Laws of Motion
Standard 9
Force
A push or pull that can cause an object to change its velocity or which can cause a
flexible object to deform.
Balanced Forces
Force F1 Force F2
F1 and F2 are acting on the rope. If both teams are evenly matched then F1 = F2. But F1 and F2 are in the opposite direction and hence cancel each other.The net force on the rope = 0.
When a number of forces act on a body such that the net force acting on the body is 0, then such a combination of forces are known as balanced forces.
Types of Forces
MuscularFrictional
Contact Forces
Non-Contact Forces
Magnetic Gravitational Electrostatic
Objects are Lazy – 1
Smooth tiled floor
Rough Ground
Football will remain at the same place unless someone kicks it.The football stops rolling after sometime as friction opposes its motion.The football travels more distance on a smooth surface as the frictional force is smaller.If there was no friction, the football would keep on rolling.
First the football does not want to move and once moving it does not want to stop (unless some force is applied).
Objects are Lazy – 2
Stack of books
Toy vehicle
Little guy
Wall
What will happen when the bus hits the wall?
The bus stops moving when it hits the wall. However the little guy on top of the bus continues moving and topples over.
Newton’s First Law of MotionAn object remains at rest or in motion in a straight line with a
constant speed until acted on by an unbalanced force.Tendency of an object to resist change is called Inertia.
Newton’s Second Law of MotionMore force is required to move or stop an object with more mass.
You need to kick harder if you want the football to travel at a greater speed / velocity.
You will have to pedal harder if you want to reach the top speed in less time.
𝑭𝒐𝒓𝒄𝒆∝𝑴𝒂𝒔𝒔∗𝑪𝒉𝒂𝒏𝒈𝒆𝒊𝒏𝒗𝒆𝒍𝒐𝒄𝒊𝒕𝒚
𝑻𝒊𝒎𝒆𝒕𝒂𝒌𝒆𝒏∝𝑴𝒂𝒔𝒔∗ 𝑨𝒄𝒄𝒆𝒍𝒆𝒓𝒂𝒕𝒊𝒐𝒏
Newton’s Second Law of Motion
Momentum is defined as the total quantity of motion.
The rate of change of momentum is directly proportional to the impressed force and takes place in the direction in which the
force acts.
Newton’s Third Law of Motion
As the air escapes from the balloon it pushes the balloon forward.
Atmospheric pressure pushes the air out
Force of air on balloon pushes it forward
Wall
Straw String
Newton’s Third Law of Motion
Ball pushes the ground downwards
Ground pushes the ball upwards
When moving down, gravity acts in the direction of motion accelerating the ball.
When moving up, gravity opposes the motion decelerating the ball.
Newton’s Third Law of Motion
For every action there is an equal and opposite reaction.Action and Reaction act on different bodies and hence do not cancel each other.
When you take off your hand the force of the water pushes the block upwards.
The harder you push the block downwards, more the force with which water pushes the block back.
Block pushes the water downwards
Water pushes the block upwards
Conservation of Momentum
If there is no net force acting on two interacting bodies then their total momentum is conserved.
Units of MeasurementPhysical Quantity SI Units CGS Units
Force kg m/s2 or Newton g cm/s2 or dyne
Momentum kg m/s g cm/s