2.0 — Force: Background Content and Teacher Information

Content Information

Force

Force is an interaction between objects that can cause a change in the acceleration, speed or direction of an object. It is helpful to have students describe the forces that exist on objects that are interacting.

Some students may think a force is a property of a specific object. The amount of force an object exerts and experiences depends on the object’s mass and acceleration, but the force itself is not stored inside the object. Students’ tendency to talk about forces as being a part of an object comes from the way we talk about forces in everyday language. If a student is explaining a wrecking ball smashing through a wall as having force, they are likely thinking of force as a property of that wrecking ball rather than the force being present in the collision of the wrecking ball and the wall. Both the wrecking ball and the wall must interact in order for the forces to be present.

Balanced and unbalanced forces explain why force can cause changes in motion. Balanced forces are in

equilibrium so the forces are equal and opposite. An example of this would be a person holding a ball in their hand. The person’s hand is pushing up on the ball and the ball is pushing back down on their hand. Once the person releases the ball it is no longer in equilibrium. Gravity is pulling down on the ball and there is no opposing force so the ball will start to fall. The ball accelerates because of the unbalanced force of gravity pulling it down. Forces can be present even when the objects are not touching. Think about a ball flying through the air in a game of catch. The pull of Earth’s gravity on the ball will bring it back down to the ground. Planets in orbit experience forces that keep them in orbit without directly touching the star it is orbiting.

Newton’s Third Law Of Motion

Newton’s Third Law is the least intuitive of Newton’s three laws of motion. Students often have difficulty understanding that all interactions involve equal forces. It does seem strange that we exert just as much force on the ground as the ground does on us when we think about how much more massive the Earth is than any single person. We can’t see any distortion or feel the pressure of our feet on the ground. When presented with the classic example of a book resting on a table, many students think that the table is just in the way of the book getting to the ground. Even if we can’t see it or feel it, forces are present in both directions. If an object is not moving, there always has to be equal and opposite forces acting on it. If there are unbalanced forces, there will always be motion. Equal and opposite forces are sometimes easier to think about in zero gravity environments, like outer space, where forces can be singled out. If an astronaut pushes the box, the motion and position of the box will change in accordance with the push, and the astronaut will also be pushed in the opposite direction from the box at the same rate.

Force Pairs (Push/Pull)

The terms “push” and “pull” are sometimes used to describe how forces change the motion of objects. How much change occurs is related to how strong the pushes and pulls are.

Patterns of Forces: Jumping Rope

Many of the lessons in this unit revolve around the physical activity of jumping rope. These lessons focus on noticing the movement of the person jumping, describing the forces related to the person jumping, and looking for the patterns in the forces of the person jumping. Below are some of the patterns your students may notice about the forces involved with jumping rope.

Forces on the person jumping:

• When the person jumps, their feet push down on the ground and the ground pushes back on them. This is the force students are most likely to label with a push sticker in the Playground Physics app.
• The person jumping moves up until gravity pulls them back down towards the ground.

Other forces present in the act of jumping rope:

• Friction between the jump rope and the ground.
• Since we are jumping rope on Earth, gravity will be pulling down on everything in the scenario, since we are jumping rope on Earth.
• The hands of the people holding the jump rope are exerting a force on the handles of the jump rope as they turn the rope.
• If the person jumping rope doesn’t jump high enough, their leg will exert a force on the rope and vice versa bringing the rope to a stop.
• Friction between the rope and the air as it turns around. The rope applies force to the air molecules.

Common Misconceptions

• Often students have trouble understanding that objects that are at rest still exert forces. Consider a book resting on a table. The book is pushing down on the table and the table is pushing up on the book. Because you can’t see the forces, they can be more difficult to think about. A great way to give students a physical example of the forces present is for them to act as the table. Have a student hold their arms straight out in front of them. Place a heavy book on their arms. To keep the book steady the student will have to push up on the book. By providing enough upward force, the student creates a state of equilibrium, which is also the state that the book and the table are in.
• Students may associate force with living things. Living beings are just one of the many things that experience force. Students’ physical experience moving things with their muscles can make it difficult for them to think of objects exerting force that do not have the same kind of muscular strength and physical activity that humans and animals do.
• It can be hard for students to conceptualize friction as a force. We are so used to friction that we don’t even think about it. A ball rolling on the ground will eventually come to a stop because of the constant resistance it experiences from friction. There is friction between your hand and a fork otherwise the fork would slide out of your hand immediately. Since all of our experiences on Earth involve friction, it can be difficult for students to imagine what would happen without it.