Pearson Edexcel GCSE (9-1) Physics revision notes
Motion and forces Revision Notes
Review revision notes for Motion and forces in Edexcel Physics.
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Motion and forces
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Weight and Gravitational Field Strength Revision Notes
Understanding Weight
Weight (W) is a force, measured in Newtons (N), that describes the pull of gravity on an object. It depends on both the object's mass and the strength of the gravitational field it is in. Unlike mass, which is a scalar quantity, weight is a vector quantity, having both magnitude and direction (always pointing towards the center of the gravitational source).
Gravitational Field Strength (g)
Gravitational field strength (g) is defined as the force per unit mass exerted by a gravitational field. It is also an acceleration due to gravity, measured in Newtons per kilogram (N/kg) or meters per second squared (m/s²). On Earth, 'g' is approximately 9.8 N/kg (often rounded to 10 N/kg for simpler calculations). Different celestial bodies have different values of 'g' due to their mass and radius.
The Relationship: W = mg
The connection between weight (W), mass (m), and gravitational field strength (g) is given by the formula W = m × g. This means that if an object's mass remains constant, its weight will vary proportionally with the gravitational field strength. For example, an object with a mass of 10 kg will have a weight of approximately 98 N on Earth (10 kg × 9.8 N/kg) but a much lower weight on the Moon due to the Moon's weaker gravitational field.
Velocity Change in Circular Motion Revision Notes
Understanding Velocity in Circular Motion
Velocity is a vector quantity, meaning it has both magnitude and direction. Speed refers only to the magnitude of the velocity. In circular motion, an object can maintain a constant speed, but its direction of movement is always changing. Therefore, its velocity is continuously changing.
Why Direction Matters for Velocity
Imagine an object moving around a circle. At one point, it's moving north; a moment later, it's moving north-east; then east, and so on. Even if its speed (e.g., 5 meters per second) remains the same, its direction is different at every instant. Because velocity accounts for direction, the constant change in direction means the velocity itself is not constant.
Implications of Changing Velocity
A change in velocity, whether in magnitude or direction, means that the object is accelerating. Therefore, an object moving in a circular path, even at a constant speed, is always accelerating. This acceleration is directed towards the centre of the circle and is known as centripetal acceleration.
Centripetal Force in Circular Motion Revision Notes
The Need for a Force in Circular Motion
For an object to move in a circle, its velocity must constantly change direction. Even if the speed is constant, a change in direction constitutes acceleration. According to Newton's Second Law, acceleration requires a resultant force. This force is essential for initiating and maintaining circular motion.
Centripetal Force: Direction and Role
The resultant force required for circular motion is called the centripetal force. It always acts towards the centre of the circular path. This force is responsible for continuously altering the object's direction, pulling it inwards and preventing it from moving off in a straight line tangent to the circle.
Consequences of No Centripetal Force
If the centripetal force acting on an object in circular motion suddenly vanishes, the object will no longer follow the circular path. Instead, it will move along a straight line path that is tangential to the circle at the point where the force was removed, due to its inertia.
Newton's Third Law Explained Revision Notes
Core Principle of Newton's Third Law
Newton's third law states that forces always occur in pairs. For every action force, there is an equal in magnitude and opposite in direction reaction force. These forces always act on different interacting objects, ensuring they never cancel each other out on a single object. The forces are always of the same type (e.g., if one is a push, the other is also a push).
Applying Newton's Third Law to Equilibrium
While Newton's third law describes interacting forces between two distinct objects, equilibrium refers to the state where the net force on a single object is zero. An object can be in static equilibrium (at rest) or dynamic equilibrium (moving at constant velocity). For an object in equilibrium, the forces acting on that specific object are balanced, but these forces are generally not Newton's third law pairs.
Distinguishing Action-Reaction Pairs from Balanced Forces
A common point of confusion is mixing up action-reaction pairs (Newton's third law) with balanced forces that result in equilibrium. A Newton's third law pair always acts on two *different* objects. Balanced forces that lead to equilibrium act on the *same* object. For instance, a book on a table has a downward gravity force and an upward normal force; these are balanced forces on the book, but not an action-reaction pair. The true action-reaction pair for the Earth's gravity on the book is the book's gravity on the Earth.
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