Measuring a force

The idea

A spring balance measures the magnitude of any force, not just weight (the gravitational force with which the Earth pulls an object); pulling its free end measures the force of the pull.

A balance at the shop

A shopkeeper hangs a bag from a spring balance and reads the weight. You can also pull on its hook with your hand.

What does the balance show when you pull the hook with your hand?

• The size of your pull
• Nothing, it only works for hanging bags
• The colour of your hand

Weigh and pull

Hang the bag, then pull the hook yourself.

Any force, not only weight

The bag stretched the spring because the Earth pulls it down. Your hand stretched it just as well. The spring stretches more for a stronger force, so the balance can tell the magnitude of any force that pulls its free end.

A spring balance measures the magnitude of any force, not just weight (the gravitational force with which the Earth pulls an object). Pulling its free end measures the force of the pull.

Notes

A spring balance measures the magnitude of any force, not just weight. Pulling its free end shows the force of the pull.

Check yourself

What is the weight of an object?

What can a spring balance measure?

You pull the free end of a spring balance and it reads 6 N. What does 6 N tell you?

A bag hangs from the balance. Which force stretches the spring?

• The force the balance exerts on it. The balance only measures a force. Weight is the pull of the Earth on the object.
• The push that the floor gives it. That is a different force, the normal force from the surface.
• The gravitational force with which the Earth pulls it — correct. Yes!
• The magnitude of any force, not just weight — correct. Yes!
• The weight of objects and nothing else. A spring balance can measure the magnitude of a force in general, such as a pull on its free end.
• The direction of a force. It reads the magnitude of a force. The direction is a separate thing to specify.
• The mass of the spring. The newton is a unit of force. The kilogram is the unit of mass.
• The magnitude of the force with which you pull — correct. Yes!
• The direction of your pull. The reading gives the magnitude of the force only.
• A magnetic force from the balance. Nothing magnetic is involved. The Earth pulls the bag by the gravitational force.
• The push of the floor on the bag. The bag is hanging in the air, and its weight pulls the spring down.
• The gravitational force of the Earth on the bag, its weight — correct. Yes!