Newtons Second Law of Motion

Newtons Second Law of Motion

Chapter 2 force and motion

Newtons Second Law of Motion

From the experiment 2.2

The acceleration is directly proportional to the force.

From the experiment 2.3

The acceleration is inversely proportional to its mass

Newtons Second Law of Motion

In order to make the formula as simple as possible, we make = 1.

Force of 1N is defining as,

1 N is the force which gives a mass of 1 kg an acceleration of 1 ms-2

Balanced Forces & Unbalanced Forces

In general, there may be several forces acting on the mass, whether parallel or anti-parallel, or in different directions.

Thus, the force, F, must be replaced with net or resultant force when there are several forces acting on the mass.

Where a is in the direction of the net or resultant force.

Balanced Forces

When the forces acting on an object are balanced, (net force = 0)

The object then behave s as if there is no forces acting on it.

Since Fnet = 0, the acceleration of the object, a =0. Thus, the object remains at rest or moves at constant velocity when there is no net force acting on it.

Example of balanced forces:

Balanced forces on a stationary gymnast

The weight of the gymnast, W, is balanced by the reaction force, R, from the beam.

The two forces are of equal magnitude but opposite in direction.

Without the beam (that is, no reaction force), the gymnast will fall to the ground because of her weight.

Balanced forces on a car moving at a constant velocity.

There are 3 horizontal forces acting on a car moving at a constant velocity.

The forward thrust, T, provided by the car engine is balanced by the frictional force on the wheels and the air resistance

T = G + Ff

The weight of the car, W, is vertically balanced by the reaction force, R, from the road.

W = R

T = G + Ff

Balanced Forces ( = 0, a =0)

Fnet = 0

(as no force acting on it)

F1 = F2

From Fnet = ma

0 = ma

(since mass, m, cannot be zero)

Balanced Forces ( = 0, a =0)

Object at rest

(v = 0 ms-1)

Zamri pushes a heavy cupboard with a force of 200N, but the cupboard does not move.

Find the frictional force acting on the cupboard.

Balanced Forces ( = 0, a =0)

Using Fnet = ma

But Fnet = 0, since a = 0

the frictional force here is known as static friction

Balanced Forces ( = 0, a =0)

Object in motion

(v 0, object is moving at constant velocity)

Linda pushes a book on a table with a force of 5 N. The book moves with a uniform velocity of 2 cm s-1. Find the frictional force acting on the book.

Balanced Forces ( = 0, a =0)

Using Fnet = ma

But Fnet = 0, since a = 0

the frictional force here is known as dynamic friction

Unbalanced Forces

When the forces acting on an object are not balanced, the object will accelerate in the direction of the net force.

The net force is known as the resultant force.

Effect of Balanced Forces and Unbalanced Forces on an Object

Balanced forces (Fnet = 0, a = 0)

Effect of Balanced Forces and Unbalanced Forces on an Object

Balanced forces on an aircraft allow it to move at constant velocity at a constant altitude.

The engine thrust is balanced by the drag due to air resistance while the weight of the aircraft is balanced by a lift from the wings.

Effect of Balanced Forces and Unbalanced Forces on an Object

Unbalanced forces (Fnet 0, a 0)

When the forces acting on an aircraft do not cancel out each other, a net force known as unbalanced force is acting on the object.

Unbalanced forces produce an acceleration to the mass on which force are acting.

Effect of Balanced Forces and Unbalanced Forces on an Object

However, the object will accelerate in the direction of the net force.

When an airplane is moving at a constant velocity, if the pilot increases the engine thrust, the forces acting horizontally are no longer balanced. There is a net force and plane will accelerate in the forward direction.