FORCES AND MOMENTUM - Chapter 1

Forces and Newton’s 1st Law

a force

A force is a push or pull that acts on an object due to interaction with another object.

SI unit of force

Newton (N)

resultant force

Sum of all forces acting on an object

when do we subtract forces

We subtract forces when they are acting in opposite directions.

4 non-contact forces.

• Gravitational force

• Magnetic force

• Electrostatic force

• Weight

5 contact forces.

• Friction

• Air resistance (drag)

• Tension

• Normal contact force

• Upthrust

SI unit of weight

Newton (N) (same as force, because weight is a force)

equation for weight

1. Weight = Mass × Gravitational field strength
   W=m×g

Newton’s First Law

An object’s velocity only changes when there is a resultant force

What do resultant forces cause?

Resultant forces cause a change in the motion of an object

Can an object with 0N resultant force be moving?

Yes, an object with 0 N resultant force can be moving at a constant velocity

Describe the motion of a rocket with a resultant force of 10,000 N upwards.

The rocket will accelerate upwards because the large resultant force causes an upward acceleration.

Describe the motion of a car with a thrust of 500 N and 500 N of friction on it.

The car will move at a constant speed because the thrust and friction forces balance each other, resulting in zero resultant force.

When do objects slow down?

Objects slow down when there is a resultant force acting opposite to their direction of motion, such as friction or air resistance.