Forces between objects
Forces are measured in newtons (N)
Objects ALWAYS exert equal and opposite forces on each other.
e.g. If you place a book on a table the weight of the book will act vertically downwards on the table. The table will exert an equal and opposite reaction force upwards on the book.
The motive force on a car is the force that makes it move. This force is due to friction between the ground and the tyre of each drive wheel. Friction acts where the tyre is in contact with the ground.
Force (N) = Mass (kg) x acceleration (m/s^2)
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.
- 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
Force and Acceleration
A resultant force always causes an acceleration.
If there is no acceleration in a particular situation the resultant force must be zero. An object can accelerate by changing its direction even if it is going at a constant speed. So a resultant force is needed to make an object change direction.
Resultant Force (N) = Mass (kg) x Acceleration m/s2
The bigger the mass of an object, the bigger the force needed to give it a particular acceleration.
The velocity of the object decreases (i.e. it decelerates) if the resultant force is opposite in direction. We say its accelerates is negative because it is opposite in direction to its velocity
On the Road
If a vehicle is travelling at a steady speed the resultant force on it is zero because the motive force of its engine is balanced by the resistive forces. 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 it travels during the driver's reaction time (the thinking distance) plus the distance it travels 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, weather conditions and the condition of the car. For example, worn tyres or worn brakes will increase braking distance.
If an object falls freely, the resultant force acting on oit is the force of gravity. We call the force of gravity 'weight', and the acceleration 'the acceleration due to gravity'.
Weight (N) = Mass (kg) x Gravitational Field Strength (N/Kg)
When an object falls through a fluid (e.g. air), the fluid exerts frictional forces (e.g. air resistance) on it, resisting its motion. The faster the object falls, the bigger the frictional force becomes. Eventually it will be equal to the weight of the object. The resultant force is now zero, so the body stops accelerating and moves at a constant velocity called 'terminal velocity'.