Bài 10. Ba định luật Niu-tơn

Newton’s
Laws of Motion
I. Law of Inertia
II. F=ma
III. Action-Reaction
While most people know what Newton`s laws say, many people do not know what they mean (or simply do not believe what they mean).
1st Law of Motion
(Law of Inertia)

An object at rest will stay at rest, and an object in motion will stay in motion at constant velocity, unless acted upon by an unbalanced force.
1st Law
Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless.
1st Law
1st Law
Unless acted upon by an unbalanced force, this an apple would sit on the table forever.
Why then, do we observe every day objects in motion slowing down and becoming motionless seemingly without an outside force?
It’s a force we sometimes cannot see – friction.
There are four main types of friction:
Sliding friction:
Rolling friction:
FluiSlidingd friction (air or liquid): air or water resistance
Static friction: initial friction when moving an object
Friction!
What is this unbalanced force that acts on an object in motion?
Newtons’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 km/hour.
2nd Law
The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.
2nd Law
F = m x a
2nd Law
The net force of an object is equal to the product of its mass and acceleration, or F=ma.
2nd Law

When mass is in kilograms and acceleration is in m/s/s, the unit of force is in newtons (N).
One newton is equal to the force required to accelerate one kilogram of mass at one meter/second/second.
2nd Law (F = m x a)
How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second?
Write the formula
F = m x a
Fill in given numbers and units
F = 1400 kg x 2 meters per second/second
Solve for the unknown
2800 kg-meters/second/second or 2800 N
If mass remains constant, doubling the acceleration, doubles the force. If force remains constant, doubling the mass, halves the acceleration.
Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.
We know that objects with different masses accelerate to the ground at the same rate.
However, because of the 2nd Law we know that they don’t hit the ground with the same force.
F = ma
98 N = 10 kg x 9.8 m/s/s
F = ma
9.8 N = 1 kg x 9.8 m/s/s
Check Your Understanding

1. What acceleration will result when a 12 N net force applied to a 3 kg object?
12 N = 3 kg x 4 m/s/s

 
2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.
16 N = 3.2 kg x 5 m/s/s
 
3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?

66 kg-m/sec/sec or 66 N

4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

 9800 kg-m/sec/sec or 9800 N
3rd Law
For every action, there is an equal and opposite reaction.
3rd Law
According to Newton, whenever objects A and B interact with each other, they exert forces upon each other.
3rd Law
There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.
Newton’s 3rd Law in Nature
Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.
The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).
3rd Law
Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.
Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards.
The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards).
Action-reaction force pairs make it possible for birds to fly.
Other examples of Newton’s Third Law
The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).
3rd Law
Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards.
3rd Law
The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases.
The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.