Newton's Laws of MotionNewton's Laws of Motion

1.1.Newton 1Newton 1ststlaw of motionlaw of motion

2.2.Newton 3Newton 3rdrdlaw of motionlaw of motion

3.3.Newton 2Newton 2ndndlaw of motionlaw of motion

Newton 1Newton 1ststlaw of motionlaw of motion

Newton's first law of motion is often Newton's first law of motion is often stated as:stated as:

An object at rest tends to stay at rest An object at rest tends to stay at rest and an object in motion tends to stay in and an object in motion tends to stay in motion with the same speed and in the motion with the same speed and in the same directionsame direction

Unless acted upon by an unbalanced Unless acted upon by an unbalanced forceforce

there are two parts to this statement :1-prediction of the behavior of stationary object

2-prediction of the behavior of moving object The two parts are summarized in the following diagram

Balanced and Unbalanced ForcesBalanced and Unbalanced Forces

consider a physics book at rest on a table top. consider a physics book at rest on a table top. There are two forces acting upon the book. There are two forces acting upon the book.

.One force - the Earth's gravitational pull - exerts Force the other force the push of the table on the book

(normal force )pushes upward on the book

downward

Since the tow forces are of equal magnitude and in opposite

Direction they balance each other the book is said to be in Equilibrium there is o unbalanced force act

upon on the bookAnd thus the book maintain its state of motion

For every action there is an equal and opposite reaction.

.In other words, when one object exerts a force on another object, the second object exerts a force of equal strength in the opposite direction on the first object

Newtons 2nd law of motion

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards the air must also be pushing the bird upwards Action-reaction force pairs make it possible for birds to fly.

Newton 2Newton 2ndndlaw of motionlaw of motion The acceleration of an object is directly The acceleration of an object is directly

proportional to the net force acting on the proportional to the net force acting on the object, is in the direction of the net force, and object, is in the direction of the net force, and is inversely proportional to the mass of the is inversely proportional to the mass of the object. object.

Example: what force is necessary to cause a Example: what force is necessary to cause a 1000 kg car to accelerate at 2 m/s21000 kg car to accelerate at 2 m/s2 ? ?

F=m a = 1000 kg x 2 m/s2 = 2000 kg-m/s2 = F=m a = 1000 kg x 2 m/s2 = 2000 kg-m/s2 = 2N2N

From our daily life, we can can observe thatFrom our daily life, we can can observe that: :

1.1. When a force When a force FF is applied on an object, is applied on an object, VV, , the change of the velocity of the object, the change of the velocity of the object, increases with the length of time delta t increases with the length of time delta t increases; increases;

2.2. The greater the force The greater the force FF, the greater , the greater VV; and ; and

3.3. The larger the body (object) is, the less The larger the body (object) is, the less easily accelerated by forces.easily accelerated by forces.

Newton's second lawNewton's second law

TheThe acceleration is directly acceleration is directly proportional to the net force applied.proportional to the net force applied.•The net force also determines the direction of the

acceleration•The direction of the acceleration is in the same direction as the net force.•The acceleration is inversely proportional to the mass of the object.

Newtons second law state thatNewtons second law state that::

""If the net force on an object is not zero, If the net force on an object is not zero, the object will acceleratethe object will accelerate. . The The direction of the acceleration is the direction of the acceleration is the same as the direction of the net forcesame as the direction of the net force. . The magnitude of the acceleration is The magnitude of the acceleration is directly proportional to the net force directly proportional to the net force applied, and inversely proportional to applied, and inversely proportional to the mass of the objectthe mass of the object."."

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Free Body DiagramsFree Body Diagrams

Some examples will have more than one Some examples will have more than one force acting on an object. It is advisable to force acting on an object. It is advisable to draw a diagram of the situation showing draw a diagram of the situation showing the direction of all forces present acting the direction of all forces present acting through one point. These are known as through one point. These are known as free body diagrams.free body diagrams.

ExamplesExamplesA woman is standing on a set of A woman is standing on a set of

bathroom scales in a stationary lift bathroom scales in a stationary lift everyday .The reading on the scales is everyday .The reading on the scales is 500 N. When she presses the ground 500 N. When she presses the ground floor button, the lift accelerates floor button, the lift accelerates downwards and the reading on the downwards and the reading on the scales at this moment is 450 N. Find scales at this moment is 450 N. Find the acceleration of the liftthe acceleration of the lift

A A ski tow pulls two skiers who are connected by ski tow pulls two skiers who are connected by a thin nylon rope along a frictionless surface. The a thin nylon rope along a frictionless surface. The tow uses a force of 70 N and the skiers have tow uses a force of 70 N and the skiers have masses of 60 kg and 80 kg.masses of 60 kg and 80 kg.FindFind

a) acceleration of the systema) acceleration of the system

b) tension in the rope.b) tension in the rope.

a) Total mass, m = 140 kg, a = F/m = 70/140 = 0.5 m s-2

b) Consider the 60 kg skier alone.Tension, T = ma = 60 x 0.5 = 30 N

Force Acting Down a PlaneForce Acting Down a Plane If an object is placed on a slope then If an object is placed on a slope then

its weight acts vertically downwards. its weight acts vertically downwards. A certain component of this force will A certain component of this force will act down the slope. The weight can act down the slope. The weight can be split into two components at right be split into two components at right angles to each other. angles to each other.