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2.7Chapter 2: Forces

Chapter 2 Review: Forces — Alhussan Physics - Stage 7 Revision Notes & Topical Questions

5 min read 7 parts Lesson 7 of 7 1 quiz

📚 Alhussan Physics - Stage 7 Hub / Topical Revision
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Lesson summary

2.7 Chapter 2 Review: Forces — Alhussan International Academy — Physics (Stages 7–9) notes: Revise mass, weight, gravitational field strength, resistance in fluids, streamlining and changing motion.Mass — the amount of matter in an object, measured in kilograms, and it does not c…

  • Values of g
  • Observation
  • Weaker gravity
  • No atmosphere
  • Streamlined shape
  • Diving bird

Part 1 of 5Mass, weight and gravitational field strength

Mass
  • Mass the amount of matter in an object, measured in kilograms, and it does not change with place.
Weight
  • Weight the force of gravity on the object, measured in newtons (N), and it depends on the strength of gravity where the object is.
Formula
  • Formula weight = mass × gravitational field strength, W = m × g.
Values of g
  • Values of g about 10 N/kg on Earth, 1.6 N/kg on the Moon and 3.7 N/kg on Mars.
    • (about one sixth of Earth's)
Astronaut
  • Astronaut one with a mass of 65 kg has a weight of 650 N on Earth, and on Mars the mass is still 65 kg but the weight is only about 240 N.

Part 2 of 5Golf on the Moon

Observation
  • Observation when Alan Shepard, the fifth person to walk on the Moon, hit a golf ball it travelled much further than on Earth.
Weaker gravity
  • Weaker gravity the ball is pulled down more slowly, so it stays in the air longer.
No atmosphere
  • No atmosphere there is no air resistance to slow the ball down.

Part 3 of 5Resistance in fluids and streamlining

Drag
  • Drag a solid moving through a liquid or gas experiences a resistive force, called water resistance or air resistance, that acts against its motion.
Streamlined shape
  • Streamlined shape it has a narrow, smooth, pointed front so the fluid flows round it easily and drag is small.
Diving bird
  • Diving bird it folds its wings close to its body before hitting the water.
  • This protects the wings from injury.
  • It makes the bird more streamlined, which reduces water resistance and lets it enter the water faster and more cleanly.
Not a forward force
  • Not a forward force streamlining increases speed by reducing drag, not by adding a forward force.

Part 4 of 5Fair test of streamlining

Method
  • Method make three different shapes from modelling clay and time how long each takes to sink to the bottom of a tank of water.
Independent variable
  • Independent variable the shape.
Dependent variable
  • Dependent variable the time to fall.
Control variables
  • Control variables the mass of clay, the depth and temperature of the water and the release height.
Repeats
  • Repeats repeat each shape and find a mean.
Prediction
  • Prediction the most streamlined shape has the least drag, so it should sink fastest.

Part 5 of 5Forces and changes in motion

Balanced forces
  • Balanced forces they leave an object stationary or moving at constant speed in a straight line.
Unbalanced forces
  • Unbalanced forces they change the speed or direction of motion.
Shape
  • Shape forces can also change the shape of an object.
Circular motion
  • Circular motion a force acting at right angles to the motion makes the object move in a circle, and in orbits the force is gravity.

Key terms

TermMeaning
Massthe amount of matter in an object, measured in kilograms.
Weightthe force of gravity acting on an object, measured in newtons.
Gravitational field strengththe force of gravity on each kilogram of mass, in N/kg.
Streamlinedshaped to reduce resistance as it moves through a fluid.
Dragthe resistive force on an object moving through a liquid or a gas.

Avoid these mistakes

1
Using mass and weight as if they were the same — mass is in kg and does not change, while weight is a force in N and depends on g.
2
Thinking an astronaut has less mass on the Moon — the mass is unchanged and only the weight is smaller.
3
Thinking streamlining increases speed by adding a forward force — it reduces drag.
4
Changing more than one variable in a fair test — for example using different amounts of clay as well as different shapes.

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