Force and motion
The topic of force and motion involves some quite counter-intuitive ideas. Informal conceptions of force are held by children and adults alike. Force and motion activities provide some rich opportunities for different representational modes: diagrams, graphs and charts, and metaphors to do with human action.
Key concepts of force and motion
The activities in this topic are designed to explore the following key concepts:
- Forces can be thought of as 'pushes' and 'pulls'.
- Forces can make things move or stop or hold things up or squeeze things.
- Friction is a common force that stops things moving or slows things down.
- Gravity is a force that makes things fall.
- Forces cause changes in motion, and are not (unlike momentum and energy) associated with motion itself.
- Forces are our way of describing the way effects such as pushes, pulls or gravity can influence the motion of things.
- A force is an effect on an object, not a property of the object or its motion. It is something that is done to, or acts on, things.
- A change in motion (speeding up, slowing down, swerving) is caused by an external effect and not by the object itself or something inside it.
- Forces occur in action-reaction pairs. Thus if you push on something it will push back on you. Your standing body pushes down on the ground, and the ground pushes back up on you.
- Pairs or sets of forces will add together to affect motion, but the addition must take into account direction. Forces can cancel each other.
- Common forces include contact forces (physical pushes, support or traction from the ground, friction, air or water resistance opposing motion, force from wind) and field forces (gravity, magnetic forces, electric field forces).
- Motion can be described by representing the distance something travels in a time interval.
Students' alternative conceptions of force and movement
Research into students' ideas about this topic has identified the following non-scientific conceptions:
- Forces are associated only with living things (i.e. people can apply force but things like gravity or friction are not forces).
- Force is thought to be the property of a moving object, rather than something acting on it from outside. Thus, constant motion requires or involves a constant force (rather than a net force causing speeding up or slowing down or deflection).
- The amount of motion is proportional to the amount of force (i.e. faster moving objects are thought to have a greater force).
- If an object is not moving there is no force acting on it, and if a body is moving there is a force acting on it in the direction of motion. (This is not true. For instance, there is no forward force on a rolling or sliding object- friction will act in a direction opposite to the motion. A stationary person standing in a room is subject to two forces.)
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