Collision explanation

When a bike rider lands after a jump, they essentially have a collision with the ground.
Use physics principles to explain fully how a suspension system makes a bike safer for landing.

For any explanation or discussion answer, try to find an equation that supports your reasoning, and make sure you refer to every term in the equation, even (especially!) if one variable doesn’t change between two scenarios.

In this situation, we can explain how the suspension reduces damage from the perspective of either force or energy. In most questions you normally only need to explore one of the ideas thoroughly for full credit.

Force:

change in momentum = Force x time

As the bike lands, the suspension compresses, increasing the time of the collision. The change in momentum is determined by the mass and velocity of the bike and rider. Their vertical momentum will effectively become zero after landing. So our change in momentum is not affected by the suspension (assuming the suspension doesn’t add much mass to the bike)

SInce change in momentum = Force x time,
Force = change in momentum / time
If the time increases, for any value of hange in momentum, the Force experienced by the bike and rider must decrease.
This decrease in Force reduces the liklihood of damage, making the landing safer.

If we compare this to a bike without suspension, the time of the collision will be shorter, and the change in momentum the same as in our previous example. Decreasing the time will increase the force experienced, and the liklihood of damage or injury.

Energy:

When the bike lands, its kinetic energy must be transferred into other forms. If the bike has suspension, some of this energy will be transferred into compressing the springs of the suspension (elastic potential energy). Some will also be transferred into sound. A small amount might be converted into heat through friction.

If the bike does not have suspension, more of this energy will be transferred into the frame of the bike, or the rider, possibly leading to damage (deformation).

If you’ve ever driven over speedbumps, you may notice that going over them slowly is more comfortable for passengers. The kinetic energy is transferred more gradually into the suspension at slow speeds. Driving over the same speedbump suddenly leads to a more rapid dispersal of energy, which the suspension may not be able to absorb as effectively, leading to more energy being transferred into the car and occupants.

There is a bit more physics in the suspension example that I’ve glossed over, but hopefully it gives you an example you might be familiar with.