Gravitational Force (Weight)

Forces and Motion

## Feeling a force of 10 N

Practical Activity for 14-16

**Demonstration**

This is a fairly elementary demonstration, but it will help students to develop a ‘feel’ for a newton of force, together with the concept of field strength.

Apparatus and Materials

- Masses, 1 kg , 2 kg , 3 kg , 4 kg
- Forcemeter, 0-50 N

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

- Get a student to hold a kilogram in one hand.
- Ask,
*"Can you feel the force of the Earth pulling down on it?"*And*"How big a force is it?"*If the student cannot answer the latter question, attach the mass to a forcemeter. - Prepare a table with 3 columns: label the first column ‘mass’ and the second column ‘gravitational force on the mass’. Leave the third column unlabelled for the moment.
- Complete the table for 2, 3, and 4 kg masses.
- Now head the third column ‘force on a unit mass of 1 kilogram’ or ‘gravitational field strength’. Ask,
*"What is the force on one kilogram of each of the masses?"*Complete the third column together.

Teaching Notes

- Students may have heard the story of Newton
discovering

gravity when an apple fell on his head as he sat thinking under an apple tree. The weight of an apple is about 1 newton. Newton's remarkable thought about gravity is that the Moon too is falling. - Students might find the question in part 5 puzzling because it is so obvious. They need only imagine the mass cut up into 1-kg lumps. The force on each kilogram is 10 N whether its mass comes in 1-kg lumps or 4-kg lumps. This is a very important demonstration because it leads to the concept of gravitational field strength. Its symbol is
*g*and its units are*N/kg*. - Physicists picture the gravitational field of force spreading out from the Earth with a ‘readiness to pull’ another mass, radially, towards the centre of the Earth. There is no actual force at a place near the Earth until you put some stuff there for the field to pull on. The field is always there.
- The force
*F*that acts on a mass*m*in a gravitational field is*F = mg*newtons. This is the weight of the body. When you hold an object and feel the pull of the Earth’s gravitational field on it, that object is not falling. It is not accelerating downwards with an acceleration*g*and it is therefore nonsense to talk of finding its weight by multiplying its mass by an*acceleration, g*. Here*g*represents the Earth’s*field strength*, 9.8*N/kg*. - See also the apparatus item:

*This experiment was safety-tested in April 2005.*