Speed
Speed is a measure of how fast an object is moving. The speed of an object can be found using the following formula:-
There are two types of Speed within Physics:-
Average speed - Measured over a long time - It is measured over a whole journey and is found using the distance travelled overall and the time taken.
Instantaneous speed - Measured over a very short time - It is the object's speed at that instant.
Measured over a long time
It is measured over a whole journey and is found using the distance travelled overall and the time taken.
Measured over a very short time
It is the object's Speed at a point in time and is found by measuring the size of an object and the time taken to pass a point.
The 100 m sprint record is 9.58 Seconds, held by Usain Bolt.
What was his average speed across the 100 m event?
S = ?
d = 100 m
t = 9.58 s
S = d / t
S = 100 / 9.58
S = 10.4 m s−1
A cart of length 0.2 m passes through a light gate in 0.05 seconds.
What is the instantaneous speed of the cart at this point?
S = ?
d = 0.2 m
t = 0.05 s
S = d / t
S = 0.2 / 0.05
S = 4 m s−1
Acceleration
When an object changes speed it is accelerating.
Acceleration is defined as :-
"The change in speed of an object over the time of 1 second."
The sign of the acceleration depends on the change in speed :-
1. Object gets faster - positive Acceleration.
2. Object gets slower - negative Acceleration.
The Acceleration of an object doesn't just depend on the change in speed, it also depends on the time it takes to make the change. A short time period for a change in speed will give a large Acceleration.
The Acceleration can be found using the following formula:-
Where:-
v = Final Speed (m s−1)
u = Initial Speed (m s−1)
t = Time (s)
a = Acceleration (m s−2)
Calculating Acceleration Experimentally
By measuring the length of the single mask (in meters) and the time taken to break the light beam (in seconds), the instantaneous speed can be found at the two light bridges. By timing how long the trolley takes to travel between the bridges, the acceleration can then be found using the above formula.
Example 3 -
A car is travelling at 15 m s−1 and then accelerates for 30 s to reach an new speed of 30 m s−1 . What is the acceleration of the car?
a = ?
v = 30 m s−1
u = 15 m s−1
t = 30 s
a = ( v - u ) / t
a = ( 30 - 15 ) / 30
a = 0.5 m s−2
Example 4
A car is travelling at 13 m s−1 when a child runs into the road. If the driver takes 1.3 seconds to stop, what is the acceleration experienced by the driver?
a = ?
v = 0 m s−1
u = 13 m s−1
t = 1.3 s
a = ( v - u ) / t
a = ( 0 - 13 ) / 1.3
a = -10 m s−2
