2.14–2.19: Newton's laws, force, mass and weight

State Newton’s first law of motion.

An object remains at rest or moves at constant velocity unless acted on by a non-zero resultant force.

What happens when the resultant force on an object is zero?

The object remains at rest or continues at constant velocity.

What happens when the resultant force on an object is non-zero?

The object’s velocity changes: its speed and/or direction changes.

What is the equation for Newton’s second law?

Resultant force = mass × acceleration.

Write Newton’s second law using symbols.

F = ma.

What is the unit of force?

Newton (N).

What is the unit of mass?

Kilogram (kg).

What is the unit of acceleration?

Metres per second squared (m/s²).

A 5 kg object accelerates at 3 m/s². Calculate the resultant force.

F = ma = 5 × 3 = 15 N.

A resultant force of 20 N acts on a 4 kg object. Calculate its acceleration.

a = F ÷ m = 20 ÷ 4 = 5 m/s².

What is inertial mass?

A measure of how difficult it is to change an object’s velocity.

How is inertial mass defined using force and acceleration?

Mass = resultant force ÷ acceleration; m = F ÷ a.

What is weight?

The gravitational force acting on an object.

What is the equation for weight?

Weight = mass × gravitational field strength; W = mg.

What is the unit of weight?

Newton (N).

What is the unit of gravitational field strength?

Newton per kilogram (N/kg).

What is the approximate gravitational field strength on Earth?

10 N/kg.

A 6 kg object is on Earth. Calculate its weight using g = 10 N/kg.

W = mg = 6 × 10 = 60 N.

What is the difference between mass and weight?

Mass is the amount of matter and is measured in kg; weight is a gravitational force measured in N.

How does weight change when gravitational field strength increases?

Weight increases in direct proportion to gravitational field strength, if mass stays constant.

Does mass change when an object moves to the Moon?

No. Mass stays the same.

Does weight change when an object moves to the Moon?

Yes. Weight decreases because the Moon’s gravitational field strength is lower.

How is weight measured?

Using a calibrated newton meter or spring balance.

How can you investigate the relationship between force, mass and acceleration using a trolley?

Apply a known resultant force to a trolley, measure its acceleration, and vary the mass while keeping the resultant force constant.

What happens to acceleration when mass increases and resultant force stays constant?

Acceleration decreases; it is inversely proportional to mass.

What happens to acceleration when resultant force increases and mass stays constant?

Acceleration increases in direct proportion to the resultant force.