Circular Motion — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions

Section 1 of 9Velocity changes in circular motion

Direction changes

• Direction changes an object moving in a circle at constant speed keeps changing direction.

Velocity is a vector, so the velocity is changing and the object is accelerating even though the speed is steady.

At every instant the object moves along the tangent to the circle.

If released, it would fly off in a straight line along that tangent (Newton's first law).

Section 2 of 9Centripetal force

Centripetal force

Centripetal force the resultant force directed towards the centre of the circle that keeps an object in circular motion.

It acts at right angles to the velocity, so it changes the direction but not the speed.

The acceleration is also towards the centre.

It is not a new kind of force but the name for the resultant force providing the motion.

Section 3 of 9Examples of centripetal force

1

Tension in a string when a ball is whirled above the head.

2

Gravity for a satellite or planet in orbit.

3

Friction between tyres and road as a car takes a bend.

4

Contact force from the wall of a spinning washing-machine drum.

Section 4 of 9If the force stops

String breaks

• String breaks the centripetal force disappears and the ball moves along the tangent, not outward from the centre.

Car on ice

• Car on ice without enough friction the car continues in a straight line instead of following the bend.

Section 5 of 9Factors affecting the force

Mass

• Mass twice the mass needs twice the force.

Speed

• Speed double the speed needs four times the force.

Radius

• Radius a larger radius (gentler curve) needs less force.

Equation (Extended)

• F = mv²/r, with F in N, m in kg, v in m/s and r in m.
  What the symbols mean: F = centripetal force towards the centre (N) · m = mass (kg) · v = speed (m s⁻¹) · ω = angular speed (rad s⁻¹) · r = radius (m)

Speed from the period

• Speed from the period v = 2πr/T, where T is the time for one revolution.

Section 6 of 9Gravity and orbits

Planets and moons

• Planets and moons gravity provides the centripetal force for nearly circular orbits.

Closer to the Sun

• Closer to the Sun the gravitational force is stronger and the orbital speed is higher.

Geostationary satellite

• Geostationary satellite its orbital period is 24 hours, so it stays above the same point on the equator.

Section 7 of 9More from the specification

1Hooke's law

• Hooke's law extension is proportional to the force applied up to the limit of proportionality, with spring constant as the gradient of a force-extension graph

2Resultant force

• Resultant force the vector sum of forces acting on an object determines its acceleration

3Circular motion

• Circular motion a force towards the centre is needed to change direction, so speed can be constant while velocity changes

4Spring measurement

• Spring measurement measure extension as new length minus original length using a ruler for each load

5Extension

• Increase in length over the original length.

6Length

• Resistance is directly proportional to the length of a wire.

7Proportional

• Changing by the same factor as another quantity.

8Balanced

• Having equal and opposite moments.

9Weight

• Gravitational force on a mass.

10Elastic

• Returns to original shape when force is removed.

11Spring constant

• Stiffness k in N/m.

12Original length

• Length of the spring with no load.

Section 8 of 9Key terms

Term	Meaning
Centripetal force	Resultant force directed towards the centre of a circular path.
Tangent	Straight line touching a circle at one point.
Orbit	Path of one body around another under gravity.
Period T	Time for one complete revolution.
Radius r	Distance from the centre to the path.

Section 9 of 9Exam technique

1

Saying an object in circular motion at constant speed is not accelerating — its direction, and so its velocity, is changing.

2

Believing the centripetal force pushes outward — it acts inward towards the centre.

3

Thinking a released object moves radially outward — it moves along the tangent.

4

Drawing centripetal force as an extra force — name the actual force such as tension, gravity or friction.

5

Forgetting that speed enters the force as a square.