Friction and dragIB MYP Physics: Revision notes
Section 1
What causes friction?
Friction is a force that opposes the movement (or attempted movement) of one surface over another. It acts along the surfaces, in the opposite direction to the motion.
Even surfaces that look smooth have tiny bumps. When two surfaces are pressed together, the bumps catch on each other, so a force is needed to make them slide. Friction transfers kinetic energy to thermal energy (heat) and wears surfaces away: rub your hands together and they warm up.
Section 2
Static and sliding friction
Static friction acts on an object that is not moving. It increases to match the push, so the object stays still, until it reaches a maximum value.
If the push is bigger than the maximum static friction, the object starts to slide. Sliding friction acts on a moving object, and it is usually smaller than the maximum static friction. This is why it is harder to start a heavy box moving than to keep it moving.
Friction depends on:
- how rough the surfaces are (rougher means more friction)
- how hard the surfaces are pressed together (a heavier object gives more friction)
If you push a box with 30 N and it does not move, the friction is 30 N, not zero and not the maximum.
Section 3
Reducing and increasing friction
Friction is sometimes useful and sometimes a nuisance.
Increasing friction: tyre treads and rough soles give grip; brake pads are made of high-friction material; sand on an icy road.
Reducing friction: lubricants such as oil or grease form a layer between surfaces so they slide more easily; wheels and rollers replace sliding with rolling; polishing and smooth surfaces; air cushions.
Reducing friction means less energy wasted as heat and less wear.
Section 4
Air resistance and fluid drag
When an object moves through a fluid (a liquid or a gas such as air), the fluid exerts a drag force that opposes the motion. In air it is called air resistance.
Drag depends on:
- speed: the faster the object moves, the greater the drag
- shape and surface area: a large or blunt front pushes more fluid out of the way, so there is more drag
Streamlining means giving an object a smooth, pointed shape so fluid flows past it easily. Streamlined cars, aeroplanes, cyclists in a crouch and swimming caps all reduce drag, saving energy and fuel.
Say "drag increases with speed" and "a larger surface area gives more drag". Quote both factors in exam answers.
Section 5
Terminal velocity
Take a skydiver falling with the parachute closed:
- At the start, weight is greater than air resistance, so the resultant force is downwards and she accelerates.
- As her speed increases, air resistance increases, so the resultant force gets smaller and the acceleration decreases.
- Eventually air resistance = weight. The forces are balanced, the resultant is zero and she falls at a constant speed called the terminal velocity.
- Opening the parachute increases the surface area, so air resistance becomes greater than weight. She decelerates until air resistance equals weight again, at a lower terminal velocity.
At terminal velocity the skydiver is still falling, and gravity still acts. She stops accelerating because the forces balance.
Must know
- Friction opposes motion; caused by surface bumps catching.
- Static friction matches the push up to a maximum; sliding friction is usually less.
- Friction increases with rougher surfaces and larger pressing force.
- Lubricants, wheels and polishing reduce friction; grip and treads increase it.
- Drag increases with speed and surface area; streamlining reduces it.
- Terminal velocity: air resistance = weight, resultant force zero, constant speed.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Friction and drag
- Cyclists in Rotterdam fit rubber brake pads that press against the metal rim of a wheel to slow the bicycle down. A cyclist notices that her brakes work much less well on a rainy day, and that her bicycle chain needs a little oil to run smoothly.Explain how putting oil on the bicycle chain reduces the energy wasted as it moves.2 marks
- A student in Cairo tries to push a heavy wooden box across a concrete floor. She pushes with a gradually increasing force. At first the box does not move at all, then it suddenly starts to slide, and she finds it easier to keep it sliding than it was to get it moving.Suggest one change the student could make so the box is easier to slide, and explain why it works.2 marks
- A skydiver in Queenstown, New Zealand, jumps from an aeroplane and falls with her parachute closed. After about 12 seconds her speed stops increasing and stays at a steady 55 m/s. Later she opens her parachute and, a few seconds after that, she is falling at a steady 5 m/s until she lands.Explain why the skydiver stops accelerating after about 12 seconds, even though she is still falling.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).