Education Always

Level: JHS, Basic 9  |  Subject: Integrated Science  |  Strand: Forces and Energy  |  Topic: Newton’s Laws of Motion and Momentum

Curriculum reference: B9.4.4.1 Understand Newton’s Laws of Motion and momentum. Indicator B9.4.4.1.1: Apply Newton’s laws of motion to explain everyday physical phenomena. All the exemplars are merged into one set of notes.

1. Newton’s First Law (Inertia)

An object stays at rest, or keeps moving in a straight line at constant speed, unless a resultant (net) force acts on it. Inertia is the tendency of an object to keep its state of motion. A heavier object has more inertia.

Examples:

• Passengers lean forward when a trotro brakes suddenly, because their bodies keep moving forward.
• Passengers lean back when the vehicle starts suddenly.
• A cloth pulled quickly from under a plate leaves the plate in place.
• A rolling ball stops only because of friction.
• Seat belts hold the body when a car stops quickly.

2. Newton’s Second Law

The resultant force on an object equals its mass multiplied by its acceleration.

F = m x a (F in newtons, m in kilograms, a in m/s squared)

• For the same force, a bigger mass gives a smaller acceleration.
• For the same mass, a bigger force gives a bigger acceleration.

Example. A car of mass 1000 kg accelerates at 3 m/s squared. F = 1000 x 3 = 3000 N.

Example. A force of 60 N acts on a 12 kg box. a = F / m = 60 / 12 = 5 m/s squared.

3. Newton’s Third Law

For every action there is an equal and opposite reaction. The two forces act on different objects.

• A swimmer pushes water backward, and the water pushes the swimmer forward.
• A gun recoils backward when the bullet shoots forward.
• A rocket pushes gas downward and the gas pushes the rocket upward.
• Walking: your foot pushes the ground backward and the ground pushes you forward.
• A balloon lets out air and moves the other way.

4. Momentum

Momentum is the quantity of motion an object has. It depends on mass and velocity.

Momentum (p) = mass x velocity = m x v (unit: kg m/s)

Momentum has direction, like velocity. A heavy lorry moving slowly can have more momentum than a fast bicycle.

Example. A 1200 kg car moves at 20 m/s. p = 1200 x 20 = 24 000 kg m/s.

5. Change of Momentum and Force

Force = change in momentum / time taken. A force acting for a longer time, or over a longer distance, gives the same change of momentum with a smaller force. This is why:

• Seat belts, crumple zones and air bags make a crash last longer and reduce the force on people.
• Athletes bend their knees when landing.
• Cricket players move their hands back when catching a ball.

6. Conservation of Momentum

When objects collide, if no outside force acts, the total momentum before the collision equals the total momentum after.

Example (objects stick together). A 3 kg trolley moving at 4 m/s hits a 1 kg trolley at rest and they stick. Momentum before = 3 x 4 = 12 kg m/s. After: (3 + 1) x v = 12, so v = 3 m/s.

Example (recoil). A gun of mass 4 kg fires a 0.02 kg bullet at 200 m/s. Bullet momentum = 0.02 x 200 = 4 kg m/s forward. The gun gets 4 kg m/s backward, so its recoil speed = 4 / 4 = 1 m/s.

Remember: First law: inertia. Second law: F = ma. Third law: action and reaction. Momentum = mass x velocity.

Education Always (education.inspiringalways.com). Original notes written to follow the NaCCA Basic 9 Integrated Science curriculum.