Forces between objects and Resultant Force
Forces are measured in Newtons(N). Objects always exert equal and opposite forces on each other. If object A exerts a force on object B, object B exerts an equal and opposite force on object A. These are sometimes called 'action and reaction' forces. Most objects have more than one force acting on them. The 'resultant force' is the single force that would have the same effect on the object as all the original forces acting together.
When the resultant force on an object is zero: if the object is at rest it will stay at rest. If the object is moving it will carry on moving at the same speed and in the same direction.
When the resultant force on an object is not zero there will be an acceleration in the direction of the force. This means that if the object is at rest it will accelerate in the direction of the resultant force. If the object is moving in the same direction as the resultant force it will accelerate in that direction. If the object is moving in the opposite direction to the resultant force it will decelerate.
If an object is accelerating, there must be a resultant force acting on it.
Force and Acceleration
A resultant force always causes an acceleration. Remember that a deceleration is a negative acceleration.
If there is no acceleration in a particular situation the resultant force must be zero.
Acceleration is a change in velocity. An object can accelerate by changing it's direction even if it is going at a constant speed. So a resultant force is needed to make an object change direction. Resultant force, mass and acceleration are related by the equation:
resultant force(N) = mass(kg) x acceleration(m/s²)
The greater the resultant force on an object, the greater its acceleration.
The bigger the mass of an object, the bigger the force needed to give it a particular acceleration.
On the road
If a vehicle is travelling at a steady speed the resultant force on it is zero. The driving forces are equal and opposite to the frictional forces.The faster the speed of a vehicle, the bigger the deceleration needed to bring it to rest in a particular distance. So the bigger the braking force needed.
The stopping distance of a vehicle is the distance is the distance it travels during the driver's reaction time (the thinking distance) plus the distance it travels under under the braking force (the braking distance). The thinking distance is increased if the driver is tired or under the influence of alcohol or drugs.
The braking distance can be increased by poorly maintained roads, bad weather conditions and the condition of the car. For example, worn tyres or worn brakes will increase braking distance.
The 'thinking distance' is the distance travelled by the vehicle in the time it takes the driver to react. The braking distance is the distance the vehicle travels unmder the barking force.
Stopping Distance = Thinking distance + Braking distance
If an object falls freely, the resultant force acting on it is the force of gravity. It will make the object accelerate at about 10m/s² close to the Earth's surface. We call the force of gravity 'weight', and the acceleration 'the acceleration due to gravity'.
The equation resultant force = mass x acceleration
becomes weight(M) = mass (kg) x acceleration due to gravity (m/s²)
If the object is on the earth, not falling, we calculate the weight using:
weight (N) = mass (kg) x gravitational field strength(N/kg)
An object falling in a fluid reaches terminal velocity.