Why resistive force will vary with speed??

I have been working on the following question. I have finished part (a) and (b) and checked the answer. But I’m confused by the answer of part(c).

A metal stake of mass 2kg is driven vertically into the ground by a blow from a sledgehammer of mass 10kg. The sledgehammer falls vertically on to the stake, its speed just before impact being 9m/s. In a model of the situation it is assumed that , after impact, the stake and the sledgehammer stay in contact and move together before coming to rest.

(a) Find the speed of the stake immediately after impact.

The stake moves 3cm into the ground before coming to rest. Assuming in this model that the ground exerts a constant resistive force of magnitude R newtons as the stake is driven down,
(b) find the value of R.

(c) State one way in which this model might be refined to be more realistic.

The answer of (c) is "the resistance (R) could be modeled as varying with speed". However, I cannot figure out the explanation of it. Since the stake is moving, the frictional force should be limiting all the time until the stake stops. Have I confused that resistive force with frictional force? Can anyone help explain it to me?

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