ScientistGrades 9–11⏱ 20 min

Newton's laws

Push a cart, add friction, change the mass, and see F = ma on a live velocity graph.

Standards: HS-PS2-1

1Wonder

A hockey puck slides across the ice for a long time, but a book pushed across a table stops almost at once. Does a moving object need a force to keep moving?

2Explore

Force and motion lab

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Turn your phone sideways, or open this lesson on a tablet or computer. The explanation and quiz below work here.

Net force 4 N Acceleration 2 m/s² Velocity 0 m/s x 0 m

Push F 4 N Mass m 2 kg Friction (μ = 0.1)

Try this

Press Play with a push of 4 N. What does the velocity graph look like?
Press Let go (F = 0) while it is moving, with friction off. What happens? Now try with friction on.
Double the mass. How does the acceleration change?
With friction on, find the smallest push that gets a 5 kg cart moving.
Push backwards (negative F) while the cart moves forwards.

3Explain

Step 1 of 5

First law: no net force, no change

An object keeps doing what it is doing (staying still, or moving in a straight line at the same speed) unless a net force acts on it. On smooth ice the puck keeps its speed: equal distances in equal times.

Newton's second law

\htmlClass{c-purple}{F_{\text{net}}} = m\,a \qquad\Longleftrightarrow\qquad a = \frac{F_{\text{net}}}{m}

1 newton is the force that gives 1 kg an acceleration of 1 m/s². With friction: $F_{\text{net}} = F - \mu m g$ .

4Practice

Question 1 of 5

A 2 kg cart is pushed with a net force of 6 N. What is its acceleration?

5Recap

First law: with no net force, an object stays at rest or keeps moving at a constant velocity.
Second law: $F_{\text{net}} = ma$ . Bigger force → bigger acceleration; bigger mass → smaller acceleration.
Third law: forces come in equal and opposite pairs.
Always add up all the forces (push, friction…) before using $F = ma$ .
On a velocity–time graph, the slope is the acceleration.