Types of Forces
What types of forces may we come across?
When we are dealing with problems involving cars or boats moving around,
all the forces acting along the line of motion can be reduced to two or three
forces.
• The driving force – the force pushing the item forwards.
• The resistance to motion – the force pulling back on the item,
eg. air resistance, friction.
• A braking force – a possible additional force acting backwards on the item.
Resistance due to air or water always act in the opposite direction to the
velocity of the object, and are usually greater for faster moving objects.
Types of Forces
What types of forces may we come across?
We looked previously at an example of a
parachute carrying a crate of supplies.
The lines joining the crate to the parachute
were in tension.
Rigid objects can also be in tension.
Consider when you grip a pencil at either end
with your fingers, and then pull on each end, or
push your hands together.
When you pull apart, the pencil is in tension.
Types of Forces
When you push the pencil
inwards, the forces on your
hands are outwards.
The pencil is in compression.
The outward force on each
hand is then called a thrust
force.
Types of Forces
When more than one force acts on an object, the resultant
force is the single force that is equivalent to all the forces
acting on the object.
If forces F1, F2,… Fn act on an object then the resultant force is
R = F 1 + F 2 + … + Fn
If the resultant force on an object is zero the object is said to
be in equilibrium. There is therefore no change in motion.
Accelerating – speed increasing.
Not in equilibrium
No acceleration. In equilibrium
No acceleration. In equilibrium
Accelerating – speed increasing.
Not in equilibrium
No acceleration. In equilibrium
No acceleration. In equilibrium
No acceleration. In equilibrium
Accelerating – direction changing
Not in equilibrium
Calculating a resultant force.
The forces (2i + 10j) N, (6i – 3j) N and (10i + 5j) N act on an object.
Find the magnitude and direction of the resultant force.
Give the direction as the angle between the resultant force and the unit vector i.
Solution:
Resultant = (2 + 6 + 10)i + (10 – 3 + 5)j
= 18i +12j
12
θ
Magnitude = √(12² + 18²)
= 21.6 N
Direction = tan-1 (12/18)
θ = 33.7˚
18