# OCR - P4: Explaining motion

 P4.1 What are forces? Background to the topic What you should be able to do: Force arises from an interaction between two objects, and when two objects interact, both always experience a force and that these two forces form an interaction pair. The two forces in an interaction pair are the same kind of force, equal in size and opposite in direction, and act on different objects (Newton's third law). Friction is the interaction between two surfaces that slide (or tend to slide) relative to each other: each surface experiences a force in the direction that prevents (or tends to prevent) relative movement. There is an interaction between an object and the surface it is resting on: the object pushes down on the surface, the surface pushes up on the object with an equal force, and this is called the normal contact force. In everyday situations, a downward force acts on every object, due to the gravitational attraction of the Earth. This is called its weight. It can be measured (in N) using a spring (or top-pan) balance. The weight of an object is proportional to its mass. Near the Earth's surface, the weight of a 1 kg object is roughly 10 N. The Earth's gravitational field strength is therefore 10 N/kg. Newton's insight that linked the force that causes objects to fall to Earth with the force that keeps the Moon in orbit around the Earth led to the first universal law of nature. 1. Recall and apply Newton's third law During your practical work you will Investigate the effect of different combinations of surfaces on the frictional forces. You should be able to: Explain how Newton's discovery of the universal nature of gravity is an example of the role of imagination in scientific discovery. 2. Recall examples of ways in which objects interact: by gravity, electrostatics, magnetism and by contact (including normal contact force and friction) 3. Describe how examples of gravitational, electrostatic, magnetic and contact forces involve interactions between pairs of objects which produce a force on each object 4. Represent interaction forces as vectors 5. Define weight 6. Describe how weight is measured 7. Recall and apply the relationship between the weight of an object, its mass and the gravitational field strength: weight (N) = mass (kg) × gravitational field strength (N/kg)