Understanding Speed, Velocity, and Acceleration
Speed is simply how fast you are going, with no regard to direction. Velocity, however, must also have the direction specified. Both are measured in m/s, km/h, or mph.
- Average speed = Distance moved ÷ Time taken (s = v × t)
- Acceleration = Change in velocity ÷ Time taken ((v-u) = a × t, v² = u² + 2as)
Distance-Time Graphs
- Flat sections indicate the object is stopped.
- A steeper graph means the object is moving faster.
- Curves represent acceleration.
- A curve getting steeper means speeding up.
- A leveling-off curve means slowing down.
- Speed = gradient = vertical ÷ horizontal
Velocity-Time Graphs
- Gradient = acceleration
- Flat sections represent steady speed.
- The steeper the graph, the greater the acceleration or deceleration.
- Uphill (/) represents acceleration.
- Downhill (\) represents deceleration.
- A curve means changing acceleration.
Gravity and Its Effects
Gravity attracts all masses. It has three important effects:
- On the surface of a planet, it makes all things accelerate towards the ground.
- It gives everything weight.
- It keeps planets, moons, and satellites in their orbits.
Mass vs. Weight
- Mass: The amount of matter in an object.
- Weight: Caused by the pull of gravity.
An object has the same mass on Earth or the Moon, but its weight will differ. A 1 kg mass will weigh less on the Moon than on Earth. Weight is measured in newtons (N) using a spring balance. Mass is not a force; it’s measured in kilograms (kg) with a mass balance.
Weight = mass × gravitational field strength (W = m × g)
Forces and Motion
Frictional force offers resistance to the applied force, opposing its motion. It always acts in the direction opposite to that of the object in motion. If a force is applied to the left, friction acts to the right.
An unbalanced force acting on an object changes its speed and/or direction of motion. If the forces are balanced, there is no net force, the object will not accelerate, and the velocity will remain constant.
Newton’s Laws of Motion
- First Law: An object will remain at rest or continue moving at a constant velocity unless acted upon by a net force.
- Second Law: If there is an unbalanced force, the object will accelerate in that direction. Force = mass × acceleration (F = m × a)
- Third Law: If object A exerts a force on object B, then object B exerts an equal and opposite force on object A.
To find the resultant force, subtract the magnitude of the smaller force from the magnitude of the larger force.
- A scalar quantity has magnitude but no particular direction.
- A vector quantity has magnitude and acts in a particular direction.
Investigating the relationship between force and extension for a spring
Method:
- Set up the apparatus as in the diagram.
- Add a 10 g mass to the holder and record the spring length.
- Add another 10 g mass and record the new spring length.
- Calculate the extension (the difference) by subtracting the previous spring length from the new length.
- Repeat by adding 10 g masses until 100 g is reached.
Stopping Distance
Stopping Distance = Thinking Distance + Braking Distance
Factors Affecting Thinking Distance
- Speed
- Reaction time (influenced by tiredness, drugs, alcohol, old age, inexperience)
Factors Affecting Braking Distance
- Speed
- Mass of the vehicle
- Brake condition
- Grip (influenced by road surface, weather conditions, tires)
Elasticity
Elasticity is the ability of a body to resist any permanent change to its shape when stress is applied. When stress application ceases, the body regains its original shape and size.