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REVIEW:• Force• Motion
• Effects of forces on the object’s motion• Forces on objects at rest
• Normal force• Friction
• Gravitational force• Scalar quantity• Vector quantity
True or False
If an object is at rest, no forces are acting upon it
Answer: False
There are many forces acting on it (gravity, friction, air pressure, etc)
but they are all BALANCED
True or False
When there is force, there is motion.
Answer: False
Force may be present but it does not necessarily result to motion. If forces are
balanced, no motion will be produced.
True or False
In the absence of force, object will continue to move constantly.
Answer: True.
Because force will slow down or stop motion.
TODAY’S LESSON
Learning ObjectiveAim high - Be able to represent forces using a force diagram
Aim higher - Describe the effect that balanced and unbalanced forces have on an object
Aim even higher - Be able to calculate the size of the resultant force
Key Words:Balanced force
Unbalanced forceResultant or Net Force
What is a force?
• A force is a push or pull that causes an object to move, stop, or change direction
• In physics a force is anything that makes an object accelerate
Force
• Forces come in pairs• Forces have a magnitude and a direction
5N, north (up) Magnitude: 5NDirection: north (up)
Balanced and Unbalanced Forces
• Forces occur in pairs and they can be either balanced or unbalanced
Balanced Forces
• Balanced forces do not cause change in motion
• They are equal in size and opposite in direction
Balanced Forces
We call this NORMAL FORCE. The book is said to be at equilibrium since there are no unbalanced forces acting upon the book and it maintains its state of motion (it still sits on the table).
Lets say you are standing on the ground. Since the floor pushes upward on you and gravity is pulling down on you, you are at equilibrium.
Unbalanced Forces• An unbalanced force always causes a change in
motion
• When unbalanced forces act in opposite directions you can find the net force– Net force
• Magnitude– The difference between the two forces
• Direction– Direction of the largest force
Unbalanced Forces
3 N, right – 6 N, left = 3N, left
Unbalanced Forces
4 N, left – 10 N, right = 6N, right
Unbalanced Forces
5 N, right + 10 N, right = 15N, right
Balanced Forces
Forces are balanced if they are the same strength but in opposing directions:
40N 40N
Balanced forces
Balanced forces
Unbalanced Forces
When forces working in opposite directions are unequal they are unbalanced:
Which direction will the box move?
100N 20N
Unbalanced forces
Normal
The forces are balanced/unbalancedThe man is moving/not moving
The forces are balanced/unbalancedThe woman is moving/not moving
Thrust
The forces are balanced/unbalanced The car is moving/not moving
Application Questions
1. A diver wears weights and dives into the water, which force is bigger, his weight or
upthrust? Draw a diagram to show the forces acting on him
2. The diver then releases the weight. Now, which force is bigger, his weight or
upthrust? Draw a diagram to show the forces acting on him.
Resultant Force
If two forces are unbalanced there is a resultant force.
It could replace all the others to give the same effect.
The difference between two forces is the resultant force.
What is the resultant force?
Hint: work out the difference between the two forces.
The resultant force is 100N – 20N = 80N
100N 20N
20 N
1.A. What is the resultant force on the satellite?
B. To which direction will the satellite move?
Resultant forces – question 1
5 N 5 N
5 N
2. a. What is the horizontal resultant force on the bird?b. What is the vertical resultant force?c. To which direction will the bird move?
5 N 5 N
Resultant forces – question 2
10 N
13 N
3. a. What is the horizontal resultant force on the
yacht?b. What is the vertical resultant force?c. To which direction will the yacht move?
10 N
10 N
Resultant forces – question 3
20 N
20 N
1. What is the resultant force on the satellite?Resultant force = 20 N – 10 N
= 10 N down
The satellite will accelerate downwards.
Resultant forces – question 1
5 N 5 N
5 N
2. What is the resultant force on the bird?
5 N 5 N
The forces acting in each direction horizontally are equal in size, so there is no resultant force in this direction.
Resultant force = 5 N – 5 N = 0 N
Resultant forces – question 2
The vertical forces are not balanced, the bird will accelerate in a downwards direction.
Resultant force = 5 N – 0 N = 5 N down
The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction. The horizontal forces are not balanced, so the yacht will accelerate to the right.
10 N
13 N
3. What is the resultant force on the yacht?
10 N
10 N
Resultant force = (20 N +10 N) – 13 N
= 17 N right
Resultant forces – question 3
20 N
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