CAMBRIDGE -PHYSICS

1. 9.5- Forces and
Movement
Sir.Muhammad Abdul Mageid

2. Learning Outcomes
After this topic you should be able to :
1- Explain what resultant forces can do to an object
2- Describe the motion of objects as they fall through air
3- Investigate the motion of objects as they fall
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3. Acceleration
If the forces are unbalanced Forces acting on objects can :
1- speed the object up
2- slow the object down
3- change the direction of travel
All three of these changes in movement are called acceleration
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4. 1.
Definition of
Acceleration
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The change in the
speed of an object
in unit time or
change in direction
or both

5. ‘’
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6. 2.
Factors
affecting the
Acceleration
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7. Factors affecting the acceleration
▣ The resultant (Net) force :
The greater the resultant force acting on an
object, the greater the acceleration will be
▣ The mass of the object :
The greater the mass of an object, the smaller
the acceleration will be
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8. Acceleration
A change in speed or
direction
This includes
Speeding up or
Slowing down
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9. How to calculate Acceleration
𝒂 =
𝒗𝒇𝒊𝒏𝒂𝒍 − 𝒗𝒊𝒏𝒕𝒊𝒂𝒍
𝒕
𝒂 =
𝑭(𝒂𝒄𝒕𝒊𝒏𝒈 𝒇𝒐𝒓𝒄𝒆)
𝑴(𝒎𝒂𝒔𝒔)
Units of acceleration
𝒎/𝒔𝟐 or 𝑵/𝒌𝒈
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10. Falling and Forces
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11.  At the moment it is dropped ,its weight is the only force
acting on the ball. So there is unbalanced force acting
on the ball, and it accelerates downwards
 As soon as the ball begins to move downwards, a
second force begins to act. This force is air resistance
(drag), caused as the ball pushes through the air
When a ball is dropped from a great height
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12.  The drag produces an upwards force acting on the ball .
The resultant force on the ball is still acting downwards
but is decreased as the drag cancels out some of the
weight. The ball now accelerates less because the
resultant force is getting smaller. The ball still moves
downwards but it is speeding up less and less
 As the ball speeds up, the air resistance becomes larger
while the weight stays the same size. Eventually the air
resistance is the same size as weight of the ball and the
forces are balanced. The balanced forces mean that the
ball stops accelerating so it says at a steady speed
When a ball is dropped from a great height
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13. The terminal velocity
The steady speed of a falling object.
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▣ The skydiver will accelerate
downwards due to the force of gravity
until the air resistance matches his
weight. When the forces reach the
same size the skydiver will have
reached terminal velocity

14. Question 1
Calculate the resultant force acting on each
of the following objects :-
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15. Question 1
Calculate the resultant force acting on each
of the following objects :-
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16. Question 2
Calculate the net force to determine which team will
win the tug of war.:-
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17. Question 3 ▣ Calculate the net force acting
on the box below.
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18. Thanks!
Any questions?
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