SCNC1112 Chap.1 Motion, Force and Gravity as Universal Laws
Motion, Force and Gravity as Universal Laws
SCNC1112 begins with four words that everyone already uses and that science has sharpened until they are exact: speed, velocity, acceleration and force. Speed is distance divided by time. Velocity adds direction. Acceleration is the rate at which velocity changes, in magnitude or in direction. Force is whatever changes an object's state of motion.
Each carries a unit, and the everyday version of each is the most reliable source of wrong answers in the opening module. The case that separates the definitions is uniform circular motion. A car travelling round a roundabout at a steady speed is accelerating continuously, because its direction is changing and direction belongs to velocity. That acceleration points towards the centre of the circle.
If you can explain why steady circular motion counts as accelerated motion, the definitions are working; if the explanation feels strange, the everyday sense of acceleration is still in the way. Newton's three laws are then learned separately and examined together. The first says an object keeps its state of motion until an unbalanced force acts.
The second says the acceleration a force produces grows in proportion to that force and shrinks as the mass grows, written as F equals ma. The third says forces act in pairs, equal in size and opposite in direction. A single ordinary event, such as a balloon-powered car leaving a bench, needs all three at once: the third supplies the forward force, the first names what that force changes, and the second fixes how much.
The gravitation lecture then makes the claim that gives the module its title. The force that pulls an apple to the ground is the same force that holds the Moon in orbit, and it acts between any two masses in proportion to each mass and in inverse proportion to the square of their separation. Two consequences carry most of the examinable content. Doubling either mass doubles the force.
Doubling the separation leaves one quarter of it. Because the gravitational constant is extremely small, attraction between everyday objects is real and negligible, and only becomes obvious when one of the two masses is a planet. The chapter closes on the distinction between mass and weight, which is dropped more often than any other in this part of the course.
Mass measures resistance to a change in motion and does not depend on location. Weight is the gravitational force on that mass and does. A student who travels to the Moon keeps the same mass and loses about five sixths of their weight, and an astronaut in orbit is weightless because nothing is pushing back, not because gravity has stopped.
The hammer and feather released together on the airless lunar surface land together, which is the observation the whole distinction was built to explain.
What this chapter covers
- 01
Speed, Velocity, Acceleration and Force
- 02
Uniform Circular Motion and Centripetal Acceleration
- 03
Newton's Three Laws Acting on One Event
- 04
Universal Gravitation and the Inverse Square
- 05
Mass Against Weight, and Free Fall
Reading a lift, a scale and a free fall with one relation
- 2Write the stationary case first. The scale reads the support force it must supply to balance gravity, which is mass times the acceleration due to gravity: 60 times 9.8, about 588 N.
- 2Add the upward acceleration. The support force now has to balance gravity and also produce the acceleration, so it is 60 times 9.8 plus 60 times 2, about 708 N. The reading rises although nothing about the student changed.
- 1Take the free fall. There is no support force at all, so the scale reads zero. This is weightlessness, and it is the absence of a push rather than the absence of gravity.
- 1Answer the question that was asked. None of the three readings is the mass; all three are forces in newtons. The mass stays 60 kg throughout, and only a balance that compares against known masses would measure it directly.
Key terms
- Velocity
- Speed together with the direction of travel, measured in metres per second. Because direction is part of it, a change of direction alone is a change of velocity.
- Centripetal Acceleration
- The acceleration of an object moving on a circular path, always directed towards the centre of the circle, present even when the speed never changes.
- Unbalanced Force
- A net force left over after all forces on an object are added, and the only thing that can change an object's state of motion.
- Inverse Square Law
- A relationship in which a quantity falls with the square of distance, so tripling the separation leaves one ninth of the original value.
- Weight
- The gravitational force acting on a mass, measured in newtons, which changes with location while the mass itself does not.
- Free Fall
- Motion under gravity alone with no support force acting, which is why an object in free fall registers zero on a spring scale.
Motion, Force and Gravity as Universal Laws FAQ
Why does a satellite accelerate if its speed never changes?
Because velocity includes direction and the direction is changing continuously. Any change in velocity counts as an acceleration, and for circular motion that acceleration points at the centre of the orbit. A question that describes constant speed is often testing whether you remember that speed and velocity are different quantities.
Is an astronaut floating in orbit outside the reach of gravity?
No. Gravity is what keeps the spacecraft in orbit rather than travelling off in a straight line. Floating happens because the craft and everything inside it fall together, so nothing presses on anything else. Weightlessness is the absence of a support force, not the absence of the gravitational force.
How much of this chapter needs actual calculation?
Very little arithmetic and a good deal of proportional reasoning. The course states that you are expected to understand what the formulas mean and to argue quantitatively from them rather than to solve complex computational problems, so being able to say what happens when a mass doubles or a distance triples matters more than a numerical answer.
Exam move
Write the four opening definitions with their units from memory, then test them on three cases the everyday meanings get wrong: a turn at constant speed, a push on a wall that does not move, and a hammer and feather dropped together in a vacuum. For gravitation, practise proportional statements out loud, because the paper asks what happens when a quantity doubles far more often than it asks for a number.
