Forces
1. Introduction to Forces
A force is a push or pull acting on an object due to interaction with another object. Forces can cause an object to:
Start or stop moving
Change direction
Change shape
Forces are vector quantities, meaning they have both magnitude and direction. They are measured in newtons (N) using a newtonmeter.
There are two main types of forces:
Contact forces (e.g. friction, air resistance, tension, normal contact force)
Non-contact forces (e.g. gravity, magnetic force, electrostatic force)
2. Weight, Mass, and Gravity
Mass is the amount of matter in an object and is measured in kilograms (kg).
Weight is the force acting on an object due to gravity.
W = m x g
On Earth, g = 9.8 N/kg
Weight acts vertically downwards and is measured using a spring balance
Mass stays the same everywhere, but weight changes depending on gravitational field strength.
3. Resultant Forces
When more than one force acts on an object, we find the resultant force. This is a single force that has the same effect as all the forces acting together.
If forces are balanced (net force = 0), the object stays at rest or moves at constant velocity.
If forces are unbalanced, the object will accelerate in the direction of the resultant force.
For perpendicular forces, we can use scale diagrams or Pythagoras’ theorem to calculate the resultant force.
4. Work Done and Energy Transfer
Work is done when a force causes movement:
W = F x d
Work is measured in joules (J)
1 J = 1 N × 1 m
Work done transfers energy. When work is done against friction, energy is transferred as thermal energy.
5. Elasticity and Hooke’s Law
When forces are applied to an object, it can stretch, compress, or bend. Objects that return to their original shape are elastically deformed.
Hooke’s Law states:
F = k x e
Spring constant is measured in N/m
Only applies within the elastic limit
The elastic potential energy stored in a spring is:
E = 0.5ke²
6. Moments, Levers, and Gears
A moment is the turning effect of a force:
M = F x d
Distance is the perpendicular distance from the pivot
Measured in newton-metres (Nm)
Levers and gears are used to increase the turning effect. A longer lever or a larger gear gives a mechanical advantage, allowing smaller forces to move larger loads.
7. Pressure in Fluids
Pressure in a fluid acts equally in all directions:
On a surface:
P = F/A
In a liquid:
P = H x D x g
Pressure increases with depth and density.
Upthrust is the upward force exerted by a fluid. If upthrust equals the object’s weight, it will float.
8. Atmospheric Pressure
Air molecules create pressure by colliding with surfaces. Atmospheric pressure decreases with altitude because:
There are fewer air molecules at higher altitudes
The weight of the air above is less
9. Motion and Speed
Speed is a scalar quantity, while velocity is a vector.
Speed = Distance/Time
Acceleration = Change in velocity/Time
Typical speeds: walking (1.5 m/s), running (3 m/s), cycling (6 m/s)
Distance–time and velocity–time graphs are useful for describing motion:
In a distance–time graph, the gradient = speed
In a velocity–time graph, the gradient = acceleration and the area under the graph = distance travelled
10. Newton’s Laws of Motion
1st Law: An object stays at rest or in uniform motion unless acted on by a resultant force (inertia).
2nd Law: The acceleration of an object depends on the resultant force and mass:
F=m×a
3rd Law: For every action, there is an equal and opposite reaction.
11. Stopping Distances
Stopping distance = Thinking distance + Braking distance
Thinking distance increases with speed and distractions (e.g., phones, alcohol)
Braking distance increases with speed, poor brakes, wet/icy roads, worn tyres
Kinetic energy of the vehicle must be transferred to the brakes:
Kinetic energy=0.5mv²
Greater speed = much greater braking distance due to the squared relationship.
12. Momentum
Momentum is the product of mass and velocity:
p=m×v
Momentum is a vector quantity
In a closed system, momentum is conserved
In collisions:
Total momentum before=Total momentum after
This principle is key in understanding car safety features (e.g., seatbelts, crumple zones, airbags), which increase the time taken to change momentum, reducing the force on occupants.