Download UNIT 5 MOTION II. ACCELERATION AND FORCES

IES MACARENA
PHYSICS AND CHEMISTRY 2º ESO
UNIT 5
MOTION II. ACCELERATION AND FORCES
1. ACCELERATION
Vocabulary of this section: speed, velocity and acceleration
Velocity is a vector with a value or speed and a direction. So velocity can change in two
ways:
-
Changing the direction. For example, when you turn the handlebar of your bicycle
you are changing the velocity’s direction.
-
Changing the value (speed). For example, if speed goes from 20 km/h to 30 km/h.
Both changes are defined as acceleration but we are going to study only the change of
speed.
Acceleration can be calculated from this
equation:
a=
𝑣𝑓 − 𝑣𝑖
𝑡
The equation above tells us how fast the
speed changes
vi is the initial speed
vf is the final speed
t is the time speed takes to change
Acceleration is positive when the moving body speeds up (speed increases).
Acceleration is negative when the moving body slows down (speed decreases). Sometimes
a negative acceleration is called deceleration.
The unit of acceleration in the International System of Units is m/s2.
o
o
QUESTION: What is the physical meaning of an acceleration of 2 m/s2?
ANSWER: It means that speed increases 2 m/s every 1 s as shown below:
Speed
Time
0 m/s
0s
2 m/s
1s
4 m/s
2s
6 m/s
3s
8 m/s
4s
2. UNIFORMLY ACCELERATED RECTILINEAR MOTION (UARM).
Vocabulary of this section: uniformly, free fall.
If a body moves with a constant acceleration in a straight line, its motion is called
Uniformly Accelerated Rectilinear Motion (UARM). Free fall is an example of UARM.
What does “constant acceleration” mean? It means that speed changes uniformly, i. e. speed
changes the same every second as in example above.
SPEED-TIME GRAPHS: In a speed-time graph, the speed of the object is plotted in the
vertical axis and time is plotted in the horizontal axis.
Which kind of motion is it described in the following graphs, URM or UARM? Why?
A
B
In graph A the speed is constant so it is a URM. In graph B speed increases uniformly so it is
a UARM.
Activity 1. An object is at rest and starts a motion with the following speeds:
Speed
Time
0 m/s
0s
5 m/s
1s
10 m/s
2s
15 m/s
3s
20 m/s
4s
Plot these data in a speed-time graph. How much does speed increase every
second? Which is the value of the acceleration?
3. HOW IS ACCELERATION PRODUCED?
Vocabulary of this section: law of motion, force, weight
The First Newton’s Law of Motion states:
-
If a body is at rest and no force is exerted on it, then the object remains at rest.
If a body is at motion and no force is exerted on it, the object goes on moving at the
same speed.
So FORCES are what can produce acceleration. This is a very important idea: no force, no
acceleration. Forces can produce deformations too. For example, when a spring is stretched.
The unit of force in the SI is Newton (N).
Activity 2. Find out the mistake of the next sentence: “If a ball is rolling on the floor it will
never stop because nobody is exerting any force”.
Activity 3. Imagine that you are floating in outer space far from any planet or star. If you
throw a bottle with a message inside, will it ever stop?
The Second Newton’s Law of Motion states:
𝐹
If a force is exerted on a body, the acceleration can be calculated by this formula: a = 𝑚
The greater the force, the greater the acceleration.
The greater the mass, the smaller the acceleration.
-
Activity 4. A force of 80 N is exerted on a 2 kg ball. Find out its acceleration.
-
Activity 5. Find out the acceleration that a 2-tone space ship reaches when its engine
exerts a 20,000 N force.
4. WEIGHT OR PULL OF GRAVITY
Acceleration of gravity, as any acceleration, is produced by a force: the pull of gravity, or
just weight. In order to calculate the weight of a body you have to multiply its mass by the
acceleration of gravity (g = 9.8 m/s2):
Weight = m·g
Activity 6: Find out the weight of a 500g-body. (You have to change grams into
kilograms in order to obtain the results in N)
Activity 7: Are the same mass and weight?
Activity 8: Is a body’s mass the same in any planet? Why?
Activity 9: Is a body’s weight the same in any planet? Why?