OCR GCSE Physics A P2.2 Newton's Laws Exam Questions

About OCR GCSE Physics A P2.2 Newton's Laws

P2.2 sits in the P2 Forces chapter of OCR Gateway Physics A and explains how forces change motion. You learn Newton's first law, including the idea of inertia, Newton's second law F = ma, and Newton's third law about interaction pairs. Momentum and its conservation are used to analyse collisions, and you link force to rate of change of momentum to explain safety features. You should also be able to describe a practical to investigate how force and mass affect acceleration. The topic builds on P2.1 Motion and is applied again in P8.1 Physics on the move. Examiners often test momentum calculations and explanations of third law pairs.

What you need to know

• Resultant force The single force with the same effect as all the forces acting. If it is zero, the object stays at rest or moves at constant velocity (Newton's first law).
• Inertia Inertia is the tendency of an object to stay at rest or keep moving uniformly. Inertial mass is the ratio of force to acceleration.
• Newton's second law Force = mass x acceleration (F = ma). Larger force gives larger acceleration; larger mass gives smaller acceleration for the same force.
• Newton's third law When two objects interact, they exert equal and opposite forces on each other, of the same type.
• Momentum Momentum = mass x velocity, measured in kg m/s. In a closed system the total momentum before an event equals the total momentum after.
• Force and momentum Force = change in momentum / time. Increasing the time of a collision reduces the force, which is why helmets, airbags and crumple zones work.
• Practical Use a trolley on a runway with light gates or a data logger, varying the force with masses on a string while keeping total mass constant, to show acceleration is proportional to force.

Common mistakes in P2.2 Newton's Laws

• Saying an object needs a resultant force to keep moving. It needs a resultant force only to change velocity.
• Putting third law pairs on the same object, such as weight and normal contact force on a book. Third law pairs act on different objects.
• Forgetting momentum is a vector. Objects moving in opposite directions have momentum of opposite signs.
• Saying crumple zones reduce the change in momentum. They increase the time of the collision, reducing the force.
• Using weight in N as mass in F = ma. Convert using weight = mass x g.

Try three P2.2 Newton's Laws exam-style questions