The core idea
The net external force changes an object’s momentum. For constant mass in an inertial frame, F_net = ma. Friction is an interaction between surfaces that opposes relative sliding or its tendency, and can help an object move.
1. Begin with one object and its interactions
A force describes an interaction: Earth attracts a bag, a table supports a book, or a hand pushes a trolley. Draw a boundary around the object you want to analyse, then draw arrows for external forces acting on that object. Arrow direction shows force direction; length can represent magnitude. Do not add a “force of motion” merely because the object is moving. Net force is the vector sum, so opposite directions subtract rather than simply add. Contact forces include support, tension and friction; gravity can act without contact. The diagram is useful because it prevents forces acting on neighbouring objects from slipping into the wrong calculation.
Sources: NCERT: Laws of Motion ↗
2. Balanced forces do not require rest
Newton’s first law says that an object remains at rest or continues with constant velocity unless a net external force acts. An inertial reference frame is one in which this law holds; a frame moving at constant velocity relative to it is also inertial. Ordinary school ground-based problems usually use that approximation. Inertia is resistance to a change in motion, and mass measures it; inertia is not an additional pushing force. A moving cycle needs continued effort on a level road because resistive forces exist, not because motion itself consumes a forward force. If forward and backward forces balance, velocity remains constant; if there is an imbalance, velocity changes.
Sources: NCERT: Laws of Motion ↗
3. Net force determines acceleration, not velocity
For a constant mass, Newton’s second law is F_net = ma: net force equals mass times acceleration. Force is measured in newtons (N), where 1 N = 1 kg·m/s². A given force produces less acceleration in a larger mass. Momentum is p = mv, with unit kg·m/s, and includes direction. More generally, average net force over an interval equals change in momentum divided by time. The product of average force and time is impulse, measured in N·s. These relationships explain why increasing stopping time reduces average force for a fixed momentum change. They describe the net force, not automatically the force of one selected hand, motor or surface.
Sources: NCERT: Laws of Motion ↗
4. Action and reaction act on different objects
If object A exerts a force on B, B simultaneously exerts an equal and opposite force on A. These are the two sides of one interaction, not a delayed response. When a shoe pushes the ground backward, the ground pushes the shoe forward. The pair does not cancel in the shoe’s force diagram because one force acts on the ground. A book’s downward weight and the table’s upward support can balance, but they are not a third-law pair: both act on the book and arise from different interactions. Their partners act on Earth and on the table respectively. Naming the receiving object prevents this common confusion.
Sources: NCERT: Laws of Motion ↗
5. Friction and support depend on the situation
Weight near Earth is mg, where g is gravitational acceleration. On a level surface with no vertical acceleration or other vertical forces, the normal support R equals mg; this equality is not universal. Static friction adjusts to oppose impending relative slip, up to a limiting value often modelled as μ_sR. Sliding friction is approximately μ_kR in a simple model. The coefficients μ_s and μ_k are dimensionless and depend on the contacting surfaces and conditions. Static friction is not always at its maximum. Friction can point forward on a walking person or driven wheel, so “friction always opposes the object’s motion” is an unreliable rule. Ask which surfaces would slip relative to each other.
Sources: NCERT: Laws of Motion ↗
6. Worked example: a push may not start motion
Illustrative level-floor model: a 10 kg trolley has normal support 100 N using g = 10 m/s². Treat the wheels as locked, so contact can be modelled as sliding once motion begins. Suppose its maximum static friction is 20 N. A horizontal push of 15 N is balanced by 15 N static friction, so acceleration is zero. Now suppose it is sliding, a 35 N push acts, and μ_k = 0.20. Sliding friction is 0.20 × 100 = 20 N. Net forward force is 35 − 20 = 15 N, giving acceleration 15/10 = 1.5 m/s². We assumed constant mass, a horizontal push, the stated sliding-friction model and negligible other resistance. Applying 35/10 would wrongly ignore friction.
Add only forces acting on the chosen object
Sources: NCERT: Laws of Motion ↗
7. Worked example: a cricket-ball stopping model
Illustrative paper problem: a 0.15 kg ball moving at +20 m/s is brought to rest in 0.10 s. Momentum changes from +3.0 kg·m/s to zero, so Δp = −3.0 kg·m/s. Average net stopping force is −3.0/0.10 = −30 N, directed opposite the initial motion. If the same change occurs over 0.20 s, average force is −15 N. The impulse remains −3.0 N·s in both cases. This explains the value of a longer stopping interval without implying the force is constant throughout contact. Actual contact forces also depend on deformation and other forces, so the model is not a measurement of a real catch.
Sources: NCERT: Laws of Motion ↗
PUT IT INTO PRACTICE
Apply it and check your reasoning
- Draw forces on an illustrative 5 kg box sliding horizontally with a 12 N push and 7 N opposing friction. State which forces act on the box rather than on the hand.
- Calculate net horizontal force and acceleration. Then identify the reaction partner of the hand’s push, including which object receives it.
- Check: net force 5 N, acceleration 1 m/s². The box pushes the hand backward; that force belongs on the hand’s diagram, not on the box’s.
Check your understanding
Can balanced forces act on a moving object?
Yes. Zero net force means zero acceleration in the stated inertial model, so constant velocity is possible. It does not require velocity itself to be zero.
Why do action and reaction not cancel for one object?
They act on different objects. A net-force calculation includes forces received by the selected object, not every force anywhere in the interaction.
Does static friction always equal its limiting value?
No. It takes the value needed to prevent slipping until the limit is reached. A smaller push can be balanced by a smaller static friction.
Why do different masses have the same ideal free-fall acceleration?
Gravitational force is proportional to mass, while acceleration is force divided by mass. The mass cancels when air resistance is neglected and the same g applies.
What stays unchanged when stopping time doubles for the same momentum change?
Impulse stays unchanged. The average net force halves, but its time integral still accounts for the same total momentum change.