SME
Grade 8 Science · Physical Science
Newton's three laws of motion
Use this for “A 4 kg cart accelerates at 2.5 m/s². What net force acts on it?”
Diagram
F = m × a = 4 kg × 2.5 m/s² = 10 N
8 blocks of 0.5 kg = 4 kg
The same pull on more mass gives less acceleration
This is an example. Type your own values to start a new problem.
F (N)
=
m (kg)
×
a (m/s²)
Formulas
F
=
m
×
a
(net
force
=
mass
×
acceleration)
10 = 4 × 2.5
Assumptions
•
The net force is all the forces on the cart added together, forces one way minus forces the other way. Friction is too small to count here.
•
First law: with no net force the motion does not change, at rest or at a steady speed.
•
Second law: F = m × a. 1 newton speeds up 1 kilogram by 1 m/s every second.
•
Third law: forces come in pairs. The rope pulls the cart forward; the cart pulls the rope back just as hard.
Step-by-step
WE KNOW
m = 4 kg (mass), a = 2.5 m/s² (acceleration)
FIND
net force (F)
Step 1 · Find net force (F)
Use F = m × a (net force = mass × acceleration)
A bigger mass or a bigger acceleration needs more force: multiply them.
F
=
4
×
2.5
F
=
10
N
Check
Put the numbers back in. Both sides should match.
10 = 4 × 2.5 ✓
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Two skaters push apart: force pairs
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