Forces and Changes in Motion
In 1687, Isaac Newton published his three laws of motion â by 2024, every engineering structure and vehicle on Earth was still designed using them.
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You are sitting on a chair right now. Are there any forces acting on you? If so, what are they and in which directions do they act?
Why does a heavy truck take longer to stop than a small car, even when both are travelling at the same speed?
â Know
- A force is a push or pull that can change an object's motion or shape.
- Forces are measured in newtons (N).
- An unbalanced force causes an object to accelerate (speed up, slow down, or change direction).
â Understand
- Forces can be contact forces or non-contact (action-at-a-distance) forces.
â Can do
- Identify forces acting on an object and describe their effects.
Slide a hockey puck across a smooth ice rink and watch: it keeps going long after your hand leaves it, gradually slowing only because of the tiny friction between ice and puck. Remove friction entirely â as in the frictionless airtrack in a physics lab â and the puck would slide at constant speed forever. That observation captures the core insight of Newton's mechanics: forces do not cause motion, they cause changes in motion. Forces are only needed to start, stop, or alter motion.
Friction is a contact force that opposes relative motion between surfaces. It arises from microscopic irregularities on surfaces interlocking and from chemical adhesion between surface molecules. Friction is essential for walking (your foot pushes backward, friction pushes you forward), driving (tyres push backward on the road, friction pushes the car forward), and holding objects (friction between your fingers and the object prevents slipping).
Balanced forces occur when all forces on an object cancel out. The object maintains constant velocity (which includes remaining at rest). Unbalanced forces occur when forces do not cancel. The object accelerates in the direction of the net force.
When you push a shopping trolley, your push is forward and friction is backward. If your push exceeds friction, the trolley accelerates forward. Once moving, if you stop pushing, friction becomes the only horizontal force. It is unbalanced and backward, so the trolley decelerates and stops. On a smooth supermarket floor, friction is small, so the trolley coasts a long way. On rough concrete, friction is large, so it stops quickly. In space, with essentially zero friction, a pushed trolley would keep moving forever.
Australian automotive engineering: Australian tyre manufacturers like Bridgestone Australia test friction coefficients on local road surfaces, from hot bitumen to wet coastal roads to red Outback dirt. The friction between tyre and road determines braking distance, cornering speed, and acceleration capability. Understanding these forces is critical for road safety design and vehicle performance standards.
Heavy objects fall faster than light objects. In everyday experience, a feather falls slower than a hammer because air resistance affects the feather more. But in a vacuum, all objects fall at the same rate regardless of mass. Apollo astronaut David Scott demonstrated this on the Moon in 1971 by dropping a hammer and a feather simultaneously - they landed together. Gravity accelerates all objects at 9.8 m/s2 near Earth surface, ignoring air resistance.
Match each scenario to the force description.
Inertia is the tendency of an object to resist changes in its state of motion. It is not a force itself, but rather a property of matter. Mass is the quantitative measure of inertia - more mass means more resistance to changes in motion.
This is why a loaded truck is harder to stop than an empty one, even at the same speed. The loaded truck has more mass, more inertia, and requires a larger force (or longer braking distance) to achieve the same deceleration. Similarly, a heavy object is harder to accelerate from rest than a light one.
Inertia explains many everyday phenomena. When a bus brakes suddenly, passengers lurch forward because their bodies tend to continue moving at the bus original speed. Seatbelts provide the force needed to overcome this inertia and stop the passenger with the bus. When a car turns, passengers feel pushed outward because their inertia tends to keep them moving in a straight line while the car changes direction.
A 20-tonne truck and a 1-tonne car both travel at 60 km/h. Both drivers brake with maximum force. The car stops in about 20 metres. The truck, with 20 times the mass and thus 20 times the inertia, requires about 20 times the force to achieve the same deceleration. Since brakes can only provide limited force, the truck stopping distance is much longer - typically 40-60 metres depending on load and brake condition. This is why trucks maintain larger following distances and lower speed limits.
Australian road safety: The Australian Design Rules for vehicles mandate minimum braking performance standards based on vehicle mass. Trucks must demonstrate stopping distances within legal limits when fully loaded. The Transport Research Centre at UNSW studies how inertia and braking forces affect heavy vehicle safety, informing regulations that reduce truck-related fatalities on Australian highways.
Inertia is a force that keeps moving objects going. This is false. Inertia is a property, not a force. It is the absence of force that keeps moving objects going, not the presence of inertia. Inertia explains why objects resist changes, but it does not cause motion. This distinction is subtle but important for correctly applying Newton laws.
- Force
- Friction
- Inertia
- Net force
- The vector sum of all forces acting on an object
- A push or pull that can change motion
- A force that opposes relative motion between surfaces
- The tendency of an object to resist changes in motion
Analysing forces requires identifying all forces acting on an object and determining their combined effect. A force diagram (free-body diagram) is the essential tool for this analysis.
To draw a force diagram:
- Represent the object as a simple dot or box.
- Draw arrows from the object representing each force.
- Make arrow length proportional to force magnitude.
- Label each arrow with the force type and magnitude.
Common forces to identify:
- Gravity (weight): Always downward, magnitude = mg.
- Normal force: Perpendicular to contact surface, opposes gravity.
- Friction: Parallel to surface, opposes motion or attempted motion.
- Applied force: Push or pull from a person, engine, etc.
- Tension: Pull from a rope or cable.
- Air resistance/drag: Opposes motion through air.
Once all forces are identified, add them as vectors to find the net force. The net force determines the acceleration via Newton Second Law: F_net = ma.
A student pulls a 10 kg suitcase with a force of 30 N at an angle of 30 degrees above horizontal. The force diagram shows: applied force (30 N, 30 degrees up), weight (98 N, down), normal force (about 83 N, up - less than weight because the upward pull component reduces the normal force), and friction (say 10 N, backward). The horizontal net force is 30 * cos(30) - 10 = 26 - 10 = 16 N. Acceleration = 16 / 10 = 1.6 m/s2. The suitcase speeds up at 1.6 metres per second every second.
Australian physics education: The Australian Curriculum requires students to analyse force diagrams and calculate net forces for simple systems. Physics teachers commonly use examples relevant to Australian students: surfing (buoyancy and drag forces), AFL football (projectile motion and collision forces), and climbing (tension and friction forces). These contexts make abstract force concepts concrete and engaging.
Normal force always equals weight. This is only true for objects at rest on horizontal surfaces with no other vertical forces. If a surface is inclined, the normal force is less than weight. If an additional upward force is applied (like lifting slightly while standing on a scale), the normal force decreases. If an additional downward force is applied (like pushing down on the object), the normal force increases. Normal force adjusts to prevent objects from passing through surfaces.
1. Name three different forces that could act on a book sitting on a table, and describe whether each is a contact or non-contact force.
2. Explain what happens to the motion of an object when the forces acting on it are unbalanced.
- Thinking that stationary objects have no forces acting on them. â Stationary objects often have balanced forces acting on them. A book on a table has gravity pulling down and the normal force pushing upâthese forces balance, so the book does not move.
- Believing that forces always cause motion. â Forces cause changes in motion. Balanced forces result in no changeâan object stays still or keeps moving at constant velocity.
đ Copy Into Your Books
âźDefinition of Force
A force is a push or pull that can change an object's motion or shape. Forces are measured in newtons (N).
Contact Forces
Contact forces require physical touching. Examples: friction, applied force, tension, normal force.
Non-Contact Forces
Non-contact forces act at a distance. Examples: gravity, magnetism, electrostatic force.
Unbalanced Forces
Unbalanced forces cause an object to accelerateâspeed up, slow down, or change direction.
You learned that forces are pushes or pulls that can change motion, and that unbalanced forces cause acceleration.
If you are sitting still in a chair, are there any forces acting on you? Explain your answer.
The hook painted a vivid picture: a fully loaded B-double needs over 100 metres to stop at highway speed, because its enormous mass resists any change in motion â even with full brakes applied.
Now that you've studied how forces cause changes in motion and how mass affects that change, how would you explain the truck's stopping distance in terms of Newton's laws? Did the mass-resistance idea match what you expected at the start?
1. What is the SI unit of force?
2. Which of the following is a non-contact force?
3. What happens when the forces on an object are balanced?
4. Friction is an example of:
5. An unbalanced force acting on an object will cause it to:
Describe two contact forces and two non-contact forces, giving an example of each. (3 marks)
Hint: Think about whether the force requires physical touching.
A car is driving at a constant speed on a straight, flat road. Describe the forces acting on the car and explain why it does not accelerate. (3 marks)
Hint: Consider the driving force, friction, air resistance, gravity, and the normal force.
Explain why a book sitting on a table does not fall through the table, using the idea of balanced forces. (3 marks)
Hint: Consider the forces acting vertically on the book.