Force direction and work
Before you start
Why does the same force add, leave unchanged or remove energy depending on its direction?
The setupA cart initially moves right at 4 m/s, constrained to a horizontal frictionless guide. Force F makes angle θ to the right. The guide supplies a vertical constraint force and does no work.
What to do and noticeThe cart becomes faster and bluer. Where does its energy come from? · At 90° there is force, so why is there no speed increase? · Turn the force to 180°: what happens?
Predict firstThe cart becomes faster and bluer. Where does its energy come from?
Why does the same force add, leave unchanged or remove energy depending on its direction?
Prerequisites: A cart initially moves right at 4 m/s, constrained to a horizontal frictionless guide. Force F makes angle θ to the right. The guide supplies a vertical constraint force and does no work.Work transfers energy
Predict firstThe cart becomes faster and bluer. Where does its energy come from?
A force transfers energy when its point of application moves along its direction. For constant force W = Fs cosθ, using N, m and J. At θ = 0°, W = Fs is positive. The whole body becomes bluer as kinetic energy grows, with ΔK = Wnet. Here the applied force supplies all horizontal net work.
How to observe
- The cart becomes faster and bluer. Where does its energy come from?
- At 90° there is force, so why is there no speed increase?
- Turn the force to 180°: what happens?
Model notes
Constant force, frictionless translation. Follow the first rightward motion until the selected distance or first stop. The guide can constrain either vertically upward or downward, so a 90° force does not lift the cart off it. Colour encodes kinetic energy, not temperature.