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5.1.3Forces

Forces as Vectors — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions

5 min read 7 parts Lesson 4 of 44

📚 AQA GCSE Combined Science Trilogy 8464 - Physics Hub / Topical Revision
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Section 1 of 7Force is a vector

Magnitude
  • Magnitude The size of a force is measured in newtons (N).
Direction
  • Direction A force always acts in a particular direction, such as left, right, up, down, or at an angle.
  • An angle is usually given from the horizontal or the vertical, for example 100 N at 40 degrees above the horizontal.
Signs along a line
  • Signs along a line When forces act on a straight line, choose one direction as positive.
  • Forces the other way are negative, which makes adding them straightforward.

Section 2 of 7Force pairs

Interaction between two objects
Always two forces
  • Always two forces Whenever two objects interact, each exerts a force on the other.
    • These are equal in size and opposite in direction.
Act on different objects
  • Act on different objects The two forces in a pair act on different bodies, which is why they do not cancel each other out.
Same type
  • Same type Both forces in a pair are the same kind, for example both gravitational or both contact forces.

Section 3 of 7Examples of force pairs

SituationForce on first objectForce on second object
Laptop on a deskLaptop pushes down on the deskDesk pushes up on the laptop
Throwing a basketballPlayer pushes the ball forwardsBall pushes back on the player
Person standing on EarthEarth pulls the person downPerson pulls the Earth up
Person pushing a rockPerson pushes the rockRock pushes the person back

Section 4 of 7Finding the resultant force

1
Draw the forces as arrows from the object, with each length matching the size of the force.
2
Choose a positive direction, for example to the right.
3
Add together forces that act in the same direction.
4
Subtract forces that act in opposite directions.
5
State the resultant, giving its size and direction. If the answer is 0 N, the forces are balanced.

Section 5 of 7Balanced and unbalanced

1Equal and opposite
  • Equal and opposite Forces of equal size acting in opposite directions have a resultant of zero.
2Balanced forces
  • Balanced forces No resultant force means no change in the motion of the object.
3Unequal forces
  • Unequal forces One force bigger than the other leaves a resultant in the direction of the larger one.
4Unbalanced
  • Unbalanced A non-zero resultant changes the motion of the object.

Section 6 of 7Key terms

TermMeaning
VectorA quantity that has both a magnitude and a direction.
ScalarA quantity that has magnitude only, with no direction.
Resultant forceThe single force that has the same effect as all the forces acting on an object.
Force pairTwo equal and opposite forces that two interacting objects exert on each other.
Balanced forcesForces that cancel so that the resultant force is zero.

Section 7 of 7Exam technique

1
Saying the two forces in a force pair cancel; they act on different objects, so they cannot cancel.
2
Adding forces that act in opposite directions instead of subtracting them.
3
Giving a force with no direction; a complete answer says how big it is and which way it acts.
4
Confusing a weight and its normal force with a force pair; they act on the same object and have different causes.

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