Physics · Both
Forces & Motion
Contact and non-contact forces, speed, acceleration, weight vs mass, and resultant force.
Forces change motion. You must distinguish mass and weight, calculate speed and resultant force, and understand balanced vs unbalanced forces.
Types of force
**Contact forces:** friction, air resistance, tension, normal contact force
**Non-contact forces:** gravity, magnetism, electrostatic
Force is measured in **newtons (N)**.
Force is a vector (has magnitude and direction).
Mass and weight
**Mass** – amount of matter (kg). Does not change with location.
**Weight** – force due to gravity (N).
Weight = mass × gravitational field strength
W = m g
On Earth g ≈ 9.8 N/kg (often use 10 N/kg in calculations).
Worked example
What is the weight of a 5 kg bag on Earth? (g = 10 N/kg)
Solution
W = 5 × 10 = **50 N**
Speed and resultant force
Average speed = distance / time
**Resultant force** = overall force when all forces are combined.
• Balanced forces (resultant = 0) → constant velocity (or at rest)
• Unbalanced forces → acceleration in the direction of the resultant force
F = m a (resultant force = mass × acceleration)
Practice questions
Try these without looking at the answers first.
Q1.Mass of an object is 12 kg. Weight on Earth (g = 10)?
Show answer
120 N
Q2.Car travels 150 m in 5 s. Average speed?
Show answer
30 m/s
Q3.What is the resultant force if forces are balanced?
Show answer
0 N (zero)