Forces are everywhere around you - from gravity pulling you...
Combined Science235 views·Updated 16 Jul 2026·17 pagesUnderstanding Forces in Combined Science: P5 Physics
J
James Hope@jameshope_gmtc
1 / 10
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of 10
Forces Basics
Forces are simply pushes or pulls that make things happen around you. Every time you kick a football, open a door, or even just stand still, forces are at work.
Forces come in two main types: contact forces (where objects must touch) like friction and tension, and non-contact forces (where objects don't need to touch) like gravity and magnetism. Think of pushing a trolley versus a magnet attracting metal from across the room.
Here's something that might surprise you - forces are vector quantities, meaning they have both size and direction. Speed is just how fast you're going, but velocity tells you how fast AND which way you're heading.
Key Point: When two objects interact, they always create equal and opposite forces on each other - this is why you can feel the ground pushing back when you walk on it!
2
of 10
Gravity, Weight and Mass
Don't get confused between mass and weight - they're completely different! Your mass is the amount of 'stuff' in your body and stays the same whether you're on Earth or the Moon. But your weight changes because it's the force of gravity pulling on you.
Weight = mass × gravitational field strength . On Earth, g = 9.8 N/kg, but on the Moon it's only 1.6 N/kg. So if you weigh 500N on Earth, you'd only weigh about 80N on the Moon - though your mass stays exactly the same!
Weight is measured in newtons using a spring balance, whilst mass is measured in kilograms using a mass balance. Remember: weight is a force, mass isn't.
Remember: Mass and weight are directly proportional - double your mass and you double your weight too!
3
of 10
Resultant Forces and Diagrams
The resultant force is the overall effect when multiple forces act on an object. It's like having several people pushing a car - some forwards, some backwards - and working out the final push.
Free body diagrams show all forces acting on an object using arrows. The length shows the force's strength, and the direction shows which way it's acting. For forces along the same line, add forces going the same way and subtract those going opposite ways.
When forces are balanced (equilibrium), the resultant force is zero. This means the object either stays still or moves at constant velocity - like when you're walking at a steady pace.
Pro Tip: Use scale drawings to find resultant forces - draw forces to scale, then measure the line from start to finish to find your answer!
4
of 10
Springs and Elasticity
Springs follow a simple rule: force = spring constant × extension . Double the force, double the extension - but only up to a point called the limit of proportionality.
Elastic deformation means the spring returns to its original shape when you remove the force. Plastic deformation means it stays permanently changed - like when you've stretched a spring too far.
The elastic potential energy stored in a spring is calculated using Ee = ½ke². This energy gets stored when you stretch the spring and released when it springs back.
Lab Alert: When investigating springs, always measure at eye level and use a marker on the bottom of the spring for accurate readings!
5
of 10
Motion: Speed, Velocity and Acceleration
Distance and speed are scalars (no direction needed), but displacement and velocity are vectors (direction matters). You could run 100m around a track but end up with zero displacement if you finish where you started!
Use s = vt to calculate distance, speed or time. Remember these typical speeds: walking , cycling , cars , and planes .
Acceleration is how quickly velocity changes, and it's also a vector. Objects rarely travel at constant speed, so most calculations give you average values over a time period.
Real World: At a roundabout, cars maintain constant speed but have changing velocity because they're constantly changing direction!
6
of 10
Newton's Laws and Momentum
Newton's First Law: Objects stay still or keep moving at constant velocity unless acted on by a resultant force. This is called inertia - your tendency to resist changes in motion.
Newton's Second Law: The bigger the force, the bigger the acceleration . Newton's Third Law: Every action has an equal and opposite reaction - when you walk, you push Earth backwards and Earth pushes you forwards.
Terminal velocity occurs when drag force equals weight, giving zero acceleration. Think of a skydiver falling at constant speed when air resistance balances their weight.
Think About It: When you're in a car that suddenly stops, your body keeps moving forward due to inertia - that's why seatbelts are essential!
7
of 10
Stopping Distance and Safety
Your stopping distance has two parts: thinking distance (how far you travel while reacting) and braking distance (how far you travel while the brakes work).
Reaction times vary between people (typically 0.2-0.9 seconds) and get worse when you're tired, distracted, or under the influence of drugs or alcohol. You can test reaction time using the ruler drop test.
Stopping distance increases dramatically with speed because kinetic energy increases with the square of velocity. This is why speed limits exist and why tailgating is so dangerous.
Safety First: Poor weather, worn brakes, and bald tyres all increase braking distance - sometimes by several car lengths!
8
of 10
Momentum and Conservation
Momentum = mass × velocity and it's always conserved in collisions. This means the total momentum before a collision equals the total momentum after.
In car crashes, momentum conservation explains why small cars suffer more damage when hitting large trucks. The momentum has to go somewhere, and lighter objects experience bigger changes in velocity.
Understanding momentum helps explain everything from how rockets work (pushing fuel backwards creates forward momentum) to why crumple zones save lives (they increase collision time, reducing the forces involved).
Key Insight: Momentum is conserved in all interactions - this fundamental principle governs everything from snooker balls to space missions!
9
of 10
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of 10
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Combined Science235 views·Updated 16 Jul 2026·17 pagesUnderstanding Forces in Combined Science: P5 Physics
J
James Hope@jameshope_gmtc
Forces are everywhere around you - from gravity pulling you down to friction stopping you from slipping. Understanding how forces work is crucial for explaining everything from why you fall when you jump to how rockets blast into space.
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of 10
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Forces Basics
Forces are simply pushes or pulls that make things happen around you. Every time you kick a football, open a door, or even just stand still, forces are at work.
Forces come in two main types: contact forces (where objects must touch) like friction and tension, and non-contact forces (where objects don't need to touch) like gravity and magnetism. Think of pushing a trolley versus a magnet attracting metal from across the room.
Here's something that might surprise you - forces are vector quantities, meaning they have both size and direction. Speed is just how fast you're going, but velocity tells you how fast AND which way you're heading.
Key Point: When two objects interact, they always create equal and opposite forces on each other - this is why you can feel the ground pushing back when you walk on it!
2
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Gravity, Weight and Mass
Don't get confused between mass and weight - they're completely different! Your mass is the amount of 'stuff' in your body and stays the same whether you're on Earth or the Moon. But your weight changes because it's the force of gravity pulling on you.
Weight = mass × gravitational field strength . On Earth, g = 9.8 N/kg, but on the Moon it's only 1.6 N/kg. So if you weigh 500N on Earth, you'd only weigh about 80N on the Moon - though your mass stays exactly the same!
Weight is measured in newtons using a spring balance, whilst mass is measured in kilograms using a mass balance. Remember: weight is a force, mass isn't.
Remember: Mass and weight are directly proportional - double your mass and you double your weight too!
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Resultant Forces and Diagrams
The resultant force is the overall effect when multiple forces act on an object. It's like having several people pushing a car - some forwards, some backwards - and working out the final push.
Free body diagrams show all forces acting on an object using arrows. The length shows the force's strength, and the direction shows which way it's acting. For forces along the same line, add forces going the same way and subtract those going opposite ways.
When forces are balanced (equilibrium), the resultant force is zero. This means the object either stays still or moves at constant velocity - like when you're walking at a steady pace.
Pro Tip: Use scale drawings to find resultant forces - draw forces to scale, then measure the line from start to finish to find your answer!
4
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Springs and Elasticity
Springs follow a simple rule: force = spring constant × extension . Double the force, double the extension - but only up to a point called the limit of proportionality.
Elastic deformation means the spring returns to its original shape when you remove the force. Plastic deformation means it stays permanently changed - like when you've stretched a spring too far.
The elastic potential energy stored in a spring is calculated using Ee = ½ke². This energy gets stored when you stretch the spring and released when it springs back.
Lab Alert: When investigating springs, always measure at eye level and use a marker on the bottom of the spring for accurate readings!
5
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Motion: Speed, Velocity and Acceleration
Distance and speed are scalars (no direction needed), but displacement and velocity are vectors (direction matters). You could run 100m around a track but end up with zero displacement if you finish where you started!
Use s = vt to calculate distance, speed or time. Remember these typical speeds: walking , cycling , cars , and planes .
Acceleration is how quickly velocity changes, and it's also a vector. Objects rarely travel at constant speed, so most calculations give you average values over a time period.
Real World: At a roundabout, cars maintain constant speed but have changing velocity because they're constantly changing direction!
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Newton's Laws and Momentum
Newton's First Law: Objects stay still or keep moving at constant velocity unless acted on by a resultant force. This is called inertia - your tendency to resist changes in motion.
Newton's Second Law: The bigger the force, the bigger the acceleration . Newton's Third Law: Every action has an equal and opposite reaction - when you walk, you push Earth backwards and Earth pushes you forwards.
Terminal velocity occurs when drag force equals weight, giving zero acceleration. Think of a skydiver falling at constant speed when air resistance balances their weight.
Think About It: When you're in a car that suddenly stops, your body keeps moving forward due to inertia - that's why seatbelts are essential!
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Stopping Distance and Safety
Your stopping distance has two parts: thinking distance (how far you travel while reacting) and braking distance (how far you travel while the brakes work).
Reaction times vary between people (typically 0.2-0.9 seconds) and get worse when you're tired, distracted, or under the influence of drugs or alcohol. You can test reaction time using the ruler drop test.
Stopping distance increases dramatically with speed because kinetic energy increases with the square of velocity. This is why speed limits exist and why tailgating is so dangerous.
Safety First: Poor weather, worn brakes, and bald tyres all increase braking distance - sometimes by several car lengths!
8
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Momentum and Conservation
Momentum = mass × velocity and it's always conserved in collisions. This means the total momentum before a collision equals the total momentum after.
In car crashes, momentum conservation explains why small cars suffer more damage when hitting large trucks. The momentum has to go somewhere, and lighter objects experience bigger changes in velocity.
Understanding momentum helps explain everything from how rockets work (pushing fuel backwards creates forward momentum) to why crumple zones save lives (they increase collision time, reducing the forces involved).
Key Insight: Momentum is conserved in all interactions - this fundamental principle governs everything from snooker balls to space missions!
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