When a monster truck lands after a jump, its downward momentum must be brought to zero without breaking the frame or injuring the driver. The landing force can be huge because the truck has a large mass and often hits the ground with high vertical speed. Engineers reduce the peak force by making the stopping time and stopping distance as large as practical.
Long-travel suspension, large tires, and shock absorbers work together to turn a violent impact into a controlled deceleration.
Key Facts
- Impulse changes momentum: J = F_avg Δt = Δp
- Increasing stopping time lowers average force: F_avg = Δp / Δt
- Work done by suspension removes kinetic energy: W = F_avg d = ΔKE
- Vertical kinetic energy before landing is KE = 1/2 mv^2
- A falling truck has impact speed v = sqrt(2gh) if air resistance is ignored
- Shock absorbers convert motion energy into thermal energy in hydraulic fluid
Vocabulary
- Impulse
- Impulse is the change in momentum caused by a force acting over a time interval.
- Momentum
- Momentum is the quantity of motion an object has and is calculated as p = mv.
- Long-travel suspension
- Long-travel suspension is a suspension design that allows the wheels to move a large distance relative to the vehicle body.
- Shock absorber
- A shock absorber is a device that resists suspension motion and converts mechanical energy into heat.
- Peak force
- Peak force is the largest force reached during an impact or collision.
Common Mistakes to Avoid
- Using only the truck weight as the landing force, which is wrong because impact force also depends on how fast the truck is moving and how quickly it stops.
- Thinking shocks make the landing energy disappear, which is wrong because energy is transformed into heat, tire deformation, sound, and vibration.
- Assuming a stiffer suspension is always safer, which is wrong because stopping over a shorter distance usually increases the peak force on parts and people.
- Ignoring tire compression, which is wrong because giant tires act like springs and add stopping distance before the suspension finishes absorbing the impact.
Practice Questions
- 1 A 4500 kg monster truck lands with a vertical speed of 8.0 m/s and comes to rest vertically in 0.50 s. What is the average upward impact force on the truck, ignoring weight during the short collision?
- 2 A truck drops from a height of 2.0 m. Ignoring air resistance, find its vertical speed just before landing using v = sqrt(2gh). Use g = 9.8 m/s^2.
- 3 Two identical monster trucks land with the same vertical speed. Truck A has 0.4 m of effective compression distance and Truck B has 0.8 m. Explain which truck has the lower average impact force and why.