Time, Distance & Speed
The average speed of a moving object is the distance it travels divided by the time it takes. At CE, this will usually be calculated in metres per second (m/s). The equation to use is…
speed = distance ÷ time (or s=d/t)
You may also need to calculate distance or time, given the other two measurements.
time = distance ÷ speed or (t=d/s)
distance = speed × time or (d=s×t)
Sometimes the data can be shown as a graph with time on the X axis and distance on the Y axis.
- The steeper the slope on the graph, the faster the object is travelling.
- A curved slope indicates that the object is accelerating or decelerating
- A straight slope means that the object is moving at a constant speed
- A flat line represents a stationary object (as time changes, the distance stays the same).
The graph above represents the journey of a toy tank over a 50 metre course. Think about the following…
(a) For how long was the tank accelerating?
(b) What was its average speed between 8s and 28s?
(c) Describe the motion of the tank between 28s and 40s.
(d) Over which 10s period was the tank travelling the fastest?
Weight v Mass
The weakest force in nature is gravity, which is the force of attraction between all objects. The greater the mass of the object, the greater is the force of attraction. It is gravity that holds us down on Earth, keeps the Moon in orbit around the Earth and the Earth in orbit about the Sun.
Mass (in gram or kilogram) is the quantity of matter within an object. The greater the mass, the greater the gravitational force it exerts and the more it is affected by gravitational forces. Weight (in newton) is a measure of the pull of gravity upon an object. Hence, your weight would be different on the Moon than it is on Earth, but your mass would be the same in both circumstances.
Mass is measured with a balance and weight is measured with a newton meter (or newton spring balance)
An apple has a mass of about 100g (0.1kg), which is equivalent to 1N on Earth because the force of gravity on Earth is approximately 10N/kg.
Balanced v Unbalanced Forces
A force is a push or a pull. A force can give an object the energy to cause it to start moving, stop moving, or change its motion.
Forces occur in pairs and can be either balanced or unbalanced. Balanced forces do not cause a change in motion. They are equal in size and opposite in direction.
In the picture above (from the BBC Bitsize revision website), the different forces acting on the car are shown with arrows. The arrow indicates the size and direction of the force. In this case, if the driving force is greater than air resistance and friction combined, the car will accelerate. As it does so, the opposing forces will increase too. When the forces are balanced the car will be travelling at a constant speed. To reduce air resistance, cars are made to be streamlined, allowing for a higher top speed.
The brakes of a car are designed to increase friction, thus slowing the car down. The road conditions will also affect the car’s ability to slow down. Wet or icy roads reduce friction meaning that it will take the car longer to stop. The overall stopping distance of a car is a combination of the driver’s reaction time to hit the brakes, the speed at which the car is going and the condition of the road.
Suggested further reading on forces…
Forces (a little more advanced but nicely explained)
As always, if you want anything explained in greater detail, either email me or leave a comment below.
Questions…
- What are the answers to (a) to (d) above?.
- What can you say about the motion of a car that has all its forces balanced?
- If the frictional forces of a moving car are greater than the force produced by the engine, what can you predict about the motion of the car?
- A parachutist has a mass of 68 kg. When he is floating down to Earth at a steady 8 m/s just before landing, what is the size of the drag force acting upon him?
Unbalanced forces are often called resultant forces. eChalk has a lovely game about resultant forces! (Same username as the usual password)
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