CLASS 8 SCIENCE · CHAPTER 5 · STUDY NOTES
Exploring Forces
Understand exploring forces through short explanations, labelled models and original practice. Read one topic, explain it in your own words, then check.
EXPLORE
A force comes from an interaction
Identify which object pushes or pulls another.Still does not mean force-free
- Force
- A push or pull arising from an interaction between objects.
- Newton (N)
- The SI unit used to measure force.
A hand can push a door; a rope can pull a load. A magnet can pull iron without touching it. Identify both interacting objects rather than treating force as something an object simply owns.
An object at rest can still experience forces. A book resting on a table has its weight downward and support upward.
Take a moment. What do you think?
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Question
A book remains still on a table. Does that show that no force acts on it?
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EXPLORE
A force can change motion or shape
Recognise a change in speed, direction or shape.Direction is part of motion
- Change in motion
- A change in speed, direction, or both.
A kick can start a ball or change where it travels. Brakes slow a bicycle. Squeezing a soft object changes its shape. More than one effect can happen at once.
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Question
A moving ball keeps the same speed but turns around a bend. Has its motion changed?
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EXPLORE
Friction acts between touching surfaces
Explain why a sliding object slows down and why grip can help.Compare movement and friction
- Contact force
- A force requiring physical contact.
- Muscular force
- A force produced through the action of muscles.
- Friction
- A contact force opposing sliding or the tendency to slide between surfaces.
A sliding box experiences friction opposite its sliding direction. Different surfaces give different friction. Air and water also resist objects moving through them.
Friction can be useful: it gives shoes grip and helps brakes work. It can also cause wear and energy loss. Muscles provide the forces used in many movements.
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Question
A box slides right across a floor. In which direction does the floor’s friction act on the box?
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EXPLORE
Forces can act across a gap
Compare magnetic, electrostatic and gravitational effects.Repel across a gap
- Non-contact force
- A force that can act without the objects touching.
- Electrostatic force
- The force associated with electric charges.
- Gravity
- The attractive interaction between objects with mass.
Magnets can attract unlike poles and repel like poles. Electric charges also have two kinds: like charges repel, unlike charges attract. A charged object can attract a neutral object through redistribution of charge.
Earth pulls objects towards itself through gravity. In this chapter’s everyday setting, gravity is attractive. A gap does not mean that no force can act.
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Question
Two similarly charged balloons move apart without touching. What does this show?
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EXPLORE
Gravity acts even at the top
Separate a ball’s direction of motion from the direction of gravity.Same force, changing motion
- Weight
- The gravitational force acting on an object.
A thrown ball rises, slows, briefly stops moving upward and then falls. Gravity pulls downward throughout this flight. At the top, zero speed does not mean zero gravity.
Air resistance can also act; the simple model here ignores it to focus on gravity.
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Question
At the highest point of a vertically thrown ball, is gravity zero?
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EXPLORE
Mass and weight are different quantities
Read a spring balance and use the correct units.Read the spaces on the scale
- Mass
- The quantity of matter in an object, measured in grams or kilograms.
- Weight
- A gravitational force, measured in newtons.
Moving an object to the Moon does not remove its matter. Its mass remains the same, but weaker gravity gives it less weight.
A spring balance measures force through stretching. Read its range and the smallest division before using it. A mass scale on such a balance assumes the gravity for which it was calibrated.
Count spaces, then read
2 N ÷ 10 = 0.2 N; 6 × 0.2 N = 1.2 NThe pointer is six small divisions below zero. Each division represents 0.2 N.
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Question
A scale has a 2 N gap between labelled marks and ten equal divisions in that gap. What does one division represent?
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EXPLORE
Water pushes upward
Compare buoyancy with weight and explain floating at rest.Balanced on the surface
- Buoyant force
- An upward force exerted by a fluid on an immersed object.
Water pushes upward on an object while gravity pulls it downward. If an object floats at rest, these forces balance. It displaces enough water to provide that upward force.
If its weight is greater than the buoyant force available when fully immersed, it sinks. Shape and enclosed air can change the volume displaced; the material name alone is not always enough.
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Question
A sealed empty bottle floats at rest. Is its buoyant force greater than its weight at that moment?
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