What Are Forces?

• Forces are pushes, pulls, twists and squeezes.

• Force, F, is measured in units called Newtons (N), and are named after Sir Isaac Newton.

• Forces are vector quantities, since they have both a size (magnitude), and a direction. For example: the weight force of a person is 750 N downwards. Forces can be represented labelled arrows. The length of the arrow shows the size of force, and the direction of the force is shown by the direction of the arrow.

• Small forces can be measured in the Science lab using Newton meters, which are sometimes known as spring balances.

• You can't see forces, but you can often see the effect of forces.

• Forces can making stationary things move, speed up moving objects, slow down moving objects, change the direction of moving objects, twist objects, squeeze objects, and change the shape of objects.

• Contact forces are forces that need to touch e.g. friction, support/reaction force, air resistance, water resistance, tension force on a string, forces caused by springs, and applied forces.

• Non contact forces act against objects that are physically separated e.g. weight force due to gravity (sometimes called gravitational force), electromagnetic force, and nuclear force. At Level 1 you will only need to worry about weight force due to gravity.

Video 1 - Introduction To Forces - How Objects Interact (Fuse School)

What is The Difference Between Mass And Weight?

• Mass is the amoung of matter (solid, liquid or gas material) in an object.

• Mass, m, is measured in kilograms, kg, but are sometimes measured in grams, g. In Physics, the SI unit for mass is the kilogram, kg, so you must convert grams, g to kilograms, kg.

• Since the amount of matter in an object stays the same, then the mass is always the same, wherever the object is. A 90kg astronaut will always have a mass of 90kg no matter where they are. They will have a mass of 90kg on Earth, 90kg on the moon, and 90kg on the International Space Station (ISS).

• In everyday life we use the term weight incorrectly in terms of Science. It is common for people to say that they weigh 66 kg. In fact, they should say that they have a mass of 66 kg. A person with a mass of 66 kg would in fact have a weight of 660 N. This is an example of when we use words in everyday life differently to how words are use in Science.

• In Science, the weight of an object is the force acting on an object. Weight is caused by gravity pulling an object downwards.

Video 2 - Weight, Force, Mass & Gravity

Balanced And Unbalanced Forces

• Forces act on everything and often there is more than one force acting. When more that one force is acting, the combination of all the forces acting will affect the motion.

• Net force F_net is the term for these combined forces

• Forces acting the SAME direction are ADDED together

• Forces acting in the OPPOSITE direction are SUBTRACTED

• The different forces on an object will either be:

1) Balanced OR 2) Unbalanced

• Balanced forces = all forces on an object that combine to give an effect of ZERO force

• Unbalanced forces = all forces on an object that combine to give an effect that is NOT ZERO force

For example, Arnold Schwarzenegger and I are trying to push a car and are applying force from opposite directions. Arnolds applied force is MUCH larger than mine..... so our forces are unblanced. The force applied by me in one direction does not equal Arnolds large force applied in the opposite direction.

Video - Newton's Second Law

NCEA Answers - Forces