Evidence-based Teaching Strategies:听Forces and Motion
Common students difficulties with forces:
An object will slow down if there is no net force (Halloun & Hestenes, 1985).
The motion will follow the path of the stronger force on the object (Halloun & Hestenes, 1985).听
Passive forces don't exist (tables don't exert a normal force) (Halloun & Hestenes, 1985).
An object with a constant net force will have a constant speed (Halloun & Hestenes, 1985).
Faster moving objects have a larger force acting on them (Halloun & Hestenes, 1985).
A constant force accelerates a body, until the body uses up all the power of the force (Halloun & Hestenes, 1985).
The net force must be in the direction of motion, so objects will travel along a line in that direction (Aguirre, 1988; Gunstone, 1987; Halloun & Hestenes, 1985).
Objects can be trained to follow a certain path by forces, and will continue along that path, even after the forces are removed (Caramazza, McCloskey, & Green, 1981; Halloun & Hestenes, 1985).
Heavier objects fall faster than light objects (Halloun & Hestenes, 1985).
Objects will point in the direction of their velocity (Aguirre, 1988).
Force must be positive, plotted above the time axis (Goldberg & Anderson, 1989).
Strings transmit (unchanged) an external force acting on one object to another object (Gunstone, 1987; McDermott, Shaffer, & Somers, 1994).
The tension in a string is the sum of the forces acting on each end (McDermott, Shaffer, & Somers, 1994).
听See also: Brown (1989) and McGuire (1983)
听
Assessment Items听to explore students'听preconceptions around the concept of forces and motions:听听
Question 1. What force makes the toy car move?
A.听Gravity
B. Air pressure
C. Ramp
D. Energy
Question 2. A ball is held in place by a peg, on a ramp, as seen below. What force makes the ball move when the peg is removed?
A.听Gravity
B. Air pressure
C. Ramp
D. Energy
(Assessment items developed by Chantier 7 project team)
