| 1.2: FORCES, MOVEMENT, SHAPE AND MOMENTUM |
|---|
Forces can change the speed, shape, or direction of motion of an object.
Types of Force
- Gravitational
- Electrostatic
- Magnetic
- Friction (including air resistance)
- Contact forces
Different types of force arise from different interactions between bodies.
Vectors and Scalars
Vector quantities have both magnitude and direction, while scalar quantities have magnitude only.
| Vectors | Scalars |
|---|---|
| Force | Speed |
| Velocity | Mass |
| Acceleration | Distance |
| Momentum | Energy |
| Displacement | Time |
Newton's Second Law
F = ma
Newton's second law states that the resultant force on a body equals its mass multiplied by its acceleration, where force F is in newtons (N), mass m in kilograms (kg) and acceleration a in metres per second squared (m/s²).
Weight
W = mg
- Weight is the gravitational force acting on a body and varies with the gravitational field strength g (about 10 N/kg on Earth)
- Mass is the amount of matter in a body and stays constant wherever the body is
Momentum
p = mv
- Momentum is a vector quantity, found from mass multiplied by velocity
- Momentum is conserved in any closed system, so the total momentum before a collision equals the total momentum after
F = Δp/t
The force on an object equals the rate of change of its momentum, which explains why safety features that extend the deceleration time reduce the force on passengers.
Stopping Distance
- Stopping distance is the sum of the thinking distance and the braking distance
- Both thinking distance and braking distance increase with speed
- Braking distance increases with the square of speed because kinetic energy is proportional to velocity squared
Terminal Velocity
Falling objects accelerate until the drag (air resistance) equals the weight, at which point the resultant force is zero and the object moves at a constant terminal velocity.
Hooke's Law
Hooke's law describes the initial linear region of a force–extension graph, where the extension of a spring is directly proportional to the force applied.
Beyond the limit of proportionality the graph is no longer a straight line, so extension is no longer proportional to force.
Elastic Behaviour
Key Definition Elastic behaviour means a material returns to its original shape after the deforming force is removed.
Newton's Third Law
Newton's third law states that when two bodies interact, they exert forces on each other that are equal in size and opposite in direction, and these forces act on different bodies.
Moments
The moment of a force is a measure of its turning effect about a pivot, and equals the force multiplied by the perpendicular distance from the pivot to the line of action of the force.
Principle of Moments
The principle of moments states that, for a body in equilibrium, the sum of the clockwise moments about any pivot equals the sum of the anticlockwise moments.
When a heavy object is placed on a beam supported at both ends, the nearer support carries the greater share of the load.