# A force is needed to change motion

Newton’s first law says that a resultant force is needed to make something start moving, speed up or slow down

If the resultant force on a stationary object is zero, the object will remain stationary. If the resultant force on a moving object is zero, it’ll just carry on moving at the same velocity

So when a train or car or bus or anything else is moving at a constant velocity, the resistive and driving forces on it must all be balanced. The velocity will only change if there’s a non-zero resultant force actin on the object.

• A non-zero resultant force will always produce acceleration in the direction of the force

• This acceleration can take five different forms: starting, stopping, speeding up, slowing down and changing direction

• On a free diagram, the arrows will be unequal

# Acceleration is proportional to the resultant force

• The larger the resultant force acting on an object, the more the object accelerates-the force and the acceleration are directly proportional. You can write this as F=a

• Acceleration is also inversely proportional to the mass of the object-so an object with a larger mass will accelerate less than one with a smaller mass

• There’s a formula that describes Newton’s second law:

F=ma, Resultant force, mass x acceleration

• You can use this to get an idea of the forces involved in everyday transport, large forces are needed to produce large acceleration