Transcript Chapter-05

Newton’s second law
an object of
mass m
Free-body diagram
Applying Newton’s second law
1.
Draw a picture of the situation.
2.
Identify the “object” that you will analyze.
3.
Identify forces on the object (think of what interacts with the
object; for tips, see next slide).
4.
Draw a force diagram, showing all forces acting on the object.
5.
Write Newton’s second law in component form.
6.
Substitute forces from your force diagram into your equations.
7.
Identify known forces and the acceleration (if it is known). Identify
the unknown variable.
8.
Do the math.
9.
Check your work.
Identifying forces
To identify all forces acting on an object, ask yourself the
following:
1. What objects are “touching” the given object? These
are contact forces.
2. What objects exert a force through “action at a
distance” such as a gravitational, electrostatic, or
magnetic force? These are action-at-a-distance
forces.
Gravitational force of Earth on a body is called weight.
Poll
What objects exert a force on box B?
1. Earth, table, box A
2. Earth, table, box A, hand
3. table, box A, hand
4. Earth, floor, hand
5. box A
Contact Forces - Surface Interactions
The component of a contact force perpendicular to the
surface(s) is called the normal component of the contact
force.
The component of a contact force parallel to the
surface(s) is called the frictional component of the
contact force.
Example
A refrigerator magnet “sticks” to a refrigerator and is
at rest. What are the forces acting on it?
Example in 1-D -- equilibrium
Suppose that the gymnast in the
picture has a mass of 70 kg and
is at rest. If each ring exerts the
same force on the gymnast, what
is the force on the gymnast by
each ring? (Assume the
ropes/straps are vertical.)
Poll
A gymnast on the rings at a
certain instant of time has a
downward acceleration. Which
force is larger in magnitude?
1. The force on the gymnast by the rings (together).
2. The gravitational force on the gymnast by Earth (i.e.
the weight of the gymnast).
3. Neither, these forces are equal.
Poll
A gymnast on the rings at a certain instant of time has an
upward acceleration. Which force is larger in magnitude?
1. The force on the gymnast by the rings (together).
2. The gravitational force on the gymnast by Earth (i.e.
the weight of the gymnast).
3. Neither, these forces are equal.
Poll
You stand on a bathroom scale in an elevator on the
first floor. When the elevator first starts moving upward,
you speed up. As it is speeding up, which is larger in
magnitude, the force on you by the scale or your
weight?
1. The force on me by the scale.
2. My weight
3. Neither, because they are equal.
Poll
You stand on a bathroom scale in an elevator on the
first floor. After initially speeding up, the elevator will
reach a constant velocity. As it moves upward with a
constant velocity, which is larger in magnitude, the
force on you by the scale or your weight?
1. The force on me by the scale.
2. My weight
3. Neither, because they are equal.
Poll
You stand on a bathroom scale in an elevator on the
first floor. When the elevator gets to the top floor, it
slows down. As it is slowing down, which is larger in
magnitude, the force on you by the scale or your
weight?
1. The force on me by the scale.
2. My weight
3. Neither, because they are equal.
Example 1-D -- constant acceleration
Analyze the forces on the girl with the yellow backpack in
the picture below. If she is speeding up, what force is
larger? (neglect friction)
Poll
Analyze the forces on the girl with the yellow backpack in
the picture below. If she is slowing down, what force is
larger? (neglect friction)
1. The force on the
girl by the guy on
the bike.
2. The force on the
girl by the guy
with the cool hat.
3. Neither, because
these forces are
equal.
Poll
Analyze the forces on the girl with the yellow backpack in
the picture below. If she is speeding up, what force is
larger? (neglect friction)
1. The force on the
girl by the guy on
the bike.
2. The force on the
girl by the guy
with the cool hat.
3. Neither, because
these forces are
equal.
http://bobsled.teamusa.org/news/article/6595
Example 1-D -- constant acceleration
A 120-kg bobsled is pushed during a
training run over a distance of 40 m in
5.0 s. (a) What is the net force on the
bobsled? (b) If there is a constant
frictional force by the track on the sled
of 100 N, what is the force on the sled
by the man?
Example 1-D -- constant acceleration
Suppose that Dr. Titus, starting at rest, has a vertical leap
of 24 in. If his mass is 80 kg and if his feet are in contact
with the floor for 0.5 s, what is the (average) force by the
floor on him during the jump?