Evidence-based Teaching Strategies: 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). 
• 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).

 See also: Brown (1989) and McGuire (1983)

Assessment Items to explore students' preconceptions around the concept of forces and motions:  

Question 1. What force makes the toy car move?

A. Gravity
B. Air pressure
C. Ramp
D. Energy

     (Assessment items developed by Chantier 7 project team)   

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. Gravity
B. Air pressure
C. Ramp
D. Energy

(Assessment items developed by Chantier 7 project team)