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Lesson · Grade 6 · Science

Newton's Second Law: Unbalanced Forces Cause Acceleration

  • Grade 6
  • Science
  • 1 min 7 s
  • British Columbia
  • BC6-SC-11

A cart rolls along the floor with a push arrow and a friction arrow above it, and a strip under the floor marks a dot where the cart is each second. Balanced forces leave equal gaps (steady speed), a push bigger than friction makes the gaps grow (acceleration), and the cart speeds up less with more mass and more with a bigger unbalanced force, ending on Newton's second law: unbalanced forces cause acceleration.

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In one sentence

The question this lesson answers

What makes a rolling cart speed up?

A push acts forward on the cart, and friction acts backward.

Transcript

What the lesson says

What makes a rolling cart speed up?

A push acts forward on the cart, and friction acts backward.

When the push and friction are the same size, the forces are balanced.

We put a dot where the cart is each second.

The gaps are equal, so the cart keeps a steady speed.

Now the push is bigger than friction, so the forces are unbalanced.

Each gap is bigger than the one before, so the cart is speeding up.

Speeding up like this is called acceleration.

Now load the cart with bricks, and keep the same push and friction.

With more mass, the cart speeds up less.

Next, make the push even bigger, so the unbalanced force is bigger.

With a bigger unbalanced force, the cart speeds up more.

This is Newton's second law: unbalanced forces cause acceleration.

Curriculum

Where this fits

British Columbia, Grade 6

Science, Physics: Newton's three laws of motionBC6-SC-11

Previous: Newton's First Law: Why Things Keep MovingNext: Newton's Third Law: Equal and Opposite Forces

The outcomes and strands follow the British Columbia curriculum.

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