The task & goals
In this task, students identify the forces acting on a ball during and after a kick. Primarily the task is addressing whether or not students know that contact forces (which might transfer energy or momentum) exist only when the objects are in contact. However, to make sense of the ideas, it also helps for students to know that unbalanced forces cause acceleration, and so acceleration will indicate the presence of an unbalanced force. Because all interesting motion happens in the "x-direction," we anticipate that students might not attend to the normal force and/or gravity.
Student responses
Student responses are modified for clarity. The first response has the movement of the ball identified as a force; the non-moving ball resists going forward and the forward-moving ball has a "forward force." The second attributes that force not as inertia-like, but as being transfered from the foot to the ball.
Student A (left) has an "inertia-like" idea. The ball has an inherent force that resists motion when it's still, but resists slowing down when it's moving forward.
Student B has a "forces-are-transfered" idea, and the force tracks with the speed of the ball.
Other ideas
In the example at left, a student notes that the foot transfers energy to the ball; the only acting force, then, is friction - but the student describes this as turning the kinetic energy back to potential. I'm curious about these kinds of moments teachers face; Kate's Suzuki lesson teacher was explaining to me "yes - I hear a few things she's doing wrong, and I'm not going to address those before the recital. They will get addressed later in Book 2." - and I agreed, saying that this happens a lot in my classes - I let so many ideas that are considered "wrong" sit uncontested in the class. (And they might not get addressed in "book 2" !) - I suspect teachers in a force unit would choose not to address the ideas here about potential energy -- but do they? Why? How?