Dissipative forces

Question 1

Recall that friction is a force that:

Answer 1

• Opposes motion

Question 2

Recall that, for an object in motion, frictional forces do what with velocity

Answer 2

• They increase
• This is because the number of particles being pushed against increases per unit time

Question 3

Describe energy transfers associated with dissipative forces at a molecular level

Answer 3

• Microscopically all surfaces are rough
• When placed in contact there will be regions where the irregularities interlock and points were high surface pressure may result in temporary bonds
• Any attempt to slide the surfaces over one another will require a certain amount of work to be done, lifting and deforming the surface
• This will require a force and the work done will increase their thermal energy

Question 4

Appreciate the orgin of viscosity in terms of the interactions between particles within a liquid

Answer 4

• There are internal forces between molecules in liquids and gases which result in viscosity - a resistance to flow

Question 5

Appreciate the link between viscosity and fluid friction

Answer 5

Viscosity is a measure of a fluid’s resistance to deformation or flow. It arises from the internal friction between adjacent layers of fluid as they move relative to each other. In liquids, particles are in constant motion due to their thermal energy. The origin of viscosity can be attributed to the cohesive forces between these particles.
Intermolecular Forces: In a liquid, molecules are attracted to each other due to intermolecular forces, such as van der Waals forces or hydrogen bonding. As one layer of liquid flows over another, these intermolecular forces resist the motion, leading to viscosity.
Collision and Transfer of Momentum: As particles move, they collide with each other and transfer momentum. This collision and momentum transfer contribute to the internal friction, which is a key factor in viscosity.

Question 6

Appreciate that dissipative forces cause what? Friction forces do what?

Answer 6

• Dissipative forces cause energy transfer from a system and reduce efficiency corrrespondingly
• Friction does negative work