# Newton's first law

##### Demonstration

This shows that a body moves with constant velocity unless an unbalanced force acts on it. The constant velocity can have any value, including zero.

#### Apparatus and materials

Crank assembly

Demonstration spring balances, 2

Plank, short

Steel rollers, 10

Brick, wooden

#### Health & Safety and Technical notes

The plank should ideally be about 75 cm long.

#### Procedure

a Put the rollers under the plank so that it can move freely on them, with very little friction.

b Attach the plank to a G-clamp at the end of the bench, with a horizontal string to keep it from moving. Insert a spring balance, which will measure the force exerted on the plank when you drag the brick along it.

c Place the brick on the plank. Use string to connect the brick to a second spring balance and to connect that to a crank assembly.

d Drag the brick along the plank by turning the crank.

e Students watch the readings of the two spring balances to see if the two forces are equal, and match this to the type of motion, accelerated or unaccelerated, of the brick.

#### Teaching notes

Newton's first law says that a body stays at rest or moves at a constant velocity unless an unbalanced force acts on it.

So:

• whenever there is no net force there is no acceleration
• whenever there is a net force there must be acceleration.

This experiment was safety-checked in March 2005