Forces and resultant forceIB MYP Sciences: Revision notes

A force is a push or a pull. It is a vector, measured in newtons (N).

• Contact forces act when objects touch: friction, air resistance, tension, normal (support) force, and pushes and pulls.
• Non-contact forces act without touching: gravity (weight), magnetic force and electrostatic force.

Forces are shown with arrows. The arrow points in the direction of the force, and a longer arrow shows a bigger force.

Mass is the amount of matter in an object. It is measured in kilograms (kg) and does not change from place to place.

Weight is the force of gravity on an object. It is measured in newtons (N) and depends on the gravitational field strength, g.

weight = mass × gravitational field strength (W = mg)

On Earth g = 10 N/kg (or 9.8 N/kg); on the Moon g is about 1.6 N/kg.

Worked example: a 60 kg astronaut has weight 60 × 10 = 600 N on Earth, but only 60 × 1.6 = 96 N on the Moon. Her mass is 60 kg in both places.

Friction is a force that opposes motion between surfaces that are in contact. It acts in the opposite direction to the movement.

Air resistance (drag) is friction with the air. It gets bigger as the speed increases and depends on the shape of the object.

Friction is useful for grip, walking and braking. It can also be a problem because it wastes energy as heat and causes wear. Streamlined shapes and lubricants reduce unwanted resistive forces.

Several forces often act on an object. The resultant force is the single force that has the same effect as all of them together.

• Forces in the same direction: add them.
• Forces in opposite directions: subtract them, and the resultant is in the direction of the larger one.

Worked example: a push of 150 N forwards and friction of 90 N backwards give a resultant of 150 − 90 = 60 N forwards.

In a force diagram, draw one arrow for each force, starting from the object, and label each with its size.

When forces are balanced, the resultant force is zero. The object stays at rest or keeps moving at a constant velocity.

When forces are unbalanced, there is a resultant force. The object accelerates in the direction of the resultant force: it speeds up, slows down or changes direction.

A crate pushed with 90 N against 90 N of friction moves at constant speed. If the push is 150 N, the resultant is 60 N forwards and the crate speeds up.

A force can stretch a spring. The extension is the new length minus the original length.

For a spring, the extension is directly proportional to the force applied (Hooke's law): double the force and the extension doubles. This works up to a limit; a spring stretched too far will not go back to its original shape.

Worked example: a 2 N load stretches a spring by 4 cm, so a 4 N load stretches it by 8 cm.