# IB DP Physics: SL复习笔记2.2.2 Newton’s First Law

### Newton’s First Law

• Newton's First Law states:

A body will remain at rest or move with constant velocity unless acted on by a resultant force

• If the net force acting on an object is zero it is said to be in translational equilibrium
• If the forces on a body are balanced (the resultant force is 0), the body must be either:
• At rest
• Moving at a constant velocity
• Since force is a vector, it is easier to split the forces into horizontal and vertical forces
• If the forces are balanced:
• The forces to the left = the forces to the right
• The forces up = the forces down
• The resultant force is the single force obtained by combining all the forces on the body

#### Worked Example

If there are no external forces acting on the car, other than friction, and it is moving at a constant velocity, what is the value of the frictional force F?