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VICPhysicsQuick questions
Unit 3: How do fields explain motion and electricity?
Quick questions on Gravitational fields and Newton's law of universal gravitation: VCE Physics Unit 3
8short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is the field model?Show answer
A gravitational field surrounds every mass. A second mass placed in the field experiences a force toward the first. The field is a vector at each point, drawn with field lines that point in the direction of the force on a positive test mass.
What is newton's law of universal gravitation?Show answer
Every pair of masses attracts with a force:
What is field strength?Show answer
The gravitational field strength $g$ at a point is the force per unit mass:
What is gravitational potential energy in a uniform field?Show answer
Close to a planet's surface, the field is approximately uniform. The change in gravitational potential energy when an object of mass $m$ is raised through height $\Delta h$ is:
What is gravitational potential energy in a non-uniform field?Show answer
Far from a planet's surface, the field varies with distance and the equation $mg\Delta h$ no longer applies. The change in potential energy between two distances $r_1$ and $r_2$ equals the work done against the gravitational force, which is the area under the $F$ vs $r$ graph between those points.
What is confusing field strength with force?Show answer
Field strength $g$ has units N/kg and is independent of the test mass. The force $F = mg$ depends on the test mass.
What is treating the sign of $\Delta U$ casually?Show answer
When an object moves outward (away from the planet), $\Delta U$ is positive (the system gains potential energy). When it falls inward, $\Delta U$ is negative.
What is forgetting that gravitational force is always attractive?Show answer
Field lines always point inward toward the source mass.