Forces and Gravity
Understand what forces are, how they are measured, the difference between weight and mass, and how to interpret force diagrams.
What is a Force?
A force is a push or a pull that acts on an object. Forces can cause an object to start moving, stop moving, change direction, change shape, or change speed.
Forces are measured in Newtons (N), named after Sir Isaac Newton. A force has both magnitude (size) and direction, making it a vector quantity.
Types of Forces
Gravity: Weight vs Mass
Gravity is a non-contact force that pulls objects with mass towards each other. On Earth, gravity pulls everything towards the centre of the planet. The gravitational field strength on Earth is approximately 9.8 N/kg (often rounded to 10 N/kg for calculations).
Mass
- The amount of matter in an object
- Measured in kilograms (kg)
- Measured using a balance
- Does NOT change with location
- A scalar quantity (size only)
Weight
- The force of gravity acting on an object
- Measured in Newtons (N)
- Measured using a spring balance (newton meter)
- Changes depending on gravitational field strength
- A vector quantity (size + direction)
Weight Formula
Weight (N) = Mass (kg) × Gravitational Field Strength (N/kg)
W = m × g
On Earth: g ≈ 9.8 N/kg. On the Moon: g ≈ 1.6 N/kg.
Balanced vs Unbalanced Forces
When multiple forces act on an object, the overall effect depends on whether the forces are balanced or unbalanced. The combined effect of all forces is called the net force (or resultant force).
Force Diagrams
Balanced Forces
Unbalanced Forces
Balanced (Net Force = 0)
The object is either stationary or moving at constant speed in a straight line. There is no acceleration.
Unbalanced (Net Force ≠ 0)
The object will accelerate (speed up, slow down, or change direction) in the direction of the larger force.
Friction and Air Resistance
Friction
A contact force that opposes motion between two surfaces that are touching. Friction acts in the opposite direction to movement.
- Rough surfaces produce more friction
- Smooth surfaces produce less friction
- Friction generates heat
- Can be useful (brakes, grip) or a nuisance (wear, energy loss)
Air Resistance (Drag)
A type of friction caused by an object moving through air. It opposes the direction of motion.
- Increases with speed
- Increases with surface area
- Streamlined shapes reduce drag
- Parachutes work by increasing air resistance
Force Diagram: Falling Object
When air resistance equals weight, the object reaches terminal velocity (constant speed).
Key Vocabulary
Newton (N)
The SI unit of force. One Newton is the force needed to accelerate 1 kg by 1 m/s².
Net Force
The overall (resultant) force on an object when all forces are combined. Also called the resultant force.
Gravity
A non-contact force of attraction between objects with mass. On Earth, g ≈ 9.8 N/kg.
Friction
A contact force that opposes the relative motion of two surfaces in contact. Always acts opposite to the direction of motion.
Terminal Velocity
The constant maximum speed reached when the driving force equals the resistive force (e.g. weight = air resistance).
Vector
A quantity that has both magnitude (size) and direction, e.g. force, velocity.
Worked Examples
Calculate the weight of a 60 kg person on Earth (g = 10 N/kg).
Step 1: Write the formula: W = m × g
Step 2: Substitute: W = 60 × 10
Answer: W = 600 N
A box has a 100 N push to the right and 40 N friction to the left. What is the net force?
Step 1: The forces act in opposite directions. Subtract the smaller from the larger.
Step 2: Net force = 100 N - 40 N = 60 N
Answer: The net force is 60 N to the right. The box will accelerate to the right.
The same 60 kg person stands on the Moon (g = 1.6 N/kg). What is their weight on the Moon? Has their mass changed?
Step 1: W = m × g = 60 × 1.6
Step 2: W = 96 N
Answer: Their weight on the Moon is 96 N (compared to 600 N on Earth). Their mass remains 60 kg — mass does not change with location.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
What is the weight of a 5 kg object on Earth? (Use g = 10 N/kg)
Question 2
A car has a 3000 N engine force forward and 1200 N of friction and air resistance backward. What is the net force?
Question 3
Which statement about mass and weight is correct?
Question 4
A skydiver falls and eventually reaches terminal velocity. What is happening to the forces at this point?
Question 5
Gravity is classified as a:
Key Concepts Summary
- ●A force is a push or pull measured in Newtons (N).
- ●Mass (kg) is the amount of matter; weight (N) is the force of gravity on that mass. W = m × g.
- ●Balanced forces (net force = 0) mean no change in motion. Unbalanced forces cause acceleration.
- ●Friction and air resistance oppose motion and are contact forces.
- ●At terminal velocity, air resistance equals weight and the object moves at constant speed.