Linear MotionI
Level : Conceptual Physics
Objectives
What are units and why are they important?
Measure speed and understand how to apply them in various situations Understand the concept of change per rate
Measure acceleration and understand how to apply them in various situations
Unit
Statement : I saw a tree yesterday and it was about 8 tall.
Question) What is wrong with the statement above?
The standard unit for distance or length is ( foot centimeter inch meter mile ) The standard unit for time is ( hour minute second )
The symbol for kilometer is ( ), for meter is ( ), for centimeter is ( ) The symbol for hour is ( ), for minute is ( ), for second is ( )
To apply measurement in the real world, we need to understand how to convert units 1km = 1000m, 1cm = 0.01m, 1hr = 3600s, 1min = 60s
1. Linear Motion when Objects move at Constant Speed - Motion moving in a straight line is called ( ) motion - If an object is in motion, it must have ( )
- Speed is related with two variables, which is ( ), ( )
Ques1) Two students simultanously run for 20 seconds. If student-A travels 30 meters, and student-B travels 50 meters, then which student was running at a higher speed?
Ques2) Two students both run a distance of 50 meters. If student-A reaches 50 meters in 10seconds, and student-B reaches 50 meters in 15 seconds, which student was running at a higher speed?
Speed
- Speed is defined as
𝑠𝑝𝑒𝑒𝑑 =
- The symbols used often in physics are the following speed = ( ), distance = ( ), time = ( )
v=
: the speed of an object moving at constant speed
- The unit for speed is meter per second = [m/s]
Practice Questions
1. A boy runs a distance of 10 meters in 2 seconds. What is the boy’s speed?
(a) 2m/s (b) 5m/s (c) 9m/s (d) 13m/s
2. A car travels a distance of 400 meters in 20 seconds. What is the speed of the car?
(a) 6m/s (b) 12m/s (c) 16m/s (d) 20m/s
3. If you run at a constant speed of 5m/s for 5 seconds, how many meters did you cover?
(a) 25m (b) 16m (c) 12m (d) 3m
4. A boat is moving at a constant speed of 10 m/s. How long will it take to move 1km(=1000m)?
(a) 120sec (b) 100sec (c) 75sec (d) 50sec
5. An animal is walking at a constant speed of 2m/s. How long will it take to move 20 meters.
(a) 10sec (b) 13sec (c) 18sec (d) 23sec
2. Linear Motion when Objects has Acceleration What is Acceleration?
- If an object changes its speed, whether speeding up or slowing down, then the object is accelerating
- An object in motion may or may not have acceleration.
Ques3) Which of the following cases is the object accelerating? There may be more than one answer
(a) A car is parked in the garage (b) A plane traveling at 500mph (c) After running the 100meter dash, the runner comes to a stop
(d) An apple falling to the ground
(e) A race car starting from rest obtains a speed of 100mph after 6 seconds
Ques4) Which object has higher acceleration?
(a) A runner starting from rest and then obtains a speed of 20mph is 5 seconds (b) An airplane moving at 300mph and then obtains a speed of 302mph in 1 hour
- Acceleration is expressed simply as ‘a’ and is defined as 𝑎 = - The symbol for ‘change in’ is ( )
𝑎 =
Ques5) If an object is initially moving at speed of 2m/s and then increase its speed to 5m/s, then what is ∆v?
- The symbol for initial speed is ( ) and final speed is ( )
=> ∆v = vf – vi
𝑎 =
[m/s2]
Ques6) If a crazy driver is driving his car at a constant speed of 120mph, what is the acceleration of the object?
(a) Since the object is in motion, there will always be acceleration!!
(b) No way~! Though the object is in motion, if there is no change in speed acceleration will be zero
Rearranging the formula for acceleration
vf = ( )
Formula summary
- When an object is moving at a constant speed, speed is expressed as v =d / t
- When an object’s speed is changing, the acceleration is expressed as a = (vf - vi) / t, or expressed as vf = vi + at
v =d / t , a = (vf – vi) / t, vf = vi + at
‘a’=acceleration, ‘vf’=final speed, ‘vi’= initial speed, ‘t’ =time
a = (vf – vi) / t, vf = vi + at
Practice Questions
1. What is the acceleration of a car that starts from rest and speeds up to 20m/s in 10 seconds?
(a) 1m/s2 (b) 2m/s2 (c) 4m/s2 (d) 11m/s2
2. What is the acceleration of a jogger that starts from rest and speeds up to 4m/s in 2 seconds?
(a) 13m/s2 (b) 8m/s2 (c) 4m/s2 (d) 2m/s2
3. A dog running at a speed of 4m/s increases its speed to 10m/s in 2 seconds, what is the acceleration?
(a) 3m/s2 (b) 11m/s2 (c) 4m/s2 (d) 0m/s2
4. A jogger starts from rest and speeds up with an acceleration of 3m/s² in 2 seconds, what is the speed at 2 seconds?
(a) 3m/s (b) 5m/s (c) 6m/s (d) 14m/s
5. A car moving at a constant speed of 30m/s. It then increases its speed at rate of 2m/s² in 5 seconds. What is the speed at 5 seconds?
(a) 1m/s2 (b) 2m/s2 (c) 4m/s2 (d) 11m/s2
a = (vf – vi) / t, vf = vi + at
6. The initial speed of an airplane is 600m/s. It increases its speed to 610m/s in 20 seconds. A jogger is running with a speed of 2m/s, and then increases its speed up to 8m/s in 1 second. Which has higher acceleration? The plane or the jogger?
