2. An object at rest tends to stay at rest and an
object in motion tends to stay in motion unless
acted upon by an unbalanced force.
Inertia is the tendency of an object to resist
changes in its velocity: whether in motion or
motionless.
3. Example- A school bus comes to a sudden stop, and all of the backpacks on
the floor start to slide forward. What force causes them to do that?
Answer: No force; the backpacks continue moving until
stopped by friction or collision.
4. Alternative Statement
In the absence of external forces, when viewed from an
inertial reference frame, an object at rest remains at
rest & an object in motion continues in motion with a
constant velocity
Newton’s 1st Law describes what happens in the absence of a net
force.
Also tells us that when no force acts on an object, the acceleration of
the object is zero.
5. Inertia: The tendency of a body to maintain its state of
rest or motion.
Mass: A measure of the inertia of a body,
Quantity of matter in a body
Quantify mass by having a standard
mass = Standard Kilogram (kg)
SI Unit of Mass = Kilogram (kg)
Weight: The force of gravity on an object
6. Newton’s second law gives the relationship of
acceleration to force and inertia.
Newton’s Second Law:
The acceleration of an object is directly
proportional to the net force acting on the object, is
in the direction of the net force, and is inversely
proportional to the mass of the object.
The acceleration of an object is always in the
direction of the net force.
7. When objects are changing direction or velocity:
If any object is changing direction or velocity (the type of motion), we say the object is
accelerating (the state of motion). This unit will show the concepts behind
accelerations, what causes acceleration (net force),
Newton's 2nd Law of Motion.
8. Experiment : The net force F on a body and the acceleration
a of that body are related.
Thousands of experiments over hundreds of years find
(object of mass m):
------- (Proportionality)
We choose the units of force so that this is not just a
proportionality but an equation:
or
m
f
a
m
F
a
maF
F = the net force acting on mass m
m = the mass (inertia) of the object.
a = acceleration of object.
Description effect of F. F is the cause of a.
9. Example : Force to stop a car.
What average net force is required to bring a 1500-kg car to
rest from a speed of 100 km/h with in a distance of 55 m?
In Figure .
First, find the acceleration (assumed constant) from the initial and final
speeds and the stopping distance;
a = -7.1 m/s2.
Then use Newton’s second law: F = -1.1 x 104 N.
10. The law of action and reaction.
For every action there is an equal and opposite
reaction.
Whenever one object exerts a force on a second
object, the second object exerts an equal (in
magnitude) and opposite (in direction) force on the
first object
11. Example :
Rocket propulsion can also be explained using Newton’s
third law: hot gases from combustion spew out of the tail of
the rocket at high speeds. The reaction force is what propels
the rocket.
Note that the rocket does not need anything
to “push” against.
11
In Figure :-
The launch of a rocket. The rocket engine pushes the
gases downward, and the gases exert an equal and
opposite force upward on the rocket, accelerating it
upward.
(A rocket does not accelerate as a result of its
propelling gases pushing against the ground.)
