Transcript 19force, gravity, friciton, circular motion - Mr-Hubeny

Objective:
Forces con’t: Gravity, Friction, Circular
Motion
Homework:
MCAS Questions
Do Now:
What is the mass of an object weighing
63 N on Earth?
Remember, weight is a Force!
A.0.1 kg
B.6.3 kg
F
Mcas formula
F = ma
sheet does not
C.73 kg
give you
a
m
formula for
D.617 kg
weight
m = F/a = 63/10 = 6.3 kg
10m/s² = Acceleration due
to gravity on Earth
1. Remember about circular motion: special force
keeps ball from flying away.
If force wasn’t there were would ball fly?
Inertia- ball will keep
going in straight path!
Force towards center of
circle, keeps objects
moving in circle
The athlete twirls the
hammer (a heavy steel
ball attached to a wire)
around several times
before releasing it.
The diagram below shows
the path ofthe ball as it
is spun in a circle before
its release.
Which arrow shows the
direction of the
centripetal force on the
ball before the
ball is released?
A.P B.Q C.R D.S
Centripetal Force:
Force towards center of
circle, keeps objects
moving in circle
Force of Gravitation problems
Mass of object 1 x mass of object 2
m1m2
What does the formula F =G
d²
Force of gravitation
(attraction between
objects)
mean?
(look in formula sheet)
Distance between objects squared
Write
in your
own
words
in note
box what this formula
More mass
and
closer
= more
force
means?
Less mass and farther apart = less force
The distance of the star Vega from Earth is
1.6 million times greater than the distance
of the Sun from Earth. Which of the
following best describes the gravitational
influence of Vega on Earth?
What 2 things determine strength of gravitational force?
A.It is roughly equal to that of the Sun.
Mass
and
distance
B.Its influence is greater than that of the Sun.
C.Its influence is small because of its distance.
D.It influences the magnitude of Earth's mass.
The diagram below represents two
identical space ships, Earth, a
space station, and the Moon.
The two ships are launched toward
the Moon. Ship 1 is launched from
Earth, and ship 2 is launched from
the space station.
It takes less force to launch ship 2.
Which of the following contributes
most to the difference in the
forces?
A.Ship 2 has less inertia in space
than ship 1 does on Earth.
B.Earth exerts less gravitational force
on ship 2 than on ship 1.
C.Ship 2 must travel a shorter
distance than ship
Why?
Less distance!
Things to remember about Friction
• Resistive Force (opposite motion)
Force Friction
Force push
• Static Friction (not moving) is always
more than Kinetic Friction (moving)
A car is parked on the side of a hill.
Which of the following most likely
prevents the car from moving down
the hill?
Friction is a
A.The car has too much mass to resistive force
(can prevent
move easily.
things from
B.There is friction in the door hingesmoving)
of the car.
Force is
C.There is friction between the tires between
surfaces in
and the road.
contact with
eachother
D.The weight of the car is mostly on
the front wheels.
Which of the following statements best
explains why it is usually easier to keep a
sliding object moving than it is to start the
Sliding objects… about friction!
object moving?
A.Kinetic friction is typically equal to static
No way jose!
friction.
B.Kinetic friction is typically less than static
friction. True, but lets check others just in case
C.Kinetic friction is a force that resists No, that’s
Static
attempts to start an object moving.
D.Kinetic friction is a force that opposes the
sliding of two objects over each other.
Yes, but not best answer to queston about why is it easier…
6. A heavy box is at rest on an inclined plane. The box
begins to slide down the inclined plane when a small
force is applied to the box. The force is removed
as soon as the box begins to slide. The speed of the
box increases as the box slides down the inclined
plane. Static friction always more than kinetic friction
Which of the following is the most likely cause of this
increasing speed?
A.The force of gravity on the box is less when the box
is not moving than when the box is moving.
B.The force of gravity on the box is greater when the
box is not moving than when the box is moving.
C.The force of friction on the box is less when the box is
not moving than when the box is moving.
D.The force of friction on the box is greater when the
box is not moving than when the box is moving.
Summarizer:
Write a sentence about one thing you forgot
but now remember for the MCAS
(something learned today)