Download Newton*s Second Law Examined

Newton’s Second Law Examined
“A change in motion is proportional to
the motive force impressed and takes
place along the straight line in which
that force is impressed”
Another way to put it…
• Acceleration is proportional to the net force
on an object and inversely proportional to its
mass
Sample Problem 1
Rocket Guy weighs 905 N and his jet pack
provides 1250 N of thrust, straight up. What
is his acceleration.
Sample Problem 2
A helicopter of mass 3770 kg can create an
upward lift force (F) when empty. It can
accelerate straight upward at a maximum of
1.37 m/s2. A careless crewman overloads the
helicopter so that it is just unable to lift off.
What is the mass of the cargo?
Sample problem 3
Len pushes toward the right on a 12 kg box with
a force of 31 N. Martina applies a 11 N force
on the box in the opposite direction. The
magnitude of kinetic friction between the box
and the floor is 4.5 N as the box slides to the
right. What is the box’s acceleration?
Newton’s Third law
• What is Newton’s Third Law of Motion?
• What does it mean?
Normal Force
• When two objects are in direct contact, the
force one object exerts in response to the
force exerted by the other. This force is
perpendicular to the objects’ contact surface.
• Huh???????????
Processing Time
What is the normal force of
the desk on the boxes? What is the
Normal force of the 9 kg box on the top two
Boxes?
Tension
Force exerted by a string, cord, twine, rope,
chain, cable, etc
Often times, tension “takes the place” of the
normal force.
Friction
• A force that resists the motion of one object
sliding past another
• Varies by materials in contact
• Proportional to normal force
• What are the different types of friction?
Static friction
• Force opposing sliding when no motion
• Balances “pushing” force until object slides
• Max static friction proportional to:
– Coefficient of static friction
– Normal force
Equation?
Sample problem 1
• Anna is pushing a 7 kg box with 7 N of force to
the left, but the box does not move. What is
the force of static friction?
• What is the max static frictional force if the
coefficient of static friction for these materials
is 0.31?
Kinetic Friction
• Friction opposing sliding in motion
• Force constant as object slides
• Proportional to:
– Coefficient of kinetic friction
– Normal force
Equation?
Sample Problem 1
• What is the force of friction of a sliding box
down a ramp if the box has a mass of 5 kg and
the coefficient of kinetic friction is 0.67?
Sample problem 2
• While rearranging your living room, you push
your couch across the floor at a constant
speed with a horizontal force of 69 N. You are
using special pads on the couch legs to help it
slide easier. If the couch has a mass of 59.5
kg, what is the coefficient of kinetic friction
between the pads and the floor?
Independent Practice