pull. Forces can act on an object even when it stays still.
- Force and energy describe different things. A push is a force; energy can be stored or passed from one thing to another.
- Earth’s gravity pulls objects toward Earth’s center. An object’s weight tells us how strongly gravity pulls on it.
- Balanced forces do not change an object’s motion. Unbalanced forces can make it speed up, slow down, or turn.
- More than one force can act at once. To understand an object’s motion, we consider all the pushes and pulls on it.
Vocabulary
- Force: A push or pull on an object.
- Gravity / Weight: Gravity is a pulling force. Weight is how strongly gravity pulls on an object.
- Newton: A unit used to measure force. A spring scale can measure a push or pull in newtons.
- Kinetic / Potential Energy: Kinetic: energy of motion. Potential: energy stored through position, arrangement, or condition.
- Magnetic Force: A push or pull caused by a magnet. It can act without touching.
- Friction: A contact force that resists motion between two surfaces
Discussion Questions
- Can you exert a force on something without making it move? Give an example and explain why.
- What is the difference between force and energy? Why might it be confusing to think of them as the same thing?
- What is the difference between a contact force and a non-contact force? Can you give an example of each?
- If two people push on a box with equal force in opposite directions, what happens? What if one person pushes harder?
- Why do engineers need to think about forces when designing something like a bridge or a building? What could happen if they didn't?
Hands-On Activity: Paper Bridge Challenge
Supply List
- 3 index cards
- A small piece of tape
- 2 books of equal height to serve as bridge supports
- Pennies or other small weights
- A ruler
Instructions
- Set two books about 4 inches apart on a flat surface. These are your bridge supports.
- Using only 3 index cards and tape, build a bridge that spans the gap between the two books.
- You may fold, roll, layer, or shape the index cards in any way — but the bridge must be built entirely from the cards and tape, and it must span the full gap.
- Once your bridge is built, place pennies in the center one at a time. Count how many pennies your bridge holds before it collapses.
- Observe where the bridge fails first. Is it the center? The edges? What force caused it to collapse?
- Rebuild using a different design and test again. Compare your two results.
Discussion: Ask learners what forces were acting on the bridge at all times (gravity pulling down, the books pushing up). What made some bridge shapes stronger than others? How do real bridge engineers solve this same problem?