3.3.1 Understanding the Frictional Force Between Two Surfaces

Question 1

Understanding the Frictional Force Between Two Surfaces

Answer 1

• Kinetic friction, , is the force between two moving objects that resists their relative motion. Its direction is opposite to the direction of the motion.
• An approximate equation for kinetic friction is , where is the dimensionless coefficient of kinetic friction.
• Static friction, fs, is the force between two stationary objects that resists their relative motion. Static friction is maximal when , where is the coefficient of static friction. is always greater than

Question 2

Which of the following best defines the terms kinetic friction and static friction?

Answer 2

Kinetic friction is the force between two moving objects that resists their relative motion. Static friction is the force between two stationary objects that resists their relative motion.

Question 3

Which of the following statements regarding kinetic friction and / or static friction is not correct?

Answer 3

Kinetic friction, fk, is the force that resists motion between two objects. The direction of kinetic friction is equal to the direction of motion.

Question 4

If a wooden block is pushed across a table, the resulting frictional force will be__________.

Answer 4

Opposite to the direction of the force on the block.

Question 5

A crude way to measure friction is to attach a scale to a wooden block and pull it across a table at a uniform speed. Which of the following statements regarding this experiment is not correct?

Answer 5

The force of the pull is greater than the force of friction, and the difference is indicated by the reading on the scale.

Question 6

The diagram shows a block of wood in motion on a table. The normal force is N and the force pushing the block is F. The weight is W and the force of kinetic friction is fk. Which of the following formulas regarding this diagram is correct?

Answer 6

fk ∝N

Question 7

A block of wood is pushed across a table. Which of the following characteristics of the wood block most affects the force of friction in this scenario?

Answer 7

The weight of the wood block.

Question 8

A wooden block is pushed across a table with an increasing force as shown on the horizontal axis. The frictional force is shown on the vertical axis. Which of the following statements is incorrect?

Answer 8

A maximum force can be applied to the block, and it is reached at the point fs, max. This maximum force settles down to fk = mkN once the block is in motion