8th grade · Science · Unit 6 of 10
Newton's Laws
Three laws of motion
What your child practices
Inertia (1st law), force = mass × acceleration (2nd law) and action–reaction pairs (3rd law), with everyday examples and a few simple calculations.
Minnesota K-12 Academic Standards benchmark: 8P.1.2.1.2; 8P.3.2.2.2 · how forces change motion, and Newton's third law in collisions
How we explain it
Children can open this short lesson before they practice.
- First law (inertia): an object keeps doing what it is doing unless an unbalanced force acts.
- Second law: force = mass × acceleration. Force is measured in newtons (N).
- Third law: every action has an equal and opposite reaction.
Example: A 3 kg ball accelerates at 5 m/s². What force acts on it? F = m × a. 3 × 5 = 15. Answer: 15 N
Newton's Laws in Minnesota
In the Minnesota K-12 Academic Standards, this is unit 6 of 10 in 8th grade Science, after Fields & Forces at a Distance and before Motion, Forces & Newton’s Laws.
Gradelings describes a child's practice on this unit as Beginning, Approaching, Meeting or Exceeding (Four practice steps). That reflects practice, not a report-card mark: the teacher decides the mark.
Sample questions
A taste of this unit. In the app, questions change every time and get easier or harder to match your child.
Question 1
Why do seatbelts keep people safe?
🚗- They make the car go faster
- They turn off gravity
- They make the car lighter
- They stop inertia from carrying you forward in a sudden stop
They stop inertia from carrying you forward in a sudden stop
Hint: When a car stops suddenly, your body keeps moving forward. The seatbelt supplies the force that stops you.
Question 2
Action and reaction forces are equal and opposite. Why don't they cancel out?
- They do cancel, so nothing ever moves
- They act on different objects
- One is always a bit bigger
- They happen at different times
They act on different objects
Hint: When you push a wall, your push acts on the wall and the wall's push acts on you. Forces only cancel when they act on the same object.
Question 3
Which of Newton's laws does each example show? Tap an item, then tap its basket.
Sort into: 1️⃣ 1st law: inertia · 2️⃣ 2nd law: F = m × a · 3️⃣ 3rd law: action and reaction
- 🚚A loaded truck needs more force to speed up than a bike
- 🏒A puck keeps gliding on smooth ice
- ⚾A harder throw makes a ball go faster
- 🚌Riders lurch forward when a bus brakes hard
- 🏊A swimmer pushes water back to move forward
- 🚀A rocket pushes gas down and the gas pushes it up
1️⃣ 1st law: inertia: A puck keeps gliding on smooth ice, Riders lurch forward when a bus brakes hard · 2️⃣ 2nd law: F = m × a: A loaded truck needs more force to speed up than a bike, A harder throw makes a ball go faster · 3️⃣ 3rd law: action and reaction: A swimmer pushes water back to move forward, A rocket pushes gas down and the gas pushes it up
Hint: 1st law: things keep doing what they're doing (inertia). 2nd law: more force or less mass means more acceleration. 3rd law: every push has an equal push back.
Question 4
A soccer ball sits still on the grass. What will make it start moving?
⚽- an unbalanced force, like a kick
- waiting long enough
- its own inertia
- balanced forces
an unbalanced force, like a kick
Hint: Newton's first law: an object at rest stays at rest unless an unbalanced force acts on it.
Question 5
Newton's third law says that for every action force there is…
- an equal and opposite reaction force
- no reaction at all
- a reaction force in the same direction
- a bigger reaction force
an equal and opposite reaction force
Hint: Forces come in pairs. If you push on a wall, the wall pushes back on you just as hard.
Question 6
A 2 kg ball and a 4 kg ball get the same push. Compared with the 4 kg ball, the 2 kg ball's acceleration is…
- four times as large
- half as large
- the same
- twice as large
twice as large
Hint: Acceleration = force ÷ mass. Half the mass means double the acceleration.