## What is a Force?

- A force is a push or pull
- Acts on an object
- Requires an agent
- Is a vector
- Is a contact force or a long range force

**Newton’s First Law**: Consider an object with no force acting on it. If it is at rest, It will remain at rest; if it is moving, it will continue to move in a straight line at a constant speed.

## What is Power?

Power is the way we measure how quickly energy is being changed. When we look at the power of a moving object, we are really looking at how fast work is happening.

### Example:

A lift motor has to move a fully laden lift 4m between floors in 1.5s.

The lift has a mass of 1850 kg (ignore friction).

**a) Calculate the weight of the fully laden lift.**

W = mg = 1850 x 10 = 18500N

**b) What is the upward force in the cable when the lift is moving at a constant speed? **

At constant speed, forces must be balanced. Upward force = downward force (weight) Upward force = 18500 N

**c) What is the work done by the motor? **

WD = F x d = 18500 x 4 so WD = 74000 J

**d) What is the minimum power of the motor to raise the lift at a steady speed?**

Steady speed means forces are balanced. ! Power = work done time = 7400 1.5

So Power = 49333 W

### Difference between force and power:

Classification |
Power |
Force |

Unit | watt = joules/second | newton |

Definition | Power is the rate at which work is done, or energy is transmitted. | A push or a pull resulting from an interaction between objects. |

Symbol | P | F |

Derivations from other quantities | P = w/t (power = work divided by time) | F = m a (force = mass multiplied by acceleration) |

Everyday Example | Horsepower (1 horsepower = 750 Watts). | Gravity, friction, magnetism. |

Relation to “Work” | Rate at which work is performed. | Force applied over a distance creates work. |

Named After | James Watt | Isaac Newton |