AQA Physics topic 5

106 Pages·2017·11.03 MB·English

by  W Richards

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Forces
AQA Physics topic 5
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5.1 – Forces and their Interactions
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Vector vs. scalar
Scalar quantities have size (“magnitude”) only and no direction.
Vector quantities have both size and direction.
Scalar or vector???
Scalar Vector
8. Power
2. Distance
1. Mass
6. Energy
7. Time
3. Acceleration
4. Speed
9. Force
5. Velocity
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Vectors
Here’s a man walking 10km north and then 10km east. Notice
that we can replace his two movements with a “displacement
vector”. Note the length and direction of this vector:
10km The same can be applied to
velocity vectors:
10km 100ms-1
14.1km
5ms-1
100.1ms-1
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Introduction to Forces
A force is a “push” or a “pull”. What forces do these pictures
represent?
Which of these forces
would you describe as
“contact forces” and
which ones are “non-
contact forces”?
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Contact or non-contact forces?
Contact Non-contact
1. Friction
2. Air resistance
3. Gravitational
forces
4. Tension
5. Electrostatic
forces
6. Reaction
7. Magnetic
forces
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Weight vs. Mass
Earth’s Gravitational Field Strength is 10N/kg. In other
words, a 1kg mass is pulled downwards by a force of 10N.
Weight = Mass x Gravitational Field Strength W
(in N) (in kg) (in N/kg)
You need to learn this equation!!
M g
1) What is the weight on Earth of a book with mass 2kg? 20N
2) What is the weight on Earth of an apple with mass 100g? 1N
3) Charles weighs 700N on the Earth. What is his mass? 70kg
4) On the moon the gravitational field strength is 1.6N/kg.
112N
What will Charles weigh if he stands on the moon?
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More information about Weight
1) How much does 1kg weigh on the Earth?
2) How much does 2kg weigh?
3) How much does 3kg weigh?
4) What are you noticing about your answers?
Whatever mass goes up by, weight goes up by the same ratio.
For example, if you double mass you double weight. This is
called “proportionality”:
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Centre of Mass
The centre of mass is defined as “the point at which an
object’s mass is centred on”.

Where is the centre of mass for these objects?
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Resultant Force
A “resultant force” is a single force that can replace all of the
other forces acting on something. Calculate and draw the
resultant force of the following:
700N 600N
500N 100N
50N
700N 700N 800N 800N
200N 100N