How to play Forklift
- Choose a load and read its mass and its distance in front of the front wheels on the card.
- Take moments about the front wheels: the load's m·g·x tips the truck forward, the truck's M·g·b holds it back.
- Type the mass, distance, counterweight, or rear-wheel force your moments give, and press Lift.
- Watch the bars and the rear wheels: while the truck's moment is larger they stay down; when the load's catches up, the rear lifts.
What tips a forklift over?
A forklift carries its load in front of its front wheels, so the load's weight tries to turn the truck forward about those wheels. The truck's own weight, acting behind them, turns it back. Taking moments about the front wheels, the edge it would tip over, the truck stays down while the load's moment m·g·x is no more than the truck's M·g·b. As the load's moment grows, the rear wheels carry less and less: R_r·L = M·g·b − m·g·x. When R_r reaches zero the rear wheels lift and the truck tips. That is why a heavier load must sit closer in, why forklifts carry a heavy counterweight at the back, and why a capacity plate always gives a load centre as well as a mass.
A focused learning model
The truck is rigid and stands on level ground. Its own mass acts at its centre of mass and the load at its centre. The forks take the load over 2 s, so its share grows steadily. Moments are taken about the front wheels, the edge the truck would tip over: R_r·1.6 = M·g·0.8 + m_c·g·1.5 − m·g·x. The truck tips exactly when the rear wheels' push falls to zero, found exactly; a load within 0.05% of the limit counts as holding. The tipping is then drawn over 0.8 s as an illustration; a truck that holds is watched for 1 s more. Plays in real time.