Friction


1. A classroom desk supported by long legs is stationary in the room. A teacher comes around and pushes upon the desk in an effort to start it into a state of motion. The desk does not budge. The desk remains at rest because ______.
a. there is a force of static friction opposing its motion
b. there is a force of kinetic or sliding friction opposing its motion
 c. there is a force of rolling friction opposing its motion
d. there are small dust mites at the desk's feet that push back on the desk to keep it at rest

2. A classroom desk supported by long legs is stationary in the room. A teacher comes around and pushes upon the desk in an effort to start it into a state of motion. The desk is finally accelerated from rest and then moves at a constant speed of 0.5 m/s. The desk maintains this constant speed because ______.
a. there is a force of static friction balancing the teacher's forward push
b. there is a force of kinetic or sliding friction balancing the teacher's forward push
c. there is a force of rolling friction balancing the teacher's forward push
d. the teacher must have stopped pushing

3. The symbol µ stands for the _____
a. coefficient of friction b. force of friction c. normal force

4. The units on µ are _____
a. Newton b. kg c. m/s/s d. ... nonsense! There are no units on µ.

5. Use the friction equation and Fnet = m•a to fill in the blanks in the following situations.

Falling with Air Resistance

As an object falls through air, it usually encounters some degree of air resistance. Air resistance is the result of collisions of the object's leading surface with air molecules. The actual amount of air resistance encountered by the object is dependent upon a variety of factors. To keep the topic simple, it can be said that the two most common factors that have a direct effect upon the amount of air resistance are the speed of the object and the cross-sectional area of the object. Increased speeds result in an increased amount of air resistance. Increased cross-sectional areas result in an increased amount of air resistance.

Why does an object that encounters air resistance eventually reach a terminal velocity? To answer this questions, Newton's second law will be applied to the motion of a falling skydiver.

A 90-kg (approx.) skydiver jumps out of a helicopter at 6000 feet above the ground. As he descends, the

force of air resistance acting upon him continually changes. The free-body diagrams below represent the

strength and direction of the two forces acting upon the skydiver at six positions during his fall. For each

diagram, apply Newton’s second law (Fnet = m•a) to determine the acceleration value.

1. At which two altitudes has the skydiver reached terminal velocity?

2. At which altitude(s) is the skydiver in the state of speeding up?

3. At which altitude(s) is the skydiver in the state of slowing down?

4. At 2900 feet, the skydiver is ___________. Choose two.

a. moving upward b. moving downward c. speeding up d. slowing down

5. Explain why air resistance increases from 6000 feet to 4500 feet.

__9. The elephant experiences less air resistance and reaches a larger terminal velocity.

__10. The feather experiences more air resistance and thus reaches a smaller terminal velocity.

__11. The elephant and the feather encounter the same amount of air resistance, yet the elephant

has a greater terminal velocity.

The diagrams above illustrate a key principle. As an object falls, it picks up speed. The increase in speed leads to an increase in the amount of air resistance. Eventually, the force of air resistance becomes large enough to balances the force of gravity. At this instant in time, the net force is 0 Newton; the object will stop accelerating. The object is said to have reached a terminal velocity. The change in velocity terminates as a result of the balance of forces. The velocity at which this happens is called the terminal velocity.

 

In situations in which there is air resistance, more massive objects fall faster than less massive objects. But why? To answer the why question, it is necessary to consider the free-body diagrams for objects of different mass. Consider the falling motion of two skydivers: one with a mass of 100 kg (skydiver plus parachute) and the other with a mass of 150 kg (skydiver plus parachute). The free-body diagrams are shown below for the instant in time in which they have reached terminal velocity.

  As learned above, the amount of air resistance depends upon the speed of the object. A falling object will continue to accelerate to higher speeds until they encounter an amount of air resistance that is equal to their weight. Since the 150-kg skydiver weighs more (experiences a greater force of gravity), it will accelerate to higher speeds before reaching a terminal velocity. Thus, more massive objects fall faster than less massive objects because they are acted upon by a larger force of gravity; for this reason, they accelerate to higher speeds until the air resistance force equals the gravity force.

The amount of air resistance an object experiences depends on its speed, its cross-sectional area, its shape and the density of the air. Air densities vary with altitude, temperature and humidity.

Newton's Laws Name:

The Elephant and the Feather Without Air Resistance

Suppose that an elephant and a feather are dropped off a very tall building from the same height at the

same time. Suppose also that air resistance could be eliminated such that neither the elephant nor the

feather would experience any air drag during the course of their fall. Which object - the elephant or the

feather - will hit the ground first? Many people are surprised by the fact that in the absence of air

resistance, the elephant and the feather strike the ground at the same time. Why is this so? Test your

understanding by identifying the following statements as being either True (T) or False (F).

__1. The elephant and the feather each have the same force of gravity.

__2. The elephant has more mass, yet they both experience the same force of gravity.

__3. The elephant experiences a greater force of gravity, yet both the elephant and the feather

have the same mass.

__4. On earth, all objects (whether an elephant or a feather) have the same force of gravity.

__5. The elephant weighs more than the feather, yet they each have the same mass.

__ 6. The elephant clearly has more mass than the feather, yet they each weigh the same.

__7. The elephant clearly has more mass than the feather, yet the amount of gravity (force) is the

same for each.

__8. The elephant has the greater acceleration, yet the amount of gravity is the same for each.

With Air Resistance

Now consider the realistic situation that both feather and elephant encounter air resistance. Which object

- the elephant or the feather - will hit the ground first? Most people are not surprised by the fact that the

elephant strikes the ground before the feather. But why does the elephant fall faster? Test your

understanding by identifying the following statements as being either True (T) or False (F).

__1. The elephant encounters a smaller force of air resistance and therefore falls faster.

__2. The elephant has the greater acceleration of gravity and therefore falls faster.

__3. Both elephant and feather have the same force of gravity, yet the acceleration of gravity is

greatest for the elephant.

__4. Both elephant and feather have the same force of gravity, yet the feather experiences a

greater air resistance.

__5. Each object experiences the same amount of air resistance, yet the elephant experiences the

greatest force of gravity.

__6. Each object experiences the same amount of air resistance, yet the feather experiences the

greatest force of gravity.

__7. The feather weighs more, and therefore will not accelerate as rapidly as the elephant.

__8. Both elephant and feather weigh the same amount, yet the greater mass of the feather leads

to a smaller acceleration.

__9. The elephant experiences less air resistance and reaches a larger terminal velocity.

__10. The feather experiences more air resistance and thus reaches a smaller terminal velocity.

__11. The elephant and the feather encounter the same amount of air resistance, yet the elephant

has a greater terminal velocity.