Name ANSWERS
Forces Workbook 1
Year 11 Science
2014
Ruawai College
Forces:
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1.
can be a push or a pull
can cause a change in motion by making objects speed up, slow down or change direction
can make object change shape
can make objects turn
usually act in pairs
have size and direction
are measured in Newtons, N
are shown by arrows
What do forces do?
make objects speed up, slow down, change direction, change shape, turn
2.
What do forces have?
size and directio
3.
What is the unit of forces?
Newton, N
4.
How are forces represented?
by arrows
5.
Draw a diagram showing a force acting towards the left?

6. In which direction is the 2 N force acting?
to the right
7. In which direction is the 4 N force acting?
to the right
8. What is the overall force called when two
forces are acting in the same direction?
resultant force
9. How do you calculate the resultant force?
add together
10. What is the resultant force when a 2N force
and a 4 N force are acting in the same direction?
6N
When forces acting on an object are balanced,
the object is stationary or moving at a constant
speed.
11.
When forces acting on an object are unbalanced,
the object accelerates, decelerates or changes
direction.
When would forces be balanced?
equal size and acting in opposite directions
12.
What happens to an object when balanced forces act on it?
is stationary or moves at a constant speed
13.
When would forces be unbalanced?
of different sizes and acting in opposite directions
14.
What happens to an object when unbalanced forces act on it?
accelerates, decelerates or changes direction
Force = mass x acceleration
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F = ma
(Fnet is the combined force or net force)
Force is measured in Newtons, N
mass is measured in kilograms, kg
acceleration is measured in metres per second squared, ms-2
15.
What is the abbreviation for force?
F
16.
What is the unit of force?
Newton, N
17.
What is the abbreviation for mass?
m
18.
What is the unit of mass?
kilogram, kg
19.
What is the abbreviation for acceleration?
a
20.
What is the unit of acceleration?
ms-2
Sometimes you may see force written as Fnet this is read as net force and means the same as F.
21.
F= m x a
22.
m= F ÷a
23.
a=F ÷m
Example:
A toy car of mass 1.5 kg is pulled by a string and accelerates at 1.5 ms-2 . Calculate the size of the
force that accelerates the car.
F
= ma
= 1.5 x 1.5
= 2.25 N
24.
What does F stand for?
force
25.
What does m stand for?
mass
26.
What does a stand for?
acceleration
27.
What is the mass of the car?
1.5 kg
28.
What is the acceleration of the car?
1.5 ms-2
29.
What formula do you use to find the force?
F=mxa
30.
What is the size of the force needed to accelerate the car?
2.25 N
Exercises:
show all working and don’t forget the units of the answer
31.
A pile of books, of mass 1.5 kg, is resting on a bench.
When the books are pushed they
-2
accelerate at 2 ms . Calculate the force needed to move the books.
F = m x a = 1.5 x 2 = 3.0 N
32.
When a single book of mass 500g is pushed, it accelerates at 3 ms-2. Calculate the force
acting on the book.
F = m x a = 0.5 x 3 = 1.5 N
33.
Joe pulls his bag along the floor, and the force on the bag is 12 N. The mass of the bag is 4
kg. Calculate the acceleration of the bag.
a = F ÷ m = 12 ÷ 4 = 3 ms-2
34.
Jill pulls her bag along the floor, and the force on the bag is 12 N. The bag accelerates at 4
-2
ms . Calculate the mass of the bag.
m = F ÷ a = 12 ÷ 4 = 3kg
35.
In question 33, Joe pulled his 4 kg bag along the floor with a force of 12 N, the acceleration
was 3 ms-2. When Peter pulled Joe’s bag, the net force on it was 24 N. Calculate the Peter’s
acceleration of the bag.
a = F ÷ m = 24 ÷ 4 = 6 ms-2
36.
Finish this sentence: Twice the force produced twice the acceleration.
37.
In question 33, Joe pulled his 4 kg bag along the floor with a force of 12 N, the acceleration
was 3 ms-2. When Joe emptied his bag of half its contents and pulled it along the floor with the
same force, what was its acceleration?
a = F ÷ m = 24 ÷ 2 = 12 ms-2
38.
Finish this sentence: When the same force is used to accelerate two different masses, the
smaller mass has the greater acceleration.
On objects of the same mass, acceleration is in direct proportion to the force applied. Doubling the
force doubles the acceleration. a and F are directly proportional.
Where the same force is applied, the smaller mass has the greater acceleration. Doubling the mass
halves the acceleration. a and m are inversely proportional.
39.
a.
The speed time graph above represents the motion of a student’s bike on a journey. The
mass of the student and bike is 100 kg. Calculate the force acting on the student and bike in the first
60 seconds of the journey. (hint – you need to do two calculations)
a = rise ÷ run = 12 ÷ 60 = 0.2 ms-2
F = m x a = 100 x 0.2 = 20 N
b.
What can you say about the forces acting on the student and bike in the part of the journey
from 60 to 120 seconds?
bike is moving at a constant speed, so forces must be balanced – equal but opposite
40.
The starter motor on Monica’s car would not work. To start the car, Monica got three of her
friends to push it. Each of her friends pushed the car with a force of 450 N. Friction can be ignored.
If the car, with Monica inside, had a mass of 900 kg, calculate the car’s acceleration.
F = 3 x 450 = 1 350 N
a = F ÷ m = 1 350 ÷ 900 = 1.5 ms-2
Mass and Weight - ARE NOT THE SAME
Mass:
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the amount of matter an object possesses
measured in grams (g) or kilograms (kg)
Weight:
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41.
force that an object pushes down whatever it is resting on
measured in Newtons (N)
mass x force of gravity (g)
mass x 10
What is the unit of mass?
Kilogram, kg
42.
What is the unit of weight?
Newtons, N
43.
How do you calculate weight?
Weight = mass x 10
44.
What is the difference between mass and weight?
Mass is amount of matter in an object, weight is force of the object – weight = mass x 10
45. What is the mass of the astronaut on Earth?
120 kg
46. What is the weight of the astronaut on
Earth?
120 N
47. How do you calculate the weight of the
astronaut on Earth?
Weight = mass x 10
48.
What is the mass of the astronaut on the Moon?
120 kg
49.
What is the weight of the astronaut on the Moon?
200 N
50.
Why is the mass of the astronaut on Earth and the Moon the same?
Mass is the amount of matter the astronaut has – the same on Earth and the Moon
51.
Why is the weight of the astronaut on Earth and Mars different?
Fw = m x g
Weight = mass x gravity
Fw = m x 10
g = 10 N kg-1
Example:
What is the weight of a person with a mass of 54 kg sitting on a park bench?
Fw
=mxg
= 54 x 10
= 540 N
52.
What is the mass of the person sitting on the park bench?
54 kg
53.
What is the weight of the person sitting on the park bench?
540 N
54.
How was the weight of the person on the park bench calculated?
54 x 10 = 540
55.
Why is the mass and weight of the person different?
mass is amount of matter in an object, weight is force of the object – weight = mass x 10
56.
What is the value of g?
10
57.
What are the units of g?
Nkg-1
Exercises:
58.
Fill in the gaps in the table below:
Mass
a.
Weight
10 kg
b.
50 kg
100 N
500 N
c.
500 g
5N
d.
1.5 kg
15 N
e.
435 kg
4350 N
59.
A Moon rock is brought back to Earth. Explain how its mass and weight on Earth compare
with its mass and weight on the Moon.
Gravity:
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60.
Sun?
force of attraction exerted by large masses eg. the Earth
gravity on Earth is 10 newtons per kilogram – 10 Nkg-1
causes objects to accelerate as they fall
acceleration due to gravity is about 10 ms-2 – 10 metres per second squared
on the Moon is about 1/6 of that on Earth
does not exist in outer space – objects are weightless
Why is the mass of the person the same when they are on Earth, the Moon, Jupiter and the
mass is the amount of matter the person has – the same on Earth, the Moon, Jupiter and the Sun
61.
How is weight calculated?
weight = mass x 10
62.
Why is the weight of the person less on the Moon?
gravity on the Moon is less than on Earth
63.
Why is the weight of the person more on Jupiter and the Sun than it is on Earth?
gravity on the Earth is less than on Jupiter and the Sun
64.
Acceleration due to gravity on a planet in a far distant galaxy is 20 ms-2.
a.
Calculate the weight of a rocket, mass 600 kg, after it has landed on the planet.
Weight = 600 x 20 = 12 000 N
b.
The planet has the same radius as Earth’s. State whether the planet’s mass would be the
same, greater or less, than that of Earth. Explain your answer.
The mass of the planet would be greater than Earth because it has greater gravity
65.
a.
In which direction is the truck moving?
To the right
b.
What word would describe the movement of the truck?
Acceleration
c.
If the mass of the truck was 2 000 kg, what would be the acceleration of the truck?
Resultant force = 100 – 60 = 40 N
a = F ÷ m = 40 ÷ 2000 = 0.02 ms-2
66.
A cyclist was pedalling along a level road at a constant speed:
a. If a cyclist and his bike had a mass of 80 kg, what would his weight force be?
weight = mass x 10 = 80 x 10 = 800 N
b. The forces acting on the bike are balanced. How do you know this?
bike is travelling at a constant speed
c. How much force would need to be applied for the bike to reach an acceleration of 6ms-2?
F = m x a = 80 x 6 = 480 N
67.
Make a glossary of terms and definitions.
a.
Keyword
force
Definition
A push or a pull
b.
Newton
The unit of force
c.
resultant force
The overall force
d.
balanced forces
Forces that are equal and acting in opposite directions
e.
unbalanced forces
Forces that are of different sizes and acting in opposite directions
f.
net force
Overall force
g.
mass
Amount of matter an object has
h.
weight
force object has when pushing down on surface resting on
i.
gravity
Force of attraction exerted by large objects eg. Earth
68.