New
New
Year 11
OCR
Higher

Newton’s Third Law (with free body diagrams in 2D)

I can draw force diagrams and identify pairs of forces that obey Newton’s Third Law.

New
New
Year 11
OCR
Higher

Newton’s Third Law (with free body diagrams in 2D)

I can draw force diagrams and identify pairs of forces that obey Newton’s Third Law.

Lesson details

Key learning points

  1. Some forces require contact between objects, while others do not.
  2. Forces are vectors and can be drawn as arrows on diagrams.
  3. A free body diagram shows all of the forces acting on a single object.
  4. For every force there is an equal sized force acting in the opposite direction.
  5. Force pairs have the same magnitude and type, and act in opposite directions between two objects.

Common misconception

Pupils may regard force as a property of an object, expressing ideas such as ‘the object has a force’ and ‘the object runs out of force’.

Describe forces as interactions between two objects, using the phrasing ‘the force of object A on object B’.

Keywords

  • Non-contact force - A force that exists between objects, even if they are not touching.

  • Contact force - A force that exists between objects when they are touching.

  • Normal contact force - The overall effect of a set of forces acting on an object is represented by a single resultant force.

  • Free body diagram - A diagram showing all of the forces acting on a single object.

  • Newton’s third law - Forces come in equal and opposite pairs, which act on pairs of different objects.

There are many types of forces which can be described in different ways. Don't worry too much about the exact names at this stage as these will be developed throughout the unit. Focus on the idea of constructing force diagrams and locating the positions of force pairs clearly.
Teacher tip

Equipment

None required.

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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6 Questions

Q1.
What is the difference between scalar quantities and vector quantities?
Both have direction, but scalar quantities also have magnitude.
Both have magnitude, but scalar quantities also have direction.
Correct answer: Both have magnitude, but vector quantities also have direction.
Both have direction, but vector quantities also have magnitude.
Q2.
Which of the following are vector quantities?
speed
energy
Correct answer: velocity
distance
Correct answer: displacement
Q3.
What is the meaning of the prefix ‘m’ in the units mm and mg?
Correct answer: milli-
micro-
Correct answer: thousandth
thousand
millionth
Q4.
What is the meaning of the prefix ‘k’ in the units km and kg?
kine-
Correct answer: kilo-
thousandth
Correct answer: thousand
hundred
Q5.
What is the meaning of the prefix ‘M’ in the units MW and MJ?
Correct answer: mega-
micro-
hundred
millionth
Correct answer: million
Q6.
Which of the following lengths are equal to 8.2 m?
820 mm
Correct answer: 8200 mm
82 mm
Correct answer: 0.0082 km
0.082 km

6 Questions

Q1.
Match the following key terms to their definition.
Correct Answer:resultant force,one force representing the overall effect of all forces on an object

one force representing the overall effect of all forces on an object

Correct Answer:contact force,a force that exists between objects only when they are touching

a force that exists between objects only when they are touching

Correct Answer:non-contact force,a force that exists between objects even if they are not touching

a force that exists between objects even if they are not touching

Q2.
A block rests on a table. Which of the following diagrams correctly shows the weight of the block?
Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz
An image in a quiz
Q3.
A footballer kicks a football. Which of the following diagrams correctly shows the force of the boot on the football?
An image in a quiz
An image in a quiz
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
Q4.
Newton’s Third Law explains that interactions can be described by a pair of forces, sometimes called a ‘third law pair’. Which of the following are characteristics of the forces in a third law pair?
Correct answer: They act between two objects only.
They act in the same direction.
Correct answer: They are the same type of force.
Correct answer: They are the same size.
Q5.
A block rests on a table. Which of the following is the correct free body diagram for the block?
An image in a quiz
Correct Answer: An image in a quiz
An image in a quiz
An image in a quiz
An image in a quiz
Q6.
The diagram shows a ball hanging from a spring without moving. Two forces are shown. Which of the following statements are correct?
An image in a quiz
Correct answer: This is a free body diagram of the ball.
This is not a free body diagram of the ball.
The two forces are a third law pair.
Correct answer: The two forces are not a third law pair.