Newton

1
Newtons First Law of MotionInertia
2
Aristotle on Motion

• Divided motion into 2 types
• Natural Motion- Motion straight up or down.
  Objects would seek their natural resting places.
  For example A boulder falling toward the ground
  or smoke rising into the clouds.
• Violent Motion- Motion resulting from forces that
  pushed or pulled. Motion due to some external
  cause. For example Cart pulled by a horse or
  ship pushed by the wind.

3
Aristotle on Motion Continued

• Aristotle also believed that circular motion was
  natural for the heavens.
• He believed that the planets and stars moved in
  perfect circles around the earth.
• Since the motions were natural, they were not
  caused by forces.

4
Copernicus and the Moving Earth

• Nicolaus Copernicus (1473-1543) believed that the
  earth and the other planets moved around the sun.
• Copernicus worked on his ideas in secret in fear
  of persecution.
• He published his ideas, De Revolutionibus, before
  his death on May 24, 1543.

5
Galileo

• Galileo was a Italian scientist during the
  late-renaissance.

6
Galileo on Motion

• Galileo demolished the notion that a force was
  necessary to keep an object moving.
• Force-Any push or pull
• Friction-Force that acts between materials that
  touch as they move past each other. If friction
  were absent, a moving object would need no force
  whatever to remain in motion.
• Galileo argued that only when friction is
  present, is a force needed to keep an object
  moving.

7
Galileo Tests His Idea

• He tested his idea by rolling balls along plane
  surfaces tilted at different angles.
• Ball rolling down an inclined plane picks up
  speed and moves in the direction of gravity.
• Ball rolling up an inclined plane, opposite to
  gravity, loses speed.
• Ball rolling on a level surface does not roll
  with or against gravity and has almost constant
  velocity. Without friction, the ball would move
  forever with no push or pull required to keep it
  moving.

8
Galileo Supports His Idea With Another Line of
Reasoning
9
Galileo Continued
10
Galileo Comes to a Conclusion

• Galileo stated that the tendency of a moving body
  to keep moving is natural and that every material
  object resists change to its state of motion.
• The property of a body to resist change is called
  inertia.

11
Netwons First Law- Inertia

• Newton develops his famous laws at the age of 24.
• 1st Law Every object continues in a state of
  rest, or of motion in a straight line at constant
  speed, unless it is compelled to change that
  state by forces exerted upon it.

12
Inertia
13
For Example

• Dishes on a tablecloth are in a state of rest.
• They tend to remain at rest, even if you snap the
  tablecloth out from beneath them.

14
Inertia
15
For Example

• If you slide a hockey puck along the surface of a
  city street, the puck will soon come to rest.
• If you slide it on the ice, it slides a longer
  distance. (Due to less friction).
• In the absences of forces, a moving object tends
  to move in a straight line indefinitely!

16
More Examples of Inertia

• Blood rushes from your head to your feet when
  riding on a descending elevator which suddenly
  stops.
• The head of a hammer can be tightened onto the
  wooden handle by banging the bottom of the handle
  against a hard surface.
• To dislodge ketchup from the bottom of a ketchup
  bottle, the bottle is often turned upside down,
  thrust downward at a high speed and then abruptly
  halted.
• Headrests are placed in cars to prevent whiplash
  injuries during rear-end collisions.
• While riding a skateboard (or wagon or bicycle),
  you fly forward off the board when hitting a
  curb, a rock or another object which abruptly
  halts the motion of the skateboard.

17
Mass-A Measure of Inertia

• Kick an empty can and it moves. Kick a can filled
  steel nails and it will not move as much and you
  will hurt your foot.
• The nail filled can has more inertia that the
  empty can. The amount inertia an object has
  depends on its mass.
• Mass- the amount of material present in an
  object. If an object has a large mass, it may or
  may not have a large volume.
• The more mass an object has, the greater its
  inertia and the more force it takes to change its
  state of motion. Mass is the measure of the
  inertia of an object.

18
More Mass Greater Inertia
19
Quiz- Which Has More Inertia?
20
Mass is not Weight!!!

• Mass the quantity of matter in an object. More
  specifically, mass is a measure of inertia, or
  laziness, that an object exhibits in response
  to any effort made to start it, stop it, or
  change its state of motion. The mass of an object
  is the same whether its on the moon, on earth or
  in space.
• Weight the force of gravity on an object. An
  objects weight would be very different on the
  moon, on earth or in outer space. On the moon,
  the objects weight would be one-sixth than that
  on earth. In space, the objects weight would be
  zero.

21
One Kilogram Weighs 9.8 Newtons

• Weight mass acceleration due to gravity. (W
  mg)
• In the US the unit for weight is pounds. 1-kg bag
  of nails has a weight of 2.2 pounds or 9.8
  Newtons.
• A Newton is the SI unit for force named after Sir
  Isaac Newton.

22
Net Force

• Net Force all the forces acting on an object.
• When you pull with a force of 10 N, the net force
  acting on the object is 10 N.
• If a friend assists you and pulls in the same
  direction with a force of 5 N, then the net force
  is the sum of the forces or 15 N.
• If your friend pulls with a force of 5 N in the
  opposite direction, then the net force is the
  difference of the forces, or 5 N.

23
Equilibrium-When Net Force Equals Zero

• What forces act on your book while it is
  motionless on a table?
• Normal force- Force perpendicular to surface.
  Often, not always, equals weight of the object.

24
Equilibrium Continued

• When you hang from a rope, the tension of the
  rope equals your weight. The rope pulls you up
  while gravity pulls you down. Since the forces
  cancel each other out, you are motionless.

25
Vector Addition of Forces

• The tension is greater in a pair of non- vertical
  spring scales and depends on their angle from the
  vertical.