Question

section 3 review
section vocabulary
force an action exerted on a body in order to change the bodys state of rest or motion; force has magnitude and direction
friction a force that opposes motion between two surfaces that are in contact
kinetic friction the force that opposes the movement of two surfaces that are in contact and are moving over each other
static friction the force that resists the initiation of sliding motion between two surfaces that are in contact and at rest

1. compare what is the difference between kinetic friction and static friction?
2. describe fill in the table below for the relative strengths of the three remaining fundamental forces. list the forces from strongest to weakest. indicate the rela - tive strength of each force.
3. describe what happens to an object when an unbalanced force acts on it?
4. apply concepts a cyclist peddling at a constant rate sees the finish line and speeds up to finish the race sooner. explain when the forces between the cyclist and the ground are balanced and when they are unbalanced.
5. explain why is it easier to move a chair with wheels across a floor than a chair without wheels?

Explanation:

1. Kinetic friction opposes moving surfaces, static friction resists motion start.
2. Strong nuclear force (1), electromagnetic force (1/137), weak nuclear force (1e - 6), gravitational force (1e - 38).
3. Unbalanced force causes object's motion change (acceleration).
4. Balanced when constant speed (forces equal), unbalanced when speeding up (net force forward).
5. Wheels replace sliding (high friction) with rolling (low friction).

Answer:

1. Kinetic friction opposes moving surfaces, static friction resists motion start.

2.

Fundamental Force	Approximate Relative Strength
Electromagnetic force	$\frac{1}{137}$
Weak nuclear force	$10^{- 6}$
Gravitational force	$10^{-38}$

1. Unbalanced force causes object's motion change (acceleration).
2. Balanced when constant speed (forces equal), unbalanced when speeding up (net force forward).
3. Wheels replace sliding (high friction) with rolling (low friction).