Gravity, frictional force, air resistance, internal-muscular force, weight.

The idea

Each named force has a distinct origin and direction. Gravity is the attractive force between masses; on Earth it gives a gravitational field strength of about 9.81 N kg⁻¹, so a 70 kg performer has a weight of about 687 N acting vertically downwards. Weight is therefore the gravitational force on that performer, measured in newtons. Frictional force arises at the contact between shoe and surface and opposes sliding; it can be static, gripping the foot to allow push-off, or kinetic, opposing sliding. Air resistance is a drag force from collisions with air particles, acting opposite to motion and increasing with speed and frontal area. Internal-muscular force is generated by muscle contraction within the performer and is transmitted through tendons and bones to the ground, producing propulsion.

What earns the marks

• Gravity is the attractive force between masses and produces a gravitational field strength of about 9.81 N kg⁻¹ near the Earth's surface.
• Weight is the gravitational force acting on a performer, calculated as mass multiplied by gravitational field strength, and acts vertically downwards through the centre of mass.
• Frictional force acts at the contact surface between shoe and ground, opposing the relative sliding tendency; static friction enables grip and push-off.
• Air resistance is a drag force opposing motion through the air, increasing with speed and frontal area.
• Internal-muscular force is generated by muscle contraction inside the performer and drives propulsion when transmitted to the ground.
• Weight and gravity are related but not identical terms: gravity is the interaction, weight is the force experienced by a particular mass.

Common mistakes and how to put them right

• Using weight and gravity as exact synonyms.

  Put it right: Gravity is the interaction between masses, while weight is the force that interaction produces on a specific mass.

• Giving weight in kilograms.

  Put it right: Weight is a force measured in newtons, so a 70 kg performer has a weight of about 687 N, not 70 N.

• Describing friction as always opposing motion.

  Put it right: Static friction at the shoe can act forwards, in the direction of travel, to propel the performer.

• Treating air resistance as constant.

  Put it right: It rises with speed and frontal area, so it matters far more in cycling or sprinting than in a slow walk.

Exam tips

• Match each option to the force definition before choosing, because distractors often swap the origin or direction of two forces.
• Check units in any numerical option: weight must be in newtons, mass in kilograms, and gravitational field strength in N kg⁻¹.
• For friction questions, decide first whether the contact is sliding or gripping, since that determines whether friction opposes motion or enables it.