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2.1Unit 3: Forces, motion and pressure

Forces and motion — Cambridge Checkpoint Physics 0893 Revision Notes & Topical Questions

5 min read 9 parts Lesson 1 of 7 1 quiz

📚 Cambridge Checkpoint Physics 0893 Hub / Topical Revision
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Lesson summary

2.1 Forces and motion — CHECKPOINT PHYSICS 0893 notes: Distinguish balanced from unbalanced forces and describe their effects on motion.Forces are present — an object that is not moving still has forces acting on itWeight — gravity pulls a rock towards the centre of the Earth wit…

  • Contact force
  • Stationary
  • Starting to move
  • Unchanged direction
  • Lower speed
  • Constant speed

Part 1 of 8Forces on a stationary object

Forces are present
  • Forces are present an object that is not moving still has forces acting on it
Weight
  • Weight gravity pulls a rock towards the centre of the Earth with a force called its weight
Contact force
  • Contact force the ground pushes up on the rock and stops it moving towards the centre of the Earth
Balanced
  • Balanced these two forces are equal in size and opposite in direction
Friction
  • Friction wind pushing on the rock is balanced by friction between the rock and the ground

Part 2 of 8Force diagrams

Arrows
  • Arrows they show the size and direction of each force
Length
  • Length the longer the arrow, the bigger the force
Balanced forces
  • Balanced forces draw arrows of the same length pointing in opposite directions

Part 3 of 8Balanced forces

Balanced forces
Balanced forces forces equal in size and opposite in direction
Effect
  • Effect balanced forces cause no change in movement
Stationary
  • Stationary an object with balanced forces may be at rest
Moving
  • Moving an object with balanced forces keeps moving at a constant speed in a straight line

Part 4 of 8Unbalanced forces

Unbalanced forces
Unbalanced forces forces not equal in size and/or not opposite in direction
Effect
  • Effect unbalanced forces change movement by speeding up, slowing down or changing direction
Starting to move
  • Starting to move a push larger than friction makes a rock start moving in the direction of the larger force
Unchanged direction
  • Unchanged direction forces at right angles that stay balanced do not change movement in that direction

Part 5 of 8Slowing down

Parachute
  • Parachute air resistance larger than the weight makes a falling object slow down
Lower speed
  • Lower speed as the object slows, air resistance decreases
Constant speed
  • Constant speed when air resistance equals the weight the forces are balanced and the object falls at constant speed

Part 6 of 8Changing direction

Tennis ball
  • Tennis ball to return the ball the hitting force must be larger than the force from the ball
Circular motion
  • Circular motion an object moving in a circle is always changing direction
Orbit
  • Orbit a constant unbalanced force is needed, such as the Sun's gravity on a planet, to keep an object moving in a circle

Part 7 of 8Summary of force combinations

SizeDirectionForcesEffect on motion
EqualOppositeBalancedNo change
EqualNot oppositeUnbalancedChange of direction
Not equalOppositeUnbalancedSpeed increases or decreases
Not equalNot oppositeUnbalancedChange of speed and direction

Part 8 of 8Investigating balanced and unbalanced forces

1
Two people each hold a newton meter joined by a tight string, with tape on the string lined up with tape on the desk.
2
Both pull with equal forces in opposite directions and the tape stays lined up.
3
Increase both forces equally and the string still does not move.
4
Make one force smaller and the string moves towards the larger force.
5
Increase the difference between the forces and the string moves more quickly.
6
Link the result to a tug of war, where the newton meters represent the two teams.

Key terms

TermMeaning
Balanced forcesForces equal in size and opposite in direction.
Unbalanced forcesForces not equal and/or not opposite.
Contact forceForce from a surface pushing up on an object.
FrictionForce between surfaces that opposes movement.
Air resistanceForce on an object moving through air.
Force diagramDiagram using arrows to show forces.

Avoid these mistakes

1
Thinking an object at rest has no forces — it has balanced forces such as weight and contact force.
2
Thinking a moving object needs an unbalanced force to keep moving — balanced forces keep constant speed.
3
Drawing balanced forces with different arrow lengths — arrows must be equal and opposite.
4
Thinking unbalanced forces always speed things up — they can slow an object or change its direction.

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