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P8.2 Forces Between Objects

GCSE Combined Science · AQA · Chapter P8: Forces in Balance

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: what forces do, sizes of forces, contact and non-contact forces, and Newton’s third law. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe what a force can do to an object and use the units N, kN and MN
  • Classify forces as contact or non-contact forces and give examples of each
  • State Newton’s third law and list the features of an interaction pair of forces
  • Use Newton’s third law to explain everyday examples such as driving, walking and pushing off

Key words

  • Force: A push or a pull on an object caused by its interaction with another object. Measured in newtons (N).
  • Contact force: A force that acts only when the objects touch, e.g. friction, air resistance (drag), tension, normal contact force.
  • Non-contact force: A force that acts between objects that are physically separated, e.g. gravitational, magnetic and electrostatic forces.
  • Normal contact force: The push of a surface on an object resting on it, at right angles to the surface.
  • Tension: The pulling force in a stretched rope, string, cable or spring.
  • Newton’s third law: Whenever two objects interact, the forces they exert on each other are equal and opposite.

What forces can do

A force is a push or a pull. A force always comes from an interaction between two objects: a foot and a ball, the Earth and a falling stone, a tyre and the road. Forces can:

  • change the shape of an object (squash, stretch, bend or twist it)
  • make a stationary object start to move, or make a moving object stop
  • change the speed or the direction of a moving object (a cyclist steering round a bend).

Force is measured in newtons (N) with a newtonmeter. Large forces are given in kilonewtons (1 kN = 1000 N) or meganewtons (1 MN = 1 000 000 N). Force is a vector: it has a size and a direction.

Worked example: converting units

3.6 kN = 3.6 × 1000 = 3600 N
0.25 MN = 0.25 × 1 000 000 = 250 000 N

Answer: 3600 N and 250 000 N

Contact and non-contact forces

Forces are sorted into two groups, depending on whether the two interacting objects have to touch. The same object can feel both kinds at once: a falling hailstone is pulled down by gravity (non-contact) and slowed by air resistance (contact).

Key fact: A contact force acts when an object is supported by or strikes another object. If the force still acts when the objects are separated (two magnets held 2 cm apart still repel), it is a non-contact force.

Newton’s third law: equal and opposite forces

Newton’s third law: whenever two objects interact, the forces they exert on each other are equal and opposite. The two forces in a third-law pair:

  • act on two different objects – one on each
  • are the same type of force (both contact, both gravitational, both electrostatic …)
  • are the same size
  • act along the same line, in opposite directions.

Because the two forces act on different objects, they never cancel each other out. Their effects can be very different: the object with the smaller mass speeds up more. When a spacecraft is pulled towards the Earth, the Earth is also pulled towards the spacecraft with the same force, but the Earth’s huge mass means it hardly moves.

Exam tip: The weight of the person and the push of the floor on the person are equal here, but they are not a third-law pair: they both act on the same object (the person), and they are different types of force.

Friction, driving forces and walking

The force that drives a car forwards is friction between the tyres of the drive wheels and the road. The engine turns the wheels; each tyre pushes backwards on the road, so the road pushes forwards on the tyre. Walking works the same way: your foot pushes back on the ground and the ground pushes you forwards. On ice there is very little friction, so the ground cannot push you forwards much and your foot slips.

Worked example: a tractor pulling a trailer

A tractor pulls on a tow bar with a force of 2400 N. The tow bar pulls back on the tractor with 2400 N (third law).
The tow bar pulls the trailer forwards with 2400 N, and the trailer pulls back on the tow bar with 2400 N.
Why can they move at all? Whether each vehicle speeds up depends on all the forces acting on it. The tractor moves forwards because the ground pushes its tyres forwards with more than 2400 N.

Answer: pairs are always equal; motion depends on the total of the forces on one object

Common mistakes

  • Saying a third-law pair cancels out. The two forces act on different objects, so they cannot cancel.
  • Pairing the weight of an object with the normal contact force on it. They act on the same object, so they are not a third-law pair.
  • Thinking the bigger or heavier object pushes harder. In an interaction the forces are always equal; only the effects differ.
  • Calling weight a contact force. Gravity acts at a distance, so weight is a non-contact force.

Revision questions

Try these, then check your answers below each one.

Question 1. Which of these is a non-contact force? (Choose from: Magnetic force, Friction, Tension, Air resistance.)

Answer: Magnetic force

Working: Magnetic force acts across a gap, so it is non-contact. Friction, tension and air resistance all need the objects to touch.

Question 2. A boxer’s glove hits a punch bag with a force of 420 N. What is the size of the force that the punch bag exerts on the glove?

Answer: 420 N

Working: By Newton’s third law the bag pushes back on the glove with an equal and opposite force: 420 N.

Question 3. True or false? The weight of a book on a table and the upward push of the table on the book are a Newton’s third law pair.

Answer: False

Working: False. Both forces act on the book, and one is gravitational while the other is a contact force. The third-law partner of the book’s weight is the book pulling the Earth upwards.

Related lessons

Continue in the full course

This lesson is section P8.2 (AGCSC3702) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.

 

 

 

 

 

 

 

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