Transcript Weight as a force - Science

Topic – Physics 2a
Weight as a force
Topic – Physics 2a
Weight
• Weight is the force due
to the attraction of
gravity.
• The force of gravity,
called weight, is pulling
you downward, toward
the center of the Earth.
• weight (N) = Mass (kg) x g ,
• g depends on the
planet and it also
depends on the distance
from the object to the
center of the planet.
Topic – Physics 2a
G on Earth
• At sea level, it is about 9.8 m/s2.
• g is measured in m/s2 = m/s/s is the unit
for acceleration
• It means that an object falling on Earth,
if we neglect air resistance, will speed
up toward the ground at a rate of 9.8
m/s2.
• Every second, the object will go 9.8m/s
faster.
Topic – Physics 2a
Questions
1. John's mass is 65kg on earth. What his metric (a) mass
and (b) weight on Mars, where g= 3.72m/s/s.
m=____kg and W = _________
1. On Mars, a falling object will speed up by ______ m/s
every second.
2. Find out your mass in Kg. _________kg.
What are your metric mass and weight on Jupiter where
g = 25.9 m/s/s ?
m = _________kg and W = ________N.
Topic – Physics 2a
Questions
4. Find the mass of a
11,500N automobile on
mars.
m = _____kg.
Topic – Physics 2a
5. A student use a balance scaled in newton to find the weight of 1kg and
then 2 kg.
He finds that 1 kg = _______ N, 2kg = ________ N, 3 kg = __________ N
... 10kg = _________N
The mass is proportional to the pull due to gravity (weight) and the constant
of proportionality is_______
Topic – Physics 2a
• We already know that mass is the amount of matter an object has, it
can’t be changed unless matter is added or removed. What can be
changed is the weight. Weight is the mass of a body acted on by
acceleration due to gravity.
• F = ma
where
Fw = weight (N)
• g = a
m = mass (kg)
• then Fweight or Fw = mg
g = acceleration due to gravity (g)
• Example:
• A 10.0 kg stone is first held on Earth (g = 9.8 ms-2) and on the moon
(g = 1.6 ms-2). What is its weight in both cases?
Fw Earth = mg = 10 x 9.8
Fw Moon = mg = 10 x 1.6
Fw Earth = 98 N
Fw Moon = 16 N
Topic – Physics 2a
Lifts
• Why do you feel heavier or lighter when a lift
starts or stops? Your acceleration (from the lift) is
added vectorially to the acceleration due to
gravity.
• When you accelerate up, gravity must be
overcome so your apparent weight is
• Fw = m (g + a)
you feel heavier
• When you accelerate down, it is helping gravity,
so your apparent weight is Fw = m (g – a)
you feel lighter
• When you are moving at a constant velocity, there
is no additional acceleration so your weight is
simplyFw = mg
Topic – Physics 2a
Example 1
Topic – Physics 2a
Example 2
Topic – Physics 2a
Free body diagrams
Topic – Physics 2a
Free body diagrams
Topic – Physics 2a
Free body diagrams
Topic – Physics 2a
Activity
Topic – Physics 2a