Name
Period
Date
Dynamics Packet 3
Friction
Read from Lessons 2 and 3 of the Newton's Laws chapter at The Physics Classroom:
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
4.
5.
The units on µ are _____
a. Newton
b. kg
c. m/s/s
c. normal force
d. ... nonsense! There are no units on µ.
Use the friction equation and Fnet = m•a to fill in the blanks in the following situations.
© The Physics Classroom, 2009
Dynamics Packet 3
page 2
Air Resistance and Terminal Velocity
Read from Lesson 3 of the Newton's Laws chapter at The Physics Classroom:
MOP Connection:
Newton's Laws: sublevel 11
1.
When falling under the influence of air resistance and
dropped from the same height, which will fall to the ground
at a faster rate?
a. a mouse
b. an elephant
c. the same
2.
Which of the following variables will have a direct effect
upon the amount of air resistance experienced by an object?
(That is, for which of these quantities will an increase lead to
a resulting increase in the air resistance force?)
a. speed
b. air density
c. cross-sectional area
3.
Consider the dragster's motion below. Speedometer readings and the forward propulsion force
(Fapp) are shown. The top (or terminal) speed is 120 mph. Draw Fair force arrows on each diagram
to illustrate how the amount of air resistance changes during the course of its motion.
4.
Draw Fair force arrows to show how the force of air resistance changes on the falling skydiver. At
A, the diver has just jumped; and at E, the diver has just reached terminal velocity.
5.
Fill in the blanks in the following paragraph.
As an object moves faster and faster, the amount of air resistance __________________ (increases,
decreases) until a state of terminal velocity is reached. Once terminal velocity is reached, the force of
air resistance is __________________ (greater than, less than, equal to) the force of gravity. Hence,
the object will _______________________________________ (continue to accelerate, stop its motion,
stop its acceleration, move back up to its starting position).
© The Physics Classroom, 2009
Dynamics Packet 3
page 3
Skydiving
Read from Lesson 3 of the Newton's Laws chapter at The Physics Classroom:
MOP Connection:
Newton's Laws: sublevel 11
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
5.
Explain why air resistance increases from 6000 feet to 4500 feet.
6.
Explain why air resistance decreases from 3000 feet to 1500 feet.
© The Physics Classroom, 2009
d. slowing down
Dynamics Packet 3
page 4
The Elephant and the Feather
Study the two animations from the Multimedia Physics Studios
MOP Connection:
Newton's Laws: sublevels 10 and 11
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.
© The Physics Classroom, 2009
Dynamics Packet 3
page 5
Newton's Third Law
Read from Lesson 4 of the Newton's Laws chapter at The Physics Classroom:
MOP Connection:
Newton's Laws: sublevel 12
A force is a push or pull resulting from an interaction between two objects.
Whenever there is a force, there are two objects involved - with both objects
pushing (or pulling) on each other in opposite directions. While the direction
of the pushes (or pulls) is opposite, the strength or magnitudes is equal. This is
sometimes stated as Newton's Third Law of motion: for every action, there is an
equal and opposite reaction. A force is a push or a pull and it always results from
an interaction between two objects. These forces always come in pairs.
1.
For each stated action force, identify the reaction force.
Bat hits ball.
2.
Man pushes car.
Bus hits bug.
Identify by words the action-reaction force pairs in each of the following diagrams.
© The Physics Classroom, 2009
Dynamics Packet 3
3.
TRUE or
page 6
FALSE:
As you sit in your seat in the physics classroom, the Earth pulls down upon your
body with a gravitational force; the reaction force is the chair pushing upwards on
your body with an equal magnitude.
If False, correct the answer.
4.
5.
Shirley Bored sits in her seat in the English classroom. The Earth
pulls down on Shirley's body with a gravitational force of 600 N.
Describe the reaction force of the force of gravity acting upon
Shirley.
Use Newton's third law (law of action-reaction) and Newton's second law (law of acceleration: a =
Fnet/m) to complete the following statements by filling in the blanks.
a.
A bullet is loaded in a rifle and the trigger is pulled. The force experienced by the bullet is
____________ (less than, equal to, greater than) the force experienced by the rifle. The resulting
acceleration of the bullet is ____________ (less than, equal to, greater than) the resulting
acceleration of the rifle.
b.
A bug crashes into a high-speed bus. The force experienced by the bug is ____________ (less
than, equal to, greater than) the force experienced by the bus. The resulting acceleration of the
bug is ____________ (less than, equal to, greater than) the resulting acceleration of the bus.
c.
A massive linebacker collides with a smaller halfback at midfield. The force experienced by the
linebacker is ____________ (less than, equal to, greater than) the force experienced by the
halfback. The resulting acceleration of the linebacker is ____________ (less than, equal to,
greater than) the resulting acceleration of the halfback.
d. The 10-ball collides with the 14-ball on the billiards table (assume equal mass balls). The force
experienced by the 10-ball is ____________ (less than, equal to, greater than) the force
experienced by the 14-ball. The resulting acceleration of the 10-ball is ____________ (less than,
equal to, greater than) the resulting acceleration of the 14-ball.
© The Physics Classroom, 2009
Dynamics Packet 3
page 7
Newton's Second Law Problem-Solving
Study from Lessons 3 of the Newton's Laws chapter at The Physics Classroom:
For the following problems, begin by constructing a free-body diagram and show your work
symbolically, only substituting numbers at the very end!
1.
A rightward force of 302 N is applied to a 28.6-kg crate to accelerate it across the floor. The
coefficient of friction between the crate and the floor is 0.750. Determine the acceleration of the crate.
2.
During a football workout, two linemen are pushing the coach on the sled. The combined mass of
the sled and the coach is 300. kg. The coefficient of friction between the sled and the grass is 0.800.
The sled accelerates at a rate of 0.580 m/s/s. Determine the force applied to the sled by the lineman.
3.
A 405-N rightward force is use to drag a large box across the floor with a constant velocity of 0.678
m/s. The coefficient of friction between the box and the floor is 0.795. Determine the mass of the
box.
4.
A 6.58 x 103 N upward tension force is exerted on a 521-kg downward-moving freight elevator.
Determine the acceleration of the elevator.
© The Physics Classroom, 2009
Dynamics Packet 3
page 8
5.
A basketball star exerts a force of 3225 N (average value) upon the gym floor in order to accelerate
his 76.5-kg body upward. (a) Determine the acceleration of the player. (b) Determine the final
speed of the player if the force endures for a time of 0.150 seconds.
6.
At the end of the Giant Drop free fall ride, riders experience a large upward normal force to bring
their falling bodies to a stop. Determine the normal force value required to accelerate a 52.1-kg
physics student with an upward acceleration of 27.4 m/s/s.
7.
A hockey player accelerates a puck (m = 0.167 kg) from rest to a velocity of 50.0 m/s in 0.121 sec.
Determine the acceleration of the puck and the force applied by the hockey stick to the puck.
Neglect resistance forces.
8.
A falling skydiver is accelerating in the downward direction at 3.29 m/s/s. The mass of the
skydiver (including parachute gear) is 67.2 kg. Determine the air resistance force on the skydiver
(and accompanying parachute).
9.
A 67.2-kg falling skydiver opens his parachute and instantly slows down at a rate of 7.20 m/s/s.
Determine the air resistance force on the skydiver (and accompanying parachute).
© The Physics Classroom, 2009