Newton's laws of motionIB MYP Physics: Revision notes
Section 1
Newton's first law: balanced forces
A force is a push or a pull, measured in newtons (N). When the forces on an object are balanced, the resultant force is zero.
Newton's first law: an object stays at rest, or keeps moving at a constant velocity (the same speed in a straight line), unless a resultant force acts on it.
- A car cruising at a steady speed has driving force = resistive forces, so the resultant is zero.
- A moving object does not need a force to keep it moving. A force is needed to change its motion.
Many students think a moving object must have a bigger forward force. At constant velocity the forces are balanced.
Section 2
Inertia and mass
Inertia is the tendency of an object to keep doing what it is already doing: staying still, or moving at constant velocity. Mass (in kg) measures inertia. The bigger the mass, the bigger the inertia, so the harder it is to speed the object up, slow it down or turn it.
A loaded lorry is much harder to stop than an empty car because it has more inertia.
Section 3
Newton's second law: F = m a
A resultant force causes an object to accelerate. Newton's second law:
F = m a
where F is the resultant force in newtons (N), m is the mass in kilograms (kg) and a is the acceleration in metres per second squared (m/s²). One newton is the force that gives a 1 kg mass an acceleration of 1 m/s².
- Acceleration is directly proportional to the resultant force: double the force, double the acceleration.
- Acceleration is inversely proportional to the mass: double the mass, half the acceleration.
Rearranged: a = F / m and m = F / a.
Always use the resultant force in F = m a, not just one of the forces. Subtract opposite forces first.
Section 4
Worked example
A car of mass 1200 kg has a driving force of 4000 N and resistive forces of 1000 N.
- Resultant force = 4000 − 1000 = 3000 N
- a = F / m = 3000 / 1200
- a = 2.5 m/s² in the direction of the driving force.
If the same resultant force acted on a car of mass 1500 kg, a = 3000 / 1500 = 2.0 m/s². More mass gives less acceleration.
Section 5
Newton's third law: force pairs
Newton's third law: when object A exerts a force on object B, object B exerts an equal and opposite force on object A.
The two forces in a pair:
- are equal in size
- act in opposite directions
- are the same type of force (for example both contact forces)
- act on different objects, so they never cancel each other.
Example: when you push a wall, the wall pushes back on you with the same size force.
A force pair never acts on the same object. Your weight and the push of the floor on you are NOT a third law pair.
Section 6
Examples: rockets, walking and seat belts
- Rockets: the engine pushes hot gas backwards, and the gas pushes the rocket forwards with an equal and opposite force (third law). The rocket does not need air to push against.
- Walking: your foot pushes backwards on the ground, and the ground pushes you forwards.
- Seat belts: in a sudden stop the car slows, but passengers keep moving forwards (first law). The belt applies a backwards force to stop them with the car, and stretching slightly makes the stopping time longer so the force is smaller.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Newton's laws of motion
- A bus travels at a constant velocity of 15 m/s along a straight, level road in Dubai. The driver then brakes hard and the bus slows down quickly. Passengers who are standing are thrown forwards as the bus slows.Explain, using Newton's first law, why the standing passengers are thrown forwards when the bus brakes.2 marks
- A space agency tests a rocket. Burning fuel produces hot gas that the engine pushes out of a nozzle at the back at very high speed. Once the rocket is in space, it accelerates forwards even though there is no air or ground for it to push against.Explain how the rocket accelerates forwards in space even though there is nothing for it to push against.2 marks
- A student investigates how the acceleration of a trolley of mass 0.50 kg depends on the resultant force on it. The trolley runs along a level, low-friction runway and is pulled by a constant force measured with a force sensor. A motion sensor records the acceleration. A resultant force of 0.25 N gave an acceleration of 0.50 m/s², 0.50 N gave 1.0 m/s², and 1.00 N gave 2.0 m/s².State the independent variable, the dependent variable and one control variable in this investigation.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).