Q1. A force of 50 N acts on a 10 kg body for 4 s. Find the impulse and the change in velocity.
Impulse J = F×t = 50 × 4 = 200 N·s
J = Δp = mΔv ⇒ Δv = 200/10
Δv = 20 m/s
Laws of Motion — Class 11
Newton's laws explain every force problem. Compute force, friction and acceleration, and practise the classic numericals.
Newton's lawsFirst: a body continues at rest/uniform motion unless acted on by a force (inertia). Second: F = ma. Third: action and reaction are equal and opposite.
Momentum & impulseMomentum p = mv; impulse = force × time = change in momentum (J = FΔt = Δp).
FrictionA force opposing relative motion, f = μN, where μ is the coefficient of friction and N the normal reaction.
Circular motionNeeds a centripetal force F = mv²/r directed towards the centre (provided by friction, tension or gravity).
Force & friction calculator Interactive
Find the net force (F = ma) and the friction force (f = μN) for a body on a surface.
Net force, F = ma—N
Friction, f = μmg—N
To actually move the body, the applied force must first exceed the maximum static friction μmg.
Numericals practice Interactive
Force, friction and impulse problems with steps.
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Solved numericals
Q2. A 1200 kg car goes round a curve of radius 40 m at 10 m/s. Find the centripetal force.
F = mv²/r
F = 1200 × 10² / 40
F = 3000 N (provided by friction)
Formula sheet
| Quantity | Formula | SI unit |
|---|---|---|
| Newton's second law | F = m a | newton (N) |
| Momentum | p = m v | kg·m/s |
| Impulse | J = F t = Δp | N·s |
| Force of friction | f = μ N | N |
| Centripetal force | F = m v² / r | N |
Common mistakes & exam wins
- Draw a free-body diagram first — mark every force acting on the body.
- Friction f = μN acts opposite to (attempted) motion; on a horizontal surface N = mg.
- Impulse = area under a force–time graph = change in momentum.
- Centripetal force is not a new force — it is provided by tension, friction or gravity.
Frequently asked questions
What is Newton's second law?
The net force on a body equals the rate of change of its momentum, which for constant mass gives F = ma.
What is the force of friction?
A contact force opposing relative motion, given by f = μN, where μ is the coefficient of friction and N the normal reaction.
What is impulse?
The product of force and the time for which it acts, equal to the change in momentum: J = FΔt = Δp.