How do forces change motion through Newton's laws, and how do momentum and stopping distances work?
Newton's three laws of motion, resultant force, weight and mass, the equation force equals mass times acceleration, momentum and its conservation, and stopping distance as the sum of thinking and braking distances.
A focused answer to the OCR Gateway GCSE Combined Science A topic P2 on Newton's laws and momentum, covering the three laws, resultant force, weight and mass, force equals mass times acceleration, momentum and its conservation, and stopping distances.
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What this topic is asking
OCR wants you to state and apply Newton's three laws, find a resultant force, distinguish weight and mass, use force equals mass times acceleration, work with momentum and its conservation, and explain stopping distances.
Newton's three laws
A resultant force is the single force that has the same effect as all the forces acting on an object combined. If the forces are balanced, the resultant is zero, and by the first law the object stays still or keeps a constant velocity (this is also why a falling object reaches terminal velocity when air resistance balances its weight). If the forces are unbalanced, there is a resultant force, and by the second law the object accelerates in the direction of that force.
Weight, mass and F = ma
So a larger resultant force gives a larger acceleration, and a more massive object needs a larger force to achieve the same acceleration. Weight changes if the gravitational field strength changes (you would weigh less on the Moon) but mass does not. Many Paper 5 questions combine with the motion equations from the previous topic, so practise switching between them.
Momentum and stopping distances
The momentum of a moving object is (mass times velocity), measured in kg m/s, and it is a vector. In a closed system, momentum is conserved: the total momentum before a collision or explosion equals the total momentum afterwards. This lets you calculate the velocity of objects after they collide and stick together, for example. A safety application is that increasing the time over which a momentum change happens reduces the force (crumple zones, air bags and crash mats all extend the time of a collision to lower the force on people).
The stopping distance of a vehicle is the total distance from spotting a hazard to stopping, and it is the sum of two parts:
- Thinking distance: the distance travelled during the driver's reaction time (increased by tiredness, alcohol, drugs or distractions, and by higher speed).
- Braking distance: the distance travelled while the brakes are applied (increased by higher speed, wet or icy roads, and worn tyres or brakes).
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksA car of mass 1200 kg accelerates at 2.5 m/s squared. Calculate the resultant force on the car, and state Newton's second law in words.Show worked answer →
A Physics Paper 5 calculation. Method: use , force N. Markers credit the correct substitution and the answer with the unit newtons. Newton's second law in words: the acceleration of an object is proportional to the resultant force acting on it and inversely proportional to its mass; equivalently, resultant force equals mass times acceleration. Markers want the calculation and a correct statement linking resultant force, mass and acceleration. A common slip is to leave the force without units or to confuse mass and weight.
OCR 20214 marksExplain what is meant by the stopping distance of a car, and give two factors that increase the braking distance.Show worked answer →
A P2 question on stopping distances. Reward: the stopping distance is the total distance a car travels from when the driver sees a hazard to when the car stops; it is the sum of the thinking distance (the distance travelled during the driver's reaction time) and the braking distance (the distance travelled while the brakes are applied). Two factors that increase the braking distance: a higher speed, a wet or icy (slippery) road, worn tyres or worn brakes, or a greater mass. Markers credit the definition as thinking plus braking distance, and two valid factors affecting braking distance specifically (not thinking distance factors such as tiredness or alcohol).
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