Forces And Their Interactions
Overzicht
Hold a magnet a centimetre above a steel paperclip and the clip jumps up to meet it. Nothing touched it. Rest the same clip on a table instead and it sits there, perfectly still, even though gravity is pulling on it the whole time. Two everyday moments, two completely different ways for one object to act on another, and between them they contain most of what this lesson is about.
You will sort every force you meet into the ones that need contact and the ones that reach across a gap, learn why a force can never exist on its own, and see why physicists insist on drawing forces as arrows rather than writing them as plain numbers. Then you will put a number on gravity itself, work out what you would weigh standing on the Moon without changing your mass by a single gram, and stretch a spring far enough to find the point where it stops behaving itself.
Doelstellingen
- Objects interact by non-contact (field) forces (including gravity, electrostatics, magnetism) and by contact forces (including friction, air resistance, tension and normal contact force).
- Friction is a force between two surfaces, which impedes motion and may result in heating. Air resistance is a form of friction.
- Pairs of objects interact to produce a force on each other, which can be represented as vectors.
- Scalars are quantities that have magnitude only. Vectors are quantities that have direction as well as a magnitude. A vector quantity may be represented by an arrow. The length of the arrow represents the magnitude and the direction of the arrow represents the direction of the vector quantity. Students should be aware that distance, speed and time are examples of scalars and displacement, velocity, acceleration, force and momentum are examples of vectors.
- Weight is the force acting on an object due to gravity. The weight of an object depends on the gravitational field strength at the point where the object is. The weight of an object can be calculated using the equation: Weight (N) = mass (kg) × gravitational field strength (N/kg) W=m×g Students will not be expected to know the value of g ; it will be given in any examination items.
- When more than one force is applied to an object they may cause a change in the shape of the object, by stretching, bending or compressing. After elastic distortions an object returns to its original shape when the forces are removed. After inelastic distortions an object does not return to its original shape.
- A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.
- For an object behaving elastically, the extension is directly proportional to the force applied, provided that the limit of proportionality is not exceeded. The relationship between the force, F, and the extension, e, is: F=k×e where k is a constant. Required practical: Investigate the relationship between force and extension for a spring.
Mindmap
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- Which of these is a non-contact force? A. Friction B. Tension C. Magnetism D. Normal contact force Answer: C
- Which quantity is a vector? A. Distance B. Speed C. Mass D. Acceleration Answer: D
- An object has a mass of 4.0 kg. The gravitational field strength is 10 N/kg. What is the weight of the object? A. 0.40 N B. 2.5 N C. 40 N D. 400 N Answer: C
- A force of 12 N stretches a spring by 0.30 m. The spring is behaving elastically. What is the spring constant of the spring? A. 0.025 N/m B. 3.6 N/m C. 40 N/m D. 360 N/m Answer: C
- Which statement describes an inelastic distortion? A. The object returns to its original shape when the forces are removed B. The object keeps its new shape when the forces are removed C. The object stores no energy while it is being distorted D. The object is distorted by only one force Answer: B
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