Forces and Motion
Forces, Motion, and Gravity
Page 55, Activity: Comparing
" Know Your Strength "
Forces Around You
Force: is a push or pull that starts, stops, or changes the direction of an object.
Transfers energy to an object
Lifting a wheel-barrow
Figure 3.1answer the question
Balancing the wheel-barrow
What has to happen?
How does the load change if it is heavier?
Force and Gravity
Activity page 56: " Calculating "
w = mg
Use this equation: m = w / g
SI unit of force: Newton ( N )
One newton is the amount of force needed to cause 1-kg mass to accelerate at a rate of 1 m/s2 of motion.
N = 1 kg x 1m/s2
Spring scale measure weight or force
Figure 3.2, page 56
Gravity – weight of an object depends on the force that pulls the object toward the Earth.
Force of gravity on the Earth is 9.8 m/s2, the weight of a 1 – kg object is 9.8 N.
Feather and a coin
Acceleration due to gravity ? ______
Figure 3.3, page 57
Motion opposed by friction
Friction: occurs when the surfaces of any kind of matter move past each other.
Terminal velocity: definition?
Shape of object affect
A curved path
Figure 3.4, page 58
The Curve Ball
Science and You: The Curve Ball
Figure 3.5, page 59
Unbalanced air forces
Reinforcement page 59 of teachers text.
Take notes in class.
The First Law of Motion
Fourth century China. They knew, but stayed in China.
Newton was first person to state laws of motion and forces.
An Object at rest will remain at rest and an object in motion will remain in motion unless acted upon by an outside force.
Inertia:comes from Latin iners = idle
The tendency of an object to resist any change in motion.
Bike rider, figure 3.7, page 61.
An outside force that resist motion when two surfaces come in contact.
Types of Friction
Sliding –Which kind of surface causes less sliding friction, a smooth surface or a rough surface?
Rolling –which tire in the picture will create more rolling friction? ( page 62 )
Fluid –air, water, oil
All above keep surfaces from direct contact and reduces friction.
Second Law of Motion
The net force on an object equals its mass times its acceleration.
F = ma
Figure 3.9, page 64
Force, Mass, and Acceleration
Figure 3.10, page 65
If both carts have the same acceleration how much force must be added to the cart on the right?
Cart A:a = 2.5 m/s2, m = 30 kg
Cart B:a = 1.5 m/s2, m = 30 kg + 10 kg + 10 kg
Next page for problems
Figure 3.11, page 65
3. How would each dog affect the acceleration of the sled? Answer:
Each dog would increase the acceleration of the sled because each dog would increase the force applied to the sled.
4. How does the acceleration of the 200 kg sled compare to the acceleration of the 100 kg sled? Answer:
The 200 kg sled accelerates half as fast as the 100 kg sled.
5. How does mass effect the acceleration of each sled? Answer:
The greater the mass, the slower the acceleration of the sled. The sled with a 100 kg mass is accelerated at twice the rate of the sled with the 200 kg mass.
Using Newton’s Second Law
Terms: mechanics, dynamics, kinematics
f = ma
a = f / m
m = f / a
Let’s do the practice problems on page 66.
Practice problem # 1, page 66
F = m x a
F = 40 kg x 2.5 m/s2
F = 100 kg x 1 m/s2
F = 100 N
Practice problem # 2, page 66
a = F / m
a = 90 N / 60 kg
a = 1.5 N / kg
kg x m / s2
Answer is in units: m / s2
Practice problem # 3, page 66
F = m x a
F = 60 kg x 2.5 m/s2
F = 150 kg x m/s2
To find the additional force, the initial force must be subtracted from the final force:
150 N – 90 N = 60 N
momentum = m x v
Momentum doesn’t change unless the velocity or mass changes.
Momentum can transfer from one object to another.
CONSIDER THIS: Should seat belts and air bags be required? Page 67. Research this and find out more about crash cars involving dummy drivers.
Forces in Circular Motion
Any force that causes and object to follow a circular path.
How does centripetal force apply to the cars to keep them moving around a curved track? Figure 3.12, page 68
More Miles for The Size
Do the activity, Skills Workout, on page 69 of the textbook, collecting data: Gas Usage
Read page 69 of textbook.
Question: How does this compare with the force needed for an 800 kg car? answer
The force needed for an 800 kg car is 2400 N. Show the work below.
800 kg x m/s2 = 2400 kg x m/s2, or 2400 N
Third Law of Motion
Equal and Opposite Forces
Balanced and Unbalanced Forces:look at figure 3.14 on page 71
How will the force of the diving board affect the diver’s performance? Answer
The greater the force exerted upon the diving board, the higher the dive will be.
Observing Newton’s Third Law: look at figure 3.15 on page 71
Why do you think it’s important for all crew members to pull their oars at the same time? Answer:
If the crew members do not work together, their own forces will balance each other, decreasing the overall unbalanced force they are trying to achieve.
From Fireworks to Outer Space, page 72
1. 500 years ago the Chinese _____________.
2. Chinese invented ____________________.
3. The ancient Chinese developed fireworks for_________.
4. All rockets require __________________.
5. What kind of fuel is used for space rockets and why?
6. Investigate rocket-propulsion systems. Use the internet for your research. Include labeled drawings and pictures to explain your research.
Let’s take a look at table 3.1 on page 73, " Universal Forces ".
Answer this question: What would happen to objects on the earth if the force of gravity was as strong as the electromagnetic force?
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