Gravity + Mass + Weight & Resultant Forces

What is gravity?

A non-contact force of attraction between objects with mass.

What is weight?

The force acting on an object due to gravity.

What is mass?

A measure of the amount of matter in an object.

What is the unit of mass?

Kilograms (kg).

What is the unit of weight?

Newtons (N).

What instrument measures weight?

A calibrated spring balance (newtonmeter).

What is gravitational field strength?

The force per unit mass acting on an object in a gravitational field.

What is the unit of gravitational field strength?

N/kg.

What is the relationship between mass and weight?

Weight is directly proportional to mass, provided gravitational field strength is constant.

Does mass change when an object travels from Earth to the Moon?

No, mass remains constant.

Does weight change when an object travels from Earth to the Moon?

Yes, weight decreases because the Moon has a lower gravitational field strength.

Where can the weight of an object be considered to act?

At its centre of mass.

What is a resultant force?

A single force that has the same effect as all the forces acting on an object together.

How do you determine the resultant of two forces acting in the same direction?

Add the forces.

How do you determine the resultant of two forces acting in opposite directions?

Subtract the smaller force from the larger, with the resultant acting towards the larger force.

What does a resultant force of zero mean?

The forces are balanced, so the object has no acceleration.

What happens to a stationary object if the resultant force is zero?

It remains stationary.

What happens to a moving object if the resultant force is zero?

It continues moving at a constant velocity.

What happens when a non-zero resultant force acts on an object?

Its velocity changes: it may speed up, slow down or change direction.

Why does a car moving at constant velocity have balanced forces?

The driving force equals the total resistive forces, so the resultant force is zero.