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Chapter 10 Forces
Chapter 10, Section 2
Friction, Gravity, & Elastic Forces
Chapter 10 Forces
Section 2:Friction, Gravity, and
Elastic ForcesWhat factors determine the strength of the friction force between two surfaces?What factors affect the gravitational force between two objects?Why do objects accelerate during free fall?When is matter considered to be elastic?
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Chapter 10 Forces
Friction A force that two surfaces exert on each other when they rub against each other
Smooth surfaces produce less friction than rough surfaces
Without friction, a moving object will not stop until it strikes another object
Table Talk: Share with a partner when friction is good, and when it is bad.
Chapter 10 Forces
Causes of FrictionThe strength of friction depends on 2 things:
Types of surfacesHow hard the surfaces push together
Try It! Rub both of your hands together, first very lightly, then harder. Voila! Friction!
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Chapter 10 Forces
Gravity A force that pulls objects toward each other
Not just on Earth, it’s also the force that keeps all planets orbiting the sun
The Universal Law of Gravitation states that the force of gravity acts between all objects in the universe
Chapter 10 Forces
Gravity Between ObjectsThe force of gravity between objects increases with greater mass and decreases with greater distance.
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Chapter 10 Forces
Mass and Distance The force of gravity between objects increases with greater mass and decreases with greater distance
Table Talk: Now that you know this….why does the sun exert such a large gravitational force on the planets?
Chapter 10 Forces
Mass and WeightThe gravitational force exerted on a person or object at the surface of a planet is known as weight.
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Chapter 10 Forces
Gravity & WeightWeight = the gravitational force exerted on a person or object at the surface of the planet.It varies with the strength of the gravitational force
Weight = mass x acceleration(due to gravity)
Weight is measured in Newtons (N)Mass is measured in kilograms (kg)Acceleration due to gravity = 9.8 m/s2
Try It! How much would a 50 kg person weigh?
50kg X 9.8 m/s2 = 490 N
Table Talk: Would you weigh more or less on the moon? Would your mass change?
Chapter 10 Forces
Gravity & Motion Free Fall = when the only force acting on an object is gravityAcceleration is in the downward
direction
ALL objects free fall accelerate at the same rate regardless of their masses
Table Talk: If this is true, why would the ball hit the ground faster than the feather when they are dropped from the same height at the same time?
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Chapter 10 Forces
Free Fall
The graph shows how the speed of an object in free fall changes with time. Use the graph to answer the following questions.
Chapter 10 Forces
Free Fall
9.8 m/s; 29.4 m/s.
Interpreting Graphs:
What is the speed of the object at 1 second? At 3 seconds?
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Chapter 10 Forces
Free Fall
The slope is 9.8. The speed increases by 9.8 m/s each second.
Calculating:
Calculate the slope of the graph. What does this number represent?
Chapter 10 Forces
Free Fall
58.8 m/s
Predicting:
Use the slope that you calculated in Step 2 to predict the object’s speed at 6 seconds.
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Chapter 10 Forces
Free Fall
The object’s speed increases at a constant rate.
Drawing Conclusions:
The graph has a constant slope. What does the slope tell you about the object’s motion?
Chapter 10 Forces
Air ResistanceFalling objects with a greater surface area experience more air resistance.
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Chapter 10 Forces
Air Resistance A type of fluid friction.
Acts on objects falling through the air.
Objects with greater surface area experience more air resistance as they fall.
Try It! Take two pieces of paper, exactly the same size. Leave one flat and crumple one into a ball. Drop them from the same height at the same time. What happens?
In looking at this picture, what else could affect how quickly the papers fall?
Chapter 10 Forces
Elastic ForcesMatter is elastic if it returns to its original shape after squeezed or stretched.
Compression = An elastic force that squeezes or pushes matter together.
Tension = An elastic force that stretches or pulls matter.
The upward force of tension balances the shoe’s weight
Table Talk: Can you think of some other things that may involve compression and tension forces?
See BrainPop “Gravity” end
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Chapter 10 Forces
End of Section:Friction, Gravity, and
Elastic Forces
