Download Newton*s Laws of Motion

Newton’s Laws of
Motion
The Three Laws of Motion
Newton’s 1st Law of Motion
Newton’s first law states that every object continues
in a state of rest, or of uniform speed in a straight
line, unless acted on by a nonzero net force.
“An object in motion tends to stay in
motion and an object at rest tends to stay at
rest.”
http://www.youtube.com/watch?v=d7iYZPp2zYY
http://www.youtube.com/watch?v=eIHv5FdKet0
Newton’s 2nd Law of Motion
Newton’s second law states that the acceleration
produced by a net force on an object is directly
proportional to the magnitude of the net force, is in the
same direction as the net force, and is inversely
proportional to the mass of the object.
F = ma
force = mass x acceleration
http://www.military.com/video/rockets/aircraft-rockets/humang-force-testing-at-1000kmh/663113048001/
Newton’s 3rd Law of Motion
Newton’s third law states that whenever one object
exerts a force on a second object, the second object
exerts an equal and opposite force on the first object.
http://www.youtube.com/watch?v=YHsEojBGojI
What does all three of Newton’s Law have in common?
Different forces
Contact Forces
Action-at-a-distance forces
Frictional Force
Gravitational Force
Tension
Electrical
Normal
Magnetic
Air Resistance
Applied
Spring
For this unit, we will only be dealing with contact forces.
What is a force?
A force is a push or pull acting upon an object as a
result of its interaction with another object.
Gravity, g
The first force we will investigate is that due to gravity, and we'll
call it the gravitational force.
We know that the acceleration due to gravity (if on Earth) is
approximately g = 9.8m/s2.
Vertically:
The force (weight), by Newton's Second Law is F = m g
Normal, FN
The normal force one which prevents objects from 'falling' into
whatever it is sitting upon. It is always perpendicular to the
surface with which an object is in contact.
For example, if there is a crate on the floor, then we say that the
crate experiences a normal force by the floor; and because of
this force, the crate does not fall into the floor. The normal force
on the crate points upward, perpendicular to the floor and equal
to the weight.
Friction, Ff
Related to the normal force is the frictional force. The two are
related because they are both due to the surface in contact with
the body. Whereas the normal force was perpendicular to the
surface, the frictional force is parallel. Furthermore, friction
opposes motion, and so its vector always points away from the
direction of movement.
Push and Pull
Another force which may act on an object could be any physical
push or pull. This could be caused by a person pushing a crate
on the floor, a child pulling on a wagon, or in the case of our
example, the wind pushing on the ship.
Tension
Tension in an object results if pulling force act on its ends, such
as in a rope used to pull a boulder. If no forces are acting on the
rope, say, except at its ends, and the rope itself is in equilibrium,
then the tension is the same throughout the rope.
Representations
http://www.youtube.com/watch?v=8vOVq_9yHS0
Free-Body Diagram
What is a free-body diagram?
A free-body diagram is a sketch of an object of interest with all
the surrounding objects stripped away and all of
the forces acting on the body shown.
Free-Body Diagram
How to draw a FBD
1.
2.
3.
4.
Draw the picture of the entire system.
ID the system
Draw the system alone
Draw and label the force vectors on the system
(Friction Force parallel
To the incline)
T
FN = F
FF
(Force perpendicular
to the incline)
F//
(Force parallel
To the incline)
Weight (mg)
Step #1
Step #2 Static crate on an incline. (no motion)
FN
T
T
Step #2 Static crate on an incline
connected to a hanging crate. (no
motion)
Fll
T
W1
W2
Net Force
Net force is the sum of the forces in a system.
To calculate for the net force on a system, add all the forces that
are going in the same or opposite direction.
25000 N
20000 N
2000 N
14000 N
Total force horizontally = 20000 – 2000 = 18000 N
Total force vertically = 25000 – 14000 = 11000 N
F Normal
Ffriction
F = ma
Weight
F Normal
Weight
Equilibrium
When objects are moving at a constant velocity, the
object is in dynamic equilibrium (all the forces on the
object balance)
When objects are not moving, the object is in static
equilibrium (all the forces on the object balance).
Dynamics
When objects move with varying speed (acceleration),
the object is not in equilibrium. There is an imbalance
in the forces acting on the object.