2.1 Forces
What is a Force?
A force is a push or a pull on an object. Forces are vectors (they have magnitude and direction) and are measured in newtons (N).
• Air Resistance
• Tension
• Normal Contact Force
• Applied Force (push/pull)
• Magnetism
• Electrostatic Force
Act through fields
A force is a push or a pull on an object. Forces are vectors (they have magnitude and direction) and are measured in newtons (N).
⚡ Key Concept:
Forces can make an object:
• Speed up (accelerate)
• Slow down (decelerate)
• Change direction
• Change shape (deform)
• Speed up (accelerate)
• Slow down (decelerate)
• Change direction
• Change shape (deform)
Speed Up
Slow Down
Change Direction
Change Shape
💡 Contact vs Non-Contact Forces:
All forces can be sorted into two groups:
• Contact Forces: Objects must be touching
• Non-Contact Forces: Act at a distance
• Contact Forces: Objects must be touching
• Non-Contact Forces: Act at a distance
Contact Forces
• Friction• Air Resistance
• Tension
• Normal Contact Force
• Applied Force (push/pull)
Non-Contact Forces
• Gravity (Weight)• Magnetism
• Electrostatic Force
Act through fields
🎯 Contact or Non-Contact Practice
Understanding the Difference
Mass and weight are often confused, but they are very different.
Mass and weight are often confused, but they are very different.
⚡ Key Definitions:
• Gravity: A non-contact force of attraction between all objects with mass
• Mass: The amount of matter in an object. Measured in kg. Same everywhere.
• Weight: The force of gravity acting on mass. Measured in N. Changes with location.
• Mass: The amount of matter in an object. Measured in kg. Same everywhere.
• Weight: The force of gravity acting on mass. Measured in N. Changes with location.
Mass
Constant
70 kg on Earth
70 kg on Moon
Always the same.
Weight
Variable
686 N on Earth
113 N on Moon
Depends on gravity.
Weight Equation:
W = m × g
W = Weight (N) | m = Mass (kg) | g = Gravitational field strength (N/kg)
Example: Calculating Weight on Earth
A person has a mass of 70 kg. What is their weight on Earth? (g = 9.8 N/kg)
Step 1: Write the formula
W = m × g
Step 2: Substitute values
W = 70 × 9.8
Step 3: Calculate
W = 686 N
Step 1: Write the formula
W = m × g
Step 2: Substitute values
W = 70 × 9.8
Step 3: Calculate
W = 686 N
Example: Same Person on the Moon
The same person (70 kg) on the Moon where g = 1.6 N/kg
W = m × g
W = 70 × 1.6
W = 112 N
Note: Mass stays 70 kg, but weight is much less.
W = m × g
W = 70 × 1.6
W = 112 N
Note: Mass stays 70 kg, but weight is much less.
🧮 Weight Calculator:
Weight = 686 N
🎯 Weight Calculation Practice
N
N
Combining Multiple Forces
Usually, more than one force acts on an object. The resultant force is the single force that has the same effect as all the individual forces combined.
Usually, more than one force acts on an object. The resultant force is the single force that has the same effect as all the individual forces combined.
⚡ Balanced vs Unbalanced:
• Balanced forces: Resultant = 0 N → Object stays still or moves at constant velocity
• Unbalanced forces: Resultant ≠ 0 N → Object accelerates (speeds up, slows down, or changes direction)
• Unbalanced forces: Resultant ≠ 0 N → Object accelerates (speeds up, slows down, or changes direction)
Example 1: Forces in the Same Direction
Two people push a car in the same direction:
Person A: 300 N right
Person B: 200 N right
Person A: 300 N right
Person B: 200 N right
300 N →
+
200 N →
=
500 N →
Resultant = 300 + 200 = 500 N right
Example 2: Forces in Opposite Directions
A ball is pushed right (10 N) but air resistance pushes left (2 N):
The ball accelerates to the right (unbalanced force).
10 N →
+
← 2 N
=
8 N →
Resultant = 10 − 2 = 8 N rightThe ball accelerates to the right (unbalanced force).
Example 3: Balanced Forces
A book resting on a table:
The book stays stationary (balanced forces).
Weight 10 N ↓
+
↑ Normal 10 N
=
0 N
Resultant = 10 − 10 = 0 NThe book stays stationary (balanced forces).
🎯 Resultant Force Practice
N
N
Visualising Forces
A free-body diagram is a simple drawing showing all the forces acting on a single object. We draw the object as a box and use arrows to show forces.
A free-body diagram is a simple drawing showing all the forces acting on a single object. We draw the object as a box and use arrows to show forces.
⚡ Drawing Free-Body Diagrams:
• Draw the object as a simple shape (usually a box)
• Draw arrows FROM the object pointing in the direction of each force
• Arrow length represents force magnitude (longer = stronger)
• Label each arrow with the force name and value
• Draw arrows FROM the object pointing in the direction of each force
• Arrow length represents force magnitude (longer = stronger)
• Label each arrow with the force name and value
Example: Book on a Table
Weight acts downward, Normal force acts upward. If equal, forces are balanced.
Example: Car Accelerating
Resultant horizontal = 500 − 200 = 300 N right. Car accelerates.
🎯 Free-Body Diagram Practice
Stretching and Compressing Objects
When you apply a force to an object (like a spring), you can deform it. The type of deformation depends on the force applied.
• Spring stretched gently
• Rubber band stretched
• Energy stored and released
• Spring stretched too far
• Bent paperclip
• Energy lost to deformation
When you apply a force to an object (like a spring), you can deform it. The type of deformation depends on the force applied.
⚡ Types of Deformation:
• Elastic Deformation: Object returns to original shape when force is removed
• Inelastic (Plastic) Deformation: Object is permanently deformed
• Inelastic (Plastic) Deformation: Object is permanently deformed
Elastic
• Returns to original shape• Spring stretched gently
• Rubber band stretched
• Energy stored and released
Inelastic
• Permanently deformed• Spring stretched too far
• Bent paperclip
• Energy lost to deformation
Hooke's Law:
F = k × x
F = Force (N) | k = Spring constant (N/m) | x = Extension (m)
Example: Using Hooke's Law
A spring has a spring constant of 100 N/m. What force is needed to stretch it by 0.1 m?
Step 1: Write the formula
F = k × x
Step 2: Substitute values
F = 100 × 0.1
Step 3: Calculate
F = 10 N
Step 1: Write the formula
F = k × x
Step 2: Substitute values
F = 100 × 0.1
Step 3: Calculate
F = 10 N
💡 Limit of Proportionality:
Hooke's Law only works up to a certain point called the limit of proportionality.
Beyond this point:
• Extension is no longer proportional to force
• Graph becomes curved
• Spring may be permanently deformed (past the elastic limit)
Beyond this point:
• Extension is no longer proportional to force
• Graph becomes curved
• Spring may be permanently deformed (past the elastic limit)
Force vs Extension Graph
Work Done Stretching a Spring:
W = ½ × F × x
W = Work done (J) | F = Force (N) | x = Extension (m)
🎯 Hooke's Law Practice
Real Life Uses:
• Vehicle suspension: Springs absorb bumps in the road
• Trampolines: Springs store and release energy
• Weighing scales: Spring extension measures weight
• Mattresses: Springs provide support and comfort
• Pens: Spring returns the button after clicking
• Vehicle suspension: Springs absorb bumps in the road
• Trampolines: Springs store and release energy
• Weighing scales: Spring extension measures weight
• Mattresses: Springs provide support and comfort
• Pens: Spring returns the button after clicking