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8.2 Forces Between Objects

GCSE Physics · AQA · Chapter 8: Forces in Balance

A free GCSE Physics lesson for AQA (8463) from OBA School: what forces do, 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 state the unit of force
  • Classify forces as contact or non-contact forces and give examples of each
  • State Newton’s third law and identify the two forces in an interaction pair
  • Explain the driving force on a car and other examples using equal and opposite forces

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, tension, normal contact force.
  • Non-contact force: A force that acts between objects that are not touching, e.g. gravity (weight), magnetic force, electrostatic force.
  • Normal contact force: The push of a surface on an object, at right angles to the surface.
  • Tension: The pulling force in a stretched rope, string, cable or spring.
  • Newton’s third law: When two objects interact, they exert equal and opposite forces on each other.

What forces can do

A force is a push or a pull. A force always comes from an interaction between two objects: the hand and the ball, the Earth and the apple, the road and the tyre. 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.

Force is measured in newtons (N) using a newton-meter (a spring balance). Force is a vector: it has a size and a direction.

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 skydiver is pulled down by gravity (non-contact) and slowed by air resistance (contact).

Key fact: A contact force happens when an object is supported by or strikes another object. If you can switch off the touching and the force still acts, it is a non-contact force.

Newton’s third law: equal and opposite forces

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

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

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. If skater A is a small child and skater B an adult, the child moves off faster even though the forces are equal.

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 towing

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.

Worked example: a recovery truck towing a van

A recovery truck pulls on a tow rope with a force of 1800 N. The rope pulls back on the truck with 1800 N (third law).
The rope pulls the van forwards with 1800 N, and the van pulls back on the rope with 1800 N.
Why can they move at all? Whether each vehicle speeds up depends on all the forces acting on it, including the forces from the ground. The truck moves forward because the road pushes its tyres forward with more than 1800 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: friction, tension, electrostatic force, air resistance.)

Answer: electrostatic force

Working: Electrostatic forces act between charges across a gap. Friction, tension and air resistance all need the objects to touch.

Question 2. A person pushes on a wall with a horizontal force of 250 N. What is the size of the force the wall exerts on the person?

Answer: 250 N

Working: By Newton’s third law the wall pushes back with an equal and opposite force: 250 N (away from the wall).

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 same object (the book) and they are different types of force. The weight pairs with the book’s gravitational pull on the Earth; the table’s push pairs with the book’s push on the table.

Related lessons

Continue in the full course

This lesson is section 8.2 (AGCPH0802) of our free GCSE Physics 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 Physics course.

 

 

 

 

 

 

 

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