Grade 6 · Forces and Motion Basics · Describing Forces
Describing Forces
Pushes, pulls, arrows and what a resultant force actually does.
Learning objectives
- Identify contact and non-contact forces.
- Draw force arrows to show size and direction.
- Predict motion from a resultant force.
NGSS alignment
MS-PS2-1
Apply Newton's third law to design a solution to a problem involving the motion of two colliding objects.
MS-PS2-2
Plan an investigation to provide evidence that the change in an object's motion depends on the sum of the forces on the object and the mass of the object.
MYP criteria
- Criterion A — Knowing and Understanding
- Criterion D — Reflecting on the Impacts of Science
ENGAGE
The book that will not move
Push a heavy book gently and nothing happens. Push a little harder and it suddenly slides.
Think about it
- What was cancelling out your push at first?
- What changed at the moment it began to move?
EXPLAIN
Forces and resultants
A force is a push or a pull measured in newtons (N). Forces have both size and direction, so we draw them as arrows: a longer arrow means a bigger force.
| Type | Needs touching? | Examples |
|---|---|---|
| Contact | Yes | Friction, air resistance, tension, normal contact force |
| Non-contact | No | Gravity (weight), magnetism, electrostatic force |
Resultant force
- Forces in the same direction add together
- Forces in opposite directions subtract
- A resultant of zero means balanced forces: the object stays still or keeps a constant speed
- A non-zero resultant changes the speed or direction
Common misconception
A moving object does not need a force to keep moving. It only needs a resultant force to change how it moves.
Watch
Newton's Laws: Crash Course Physics
CrashCourse · 10 min
Introduces forces and what they do to motion.
Before you watch: Does a moving object need a force to keep moving?
- Give one contact and one non-contact force.
- What happens when the resultant force is zero?
Interactive simulation · PhET
Forces and Motion: Basics
Open the Net Force tab. Add pullers to each side and switch on the force values.
While you explore
- When is the net force zero?
- What happens to the cart when the net force is not zero?
Key vocabulary
- Resultant force
- The single force left over after adding all forces.
- Force
- A push or a pull on an object.
Practice questions
0/2 correct
Level 1 · Criterion A
Which of these is a non-contact force?
Level 2 · Criterion A
A box is pushed with 12 N while friction acts with 5 N. The resultant force is:
MYP criterion tasks
Level 3 · Criterion D
A skydiver falls at a constant speed with an open parachute. Explain what this tells you about the forces acting.
Reflect & track
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