Chapter 4:
Universal Physical Laws
(Motion, Energy & Gravity)
4.1 Motion
• How do we describe motion?
• How is mass different from weight?
How do we describe motion?
• speed: rate at which object moves
⎞
speed = distance ⎛⎜⎝units of m
⎟
s
⎠
time
example: speed of 10 m/s
• velocity: speed and direction
example: 10 m/s, due east
• acceleration: any change in velocity
(either in speed or in direction)
All falling objects
accelerate at the
same rate
Galileo
All falling objects
accelerate at the
same rate
(if we ignore the
effects of air
resistance)
Galileo
The Acceleration of Gravity (g)
• Galileo showed
that g is the
same for all
falling objects
on the Earth,
regardless of
their mass.
Apollo 15 demonstration
Acceleration of Earth’s Gravity: g≈10 m/s2
• On Earth, g ≈ 10 m/s2.
• Speed increases
by 10 m/s with each
second of falling.
• Hence, acceleration
has units of
(meters/sec) per sec
= m/s2
Momentum and Force
• Momentum = mass × velocity
p=mv
• A net force changes momentum,
which leads to an acceleration (change in
velocity).
• A change in momentum (an acceleration) only
occurs when the overall force on an object is
non-zero.
QUESTION: Is there a net
force in these situations? Y/N
1. A car coming to a stop.
2. A bus speeding up.
3. An elevator moving up at constant
speed.
4. A bicycle going around a curve.
5. A moon orbiting Jupiter.
QUESTION: Is there a net
force in these situations? Y/N
1. A car coming to a stop. Y
2. A bus speeding up. Y
3. An elevator moving up at constant
speed. N
4. A bicycle going around a curve. Y
5. A moon orbiting Jupiter. Y
What force is acting on a moon orbiting Jupiter?
How is mass different from weight?
• Mass: the amount of matter in an object
• Weight: the force that acts upon an object
(what a scale reads when you stand on it)
QUESTION: On the Moon…
A.
B.
C.
D.
My weight is the same, my mass is less.
My weight is less, my mass is the same.
My weight is more, my mass is the same.
My weight is more, my mass is less.
QUESTION: On the Moon…
A.
B.
C.
D.
My weight is the same, my mass is less.
My weight is less, my mass is the same.
My weight is more, my mass is the same.
My weight is more, my mass is less.
Why are astronauts weightless in space?
• There IS gravity in space…
• Weightlessness is due to a
constant state of free-fall.
• Gravity + tangential velocity
can cause an object to
move in an orbit.
Brief recap of motion
• How do we describe motion?
• Speed = distance ÷ time
(e.g. 60 miles/hr)
• Speed & direction => velocity (v)
• Change in velocity => acceleration (a)
• Momentum = mass × velocity
• Force causes a change in momentum,
which leads to an acceleration.
• How is mass different from weight?
• Mass = quantity of matter (fundamental quantity)
• Weight = force acting on mass
• Objects are weightless when in free-fall
4.2 Newton’s Laws of Motion
• How did Newton change our view of the Universe?
• What are Newton’s three laws of motion?
How did Newton change our view of the Universe?
• Discovered laws of motion
and gravity
• Studied the nature of light.
• Developed the first reflecting
telescope
• Invented calculus
Sir Isaac Newton
(1642-1727)
“If I have seen farther than
others, it is because I have
stood on the shoulders of
giants.”
How did Newton change our view of the Universe?
• Aristotle: the Earth and the
heavens are distinct.
• Newton: realized the same
physical laws that operate
on Earth also operate in the
heavens
⇒ one universe
Sir Isaac Newton
(1642-1727)
• 1687: Principia published.
Contained the laws of motion & gravity.
Likely the most influential physics book
ever written.
Newton’s first law of motion:
An object moves at constant velocity unless a
net force acts to change its speed or direction.
• An object will move forever in a straight line unless an
external force acts on it.
– Astronomical objects do not need any fuel to travel
through the Universe.
– Spacecraft continually travel through outer space
without any fuel.
• Aristotle asserted that the natural state of an object
was at rest. This was wrong. In fact, the natural state
is to maintain its motion.
Newton’s second law of motion:
When a force acts on an object, it produces an
acceleration equal to the force divided
by the mass, or Force = mass x acceleration:
F = ma
m = mass of the object
a = acceleration
F = force
•
For the same amount of force, you produce less acceleration for a more
massive object. (You can throw a wad of paper farther than a rock.)
•
For an object of a given mass, the larger force produces more
acceleration. (The Sun has more influence on comets than the Earth.)
Newton’s third law of motion:
For every force, there is always an
equal and opposite reaction force.
• Examples:
– Standing up on the Earth.
– Ball dropped to the ground.
Newton’s third law of motion:
For every force, there is always an
equal and opposite reaction force.
• More examples
– Rocket launches: to balance the force of the gas
leaving the back of the rocket, an equal and opposite
force propels the rocket forward.
– If you were floating motionless in outer space, you
could propel yourself by throwing something in the
opposite direction.
QUESTION: A compact car and a Mack truck
have a head-on collision. Are the following true
or false?
1. The force of the car on the truck is equal and
opposite to the force of the truck on the car.
2. The momentum transferred from the truck to
the car is equal and opposite to the momentum
transferred from the car to the truck.
3. The change of velocity of the car is the same
as the change of velocity of the truck.
QUESTION: A compact car and a Mack truck
have a head-on collision. Are the following true
or false?
1. The force of the car on the truck is equal and
opposite to the force of the truck on the car. TRUE
2. The momentum transferred from the truck to
the car is equal and opposite to the momentum
transferred from the car to the truck. TRUE
3. The change of velocity of the car is the same
as the change of velocity of the truck. FALSE
Conservation Laws
Important conservation laws:
• Conservation of momentum (linear momentum and
angular momentum)
• Conservation of energy
Fundamental laws of nature &
critical to understanding how the Universe works!
Conservation Laws
• The conservation laws are embodied in Newton’s
laws, but offer a different and sometimes more
powerful way to consider motion of objects.
• Conservation of momentum is reflected in
Newton’s Second & Third Laws.
• For example, when 2 objects collide:
• Object 1 exerts a force Object 2 and changes its momentum
(Newton’s Second Law).
• However, Object 2 exerts an equal and opposite change in
the momentum of Object 1 (Newton’s Third Law).
• Overall, the total momentum of the 2 objects is unchanged.