7. NORMAL WEIGHT
We measure weight
with a scale that
measures normal
force.
W = mg
Weight is related to
mass by the
gravitational field g.
8. FALSE WEIGHT
A vertical acceleration can change
the weight.
The normal force on the floor is
our sense of weight.
Downward acceleration
reduces weight
Upward acceleration increases
weight
Mass is unchanged.
Newton’s law of acceleration
F = ma
net force, F = -mg + FN.
Solve for the normal force
-mg + FN = ma
FN = ma + mg
FN = m (a + g)
Apparent mass based on g
mapp = FN / g
9. APPARENT WEIGHT (NORMAL FORCE)
Recall:
SF = m a
Consider person accelerating up in an elevator.
Draw FBD
Apply NSL
N – mg = ma
N = m(g+a)
mg
N
•Apparent weight is normal force from scale or floor.
•Note: in free fall a=-g so N=0
10. WEIGHTLESSNESS
If the elevator accelerated downward at g, the normal force would
become 0.
FN = m (g – a) = m (g – g) = 0
The person would feel weightless.
An object in free fall is weightless, but not massless.
11. FREE-FALL ACCELERATION
Have you heard this claim?
“Astronauts are weightless in space,
therefore there is no gravity in space.”
12. FREE-FALL ACCELERATION
Have you heard this claim?
“Astronauts are weightless in space,
therefore there is no gravity in space.”
It’s true that if an astronaut on the International Space Station
(ISS) steps on a scale, he/she will “weigh” nothing!
13. FREE-FALL ACCELERATION
Have you heard this claim?
“Astronauts are weightless in space,
therefore there is no gravity in space.”
It’s true that if an astronaut on the International Space
Station (ISS) steps on a scale, he/she will “weigh” nothing!
It may seem reasonable to think that if weight = mg, since
weight = 0,
g = 0, but this is NOT true!!
14. FREE-FALL ACCELERATION
As we’ve already discussed, if you stand on a scale in an
elevator & the cables are cut, you’ll also “weigh” nothing
S F = ma = N – mg
but in free-fall, a = g, so normal force N = 0!.
This does not mean that g = 0!
Astronauts in orbit are in free-fall around the Earth, just
as you would be in the elevator. They don’t fall to Earth,
only because of their very high tangential speed.
15. WEIGHTLESSNESS
As just mentioned, objects in a satellite, including people,
experience “Effective Weightlessness”. What causes
this?
As also already mentioned, This does NOT Mean that the
gravitational force on them is zero!
They still have a gravitational force acting on them!
As already mentioned, the satellite & all of its contents are
in “free fall”, so for people inside,
16. WEIGHTLESSNESS
There is no normal force FN.
This is what leads to the experience of “weightlessness”.
More properly, this effect should be called apparent or
effective weightlessness, because the gravitational force
still exists.
This can be experienced on Earth also, but only briefly.
