Newton's Laws · Force & Acceleration · Friction · Action-Reaction · F = ma
In Chapter 4, you learnt to describe the motion of an object in terms of its position, velocity and acceleration. But what causes motion? What makes an object speed up, slow down, change direction, or stay still? In this chapter, we investigate what causes changes in the motion of objects.
A force can make an object move from rest, change the speed and direction of a moving object, and even change the shape of an object.
A ball at rest starts moving when you apply a force by kicking it.
The force applied by a cricket bat changes the direction of the ball.
The force applied by your fingers changes the shape of the lemon.
A spring balance can be used to measure the magnitude of a force. When you pull on its free end, it measures the force with which you pull on the spring inside. The weight of an object (gravitational force by the Earth) can also be measured using a spring balance.
If you hold a 100 g mass in your palm, the upward force your palm applies on it is around 1 N. That is how a force of 1 N "feels"!
In everyday life, the smallest forces we can feel are of the order of millinewtons (10⁻³ N). Scientists can measure down to yoctonewtons (10⁻²⁴ N)!
In real life, more than one force usually acts on an object at the same time. What happens depends on whether these forces are balanced or unbalanced.
Equal forces, opposite directions -- rope doesn't move
One team pulls harder -- rope moves towards the stronger team
The overall combined effect of all forces acting on an object
Net force = Sum of forces
Two people pushing a car in the same direction: net force = F₁ + F₂
Net force = Difference of forces (in direction of larger force)
Tug of war with unequal pulls: net force = F₁ - F₂
Two forces of 10 N and 6 N act on a block:
(a) Both in same direction (right): Net force = 10 N + 6 N = 16 N towards right
(b) Opposite directions (10 N right, 6 N left): Net force = 10 N - 6 N = 4 N towards right
(c) Opposite directions (10 N left, 6 N right): Net force = 10 N - 6 N = 4 N towards left
Two forces on the barbell:
• Gravitational force (weight) acting downwards
• Force applied by the weightlifter acting upwards
• Since the barbell is steady (not moving), the net force is zero. So yes, these forces are balanced!
Enter Force 1 (left team) and Force 2 (right team). Watch the rope and object respond!
Friction is a force that acts between the surfaces of objects in contact, opposing the motion of the object. It is always present and often overlooked!
When you stop pushing a ball, friction gradually slows it down and brings it to rest. You must continuously apply force to counter friction.
When you stop pedalling, friction between the tyres and road (plus air resistance) gradually decelerates the bicycle until it stops.
The force of friction depends on the nature of the surfaces in contact. Smooth surfaces have less friction, rough surfaces have more. This is demonstrated in the NCERT coin-and-rubber-band activity:
Rough surface = more friction = coins travel shorter distance
Smoother surface = less friction = coins travel farther
Very smooth surface = least friction = coins travel farthest
• You could never walk -- your feet would slip on the ground!
• Vehicles could never stop -- brakes rely on friction
• Objects once set in motion would never come to rest
• You could not hold anything -- objects would slip from your hands
• Writing would be impossible -- pens rely on friction with paper
Sort these surfaces into "High Friction" or "Low Friction" by clicking on each chip. Click a placed chip to move it back.
In other words, if the net force acting on an object is zero, the object cannot begin to move or change its velocity. Its acceleration is zero.
If net force = 0, an object at rest stays at rest. Zero velocity remains zero.
If net force = 0, a moving object keeps moving in a straight line with constant speed.
Q: A person pushes a moving box forward with a force equal to friction. Will the box continue moving or stop?
A: The two forces (applied force forward, friction backward) are equal and opposite, so they balance each other. The net force is zero. By Newton's first law, the box will continue moving with constant velocity.
In the 17th century, Galileo argued that if all impediments to motion are removed, a body moving along a horizontal plane will continue to move indefinitely. This challenged centuries of wrong thinking!
Newton used the word "inertia" to describe the tendency of objects to resist change. He presented three laws of motion in 1687 -- a defining moment in science. The unit of force (newton) is named after him.
No! According to Newton's first law, if an object is moving with constant velocity, the net force acting on it is zero. There may be multiple forces acting, but they must be balanced (cancelling each other out).
Newton's first law tells us what happens when net force is zero. But what happens when a net force acts on an object? A force produces acceleration. Newton's second law gives us the exact relationship between force, mass, and acceleration.
When a net force acts on an object, the object accelerates in the direction of the net force. The acceleration is proportional to the force and inversely proportional to the mass.
A stronger push on a ball gives it a larger acceleration (moves faster from rest).
With the same force, a lighter object accelerates more than a heavier one.
A fielder pulls their hands backward while catching a fast ball. This increases the time over which velocity reduces to zero, reducing acceleration and thus reducing the force on hands. Less chance of injury!
During a collision, airbags inflate into a soft cushion. The passenger's head pushes into the bag, increasing the stopping time. This reduces acceleration and hence reduces force, lowering injury risk.
A coconut is brought down at very high velocity to hit a hard surface. It stops in a very short time, so the ground exerts a very large force on it -- enough to break the shell!
Q: A weightlifter holds a barbell with 10 kg on each side. The bar itself is 10 kg. How much force does she apply?
A: Total mass = 30 kg
Gravitational force = mg = 30 × 9.8 = 294 N (downward)
To keep the barbell steady, she must apply 294 N upward.
Q: A 25 kg block has maximum friction of 50 N. Find displacement in 2 s if pushed with (i) 50 N and (ii) 55 N.
(i) Force = 50 N: Net force = 50 - 50 = 0 N. Block remains stationary. Displacement = 0 m.
(ii) Force = 55 N: Net force = 55 - 50 = 5 N.
a = F/m = 5/25 = 0.2 m s⁻²
s = ut + ½at² = 0 + ½ × 0.2 × 4 = 0.4 m forward.
By F = ma, for the same acceleration (a), a larger mass (m) requires a larger force (F).
So, you would need to apply a larger force on the heavier child to impart the same acceleration.
Enter any two values and leave the third blank. The calculator will find the missing value with step-by-step working!
Have you experienced that when you kick a ball, you feel a force on your foot? When you push a table, the table pushes you back? This is Newton's third law in action!
Feet push ground backward; ground pushes you forward (friction!)
Paddle pushes water backward; water pushes canoe forward
Engine expels gas downward; gas pushes rocket upward
Legs push trunk down; friction pushes person upward
In the NCERT activity, an inflated balloon on a straw moves in the opposite direction to the escaping air. This is exactly how a rocket works! The engine expels gas downward, and the gas pushes the rocket upward. If the upward force exceeds the rocket's weight, the net force is upward and the rocket lifts off.
The Vikram lander fired its engine in the direction of motion to slow down. The exhaust gases pushed forward, but the reaction force pushed the lander backward -- enabling a soft landing near the Moon's south pole!
Q: The Earth and a fruit apply equal and opposite gravitational forces on each other. Then why does the fruit fall towards the Earth while the Earth doesn't seem to move towards the fruit?
A: Though forces are equal, the mass of the Earth is enormously larger than the fruit. By F = ma, the acceleration of the Earth = F / MEarth, which is extremely small -- too small to be noticed!
Q: A 0.1 kg bullet is fired from a 5 kg gun with a force of 2 N. What are the initial accelerations?
A: By Newton's 3rd law, recoil force on gun = 2 N.
Acceleration of bullet = 2 / 0.1 = 20 m s⁻²
Acceleration of gun = 2 / 5 = 0.4 m s⁻²
Equal forces, but very different accelerations because of different masses!
Newton's third law applies to all types of forces -- contact and non-contact. It works for magnetic forces (bar magnets), electrostatic forces (charged balloons), and gravitational forces (Earth and fruit).
Two bar magnets exert equal and opposite magnetic forces on each other.
Two similarly charged balloons exert equal and opposite electrostatic forces on each other.
Earth and fruit exert equal and opposite gravitational forces on each other.
Click the tabs to explore action-reaction pairs in Magnetic, Electrostatic & Gravitational forces. Drag objects to see forces change!
By using Newton's third law! The spacecraft can fire its engine to expel gas in one direction. By Newton's third law, the gas pushes the spacecraft in the opposite direction, changing its velocity. This is exactly how rockets manoeuvre in space where there is nothing to push against!
Newton's laws can also be applied to two or more objects connected together (a system). The key idea is to treat all connected objects as a single system.
Two boxes of masses m₁ and m₂ are on a frictionless surface, connected by a string. A force F pulls Box 1.
Instead of analysing each box separately, we can treat them as a single system of mass (m₁ + m₂).
Internal forces (tension in the string) cancel out. Only external forces (the applied force F) determine the system's acceleration.
Set the masses and applied force, then watch both boxes accelerate together. See force diagrams for each object!
Enter mass and velocity for two objects. See their momentum and watch a collision animation showing conservation of momentum!
Positive velocity = moving right, Negative velocity = moving left
Watch the animated scenario, then identify which Newton's Law applies. Get 8/8 to be a Newton Master!
A vector quantity (magnitude + direction). Can set objects in motion, change speed/direction, or change shape. SI unit: newton (N).
Balanced forces: Equal magnitude, opposite direction -- no change in motion. Unbalanced forces: Non-zero net force -- acceleration occurs.
Acts opposite to direction of motion. Depends on surface nature. Without friction, a moving object would never stop!
Object at rest stays at rest; object in motion stays in motion with constant velocity -- unless a net force acts.
F = ma. Acceleration is proportional to net force and inversely proportional to mass. 1 N = 1 kg m s⁻².
Every action has an equal and opposite reaction. Forces always occur in pairs, but act on two different objects.
p = mv. Rate of change of momentum equals the net force acting on the object (complete form of 2nd law).
Connected objects can be treated as a single system. Only external forces matter; internal forces cancel out.
Newton's First Law (Law of Inertia): An object at rest remains at rest, and an object in motion continues moving with constant velocity, unless a net force acts on it. Example: A book on a table stays at rest until pushed. A ball rolling on a frictionless surface would never stop.
Newton's Second Law (F = ma): When a net force acts on an object, it accelerates in the direction of the force. The acceleration is proportional to the force and inversely proportional to mass. Example: Pushing a heavy box requires more force than pushing a light one for the same acceleration.
Newton's Third Law (Action-Reaction): Whenever one object exerts a force on a second object, the second simultaneously exerts an equal and opposite force on the first. Example: When you walk, your feet push the ground backward and the ground pushes you forward.
Relationship: The first law defines what happens without a net force (no acceleration). The second law quantifies what happens when a net force is present (a = F/m). The third law explains that forces always come in pairs between interacting objects. Together, they provide a complete framework for understanding motion.
Friction is a force that acts between surfaces in contact, opposing the relative motion. It depends on the nature of the surfaces in contact.
NCERT Activity 6.1: A stack of coins is launched by a rubber band on different surfaces (wood, laminate, marble). On smoother surfaces, the coins travel farther because friction is less, so the velocity decreases more slowly.
NCERT Activity 6.2: A spring balance is used to pull a wooden block on different surfaces. The reading when the block just starts moving gives an approximate measure of friction. Smoother surfaces give smaller readings.
Galileo's thought experiment: If friction were zero, a moving object would never stop -- it would move forever with constant velocity. This revolutionary idea led to Newton's first law.
Importance: Without friction, we could not walk (feet would slip), vehicles could not stop (brakes rely on friction), and we could not hold objects. Grooves on shoe soles and treads on tyres increase friction for safety. However, friction also causes wear and energy loss, so reducing it (using lubricants, streamlining) is sometimes important.
(i) 0 to 5 s: u = 0, v = 10 m s⁻¹, t = 5 s
a = (v - u)/t = (10 - 0)/5 = 2 m s⁻²
F = ma = 1500 × 2 = 3000 N (towards east)
(ii) 5 to 10 s: Constant velocity means a = 0
F = ma = 1500 × 0 = 0 N (no net force)
(iii) 10 to 15 s: u = 10 m s⁻¹, v = 0, t = 5 s
a = (0 - 10)/5 = -2 m s⁻²
F = 1500 × (-2) = -3000 N (towards west, opposite to motion)
System approach: Instead of analysing each box separately, we treat the two boxes and the string as a single system.
The internal force (tension T in the string) acts between the two boxes within the system. The external force is F (the applied force).
By Newton's second law for the system: a = F / (m₁ + m₂)
The system accelerates as if it were a single object of total mass (m₁ + m₂).
Why simpler: In the system approach, internal forces (tension) cancel out and we don't need to calculate them. If we analysed each box individually, we would need to find the tension first, then use it to find the acceleration of each box -- more work for the same result! This shows the power of Newton's laws in studying complex systems.