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Newton’s Laws
Review Sheet
Newton’s 1st Law
• An object at rest will remain at rest unless acted on by an outside force
• An object in motion will continue in motion unless acted on by an outside force.
• An object in motion will continue to move in a straight line unless acted on by an
outside force.
• Newton’s 1st law is also called the Law of inertia.
• Does it take more force to stop a bowling ball or a tennis ball? Why?
It takes more force to stop a bowling ball because the bowling ball has more mass
thus has a greater tendency to resist changes to its state of motion.
• Why can you remove the bottom coin from a tall stack of coins without toppling
the entire stack?
Because the coins have inertia.
• It takes less force to stop a tennis ball because it has less inertia.
The more mass an object has the more inertia the object has. (inertia is directly related
to the mass)
Newton’s 2nd Law
• Force = Mass x Acceleration
• What is the definition of force?
A push or pull.
The definition of net force is the sum of all forces acting on an object.
• Deceleration means an object slows down.
• What are the units used to measure force
Newtons.
• The units used to measure mass is the kilogram.
• What is the net force of an object at rest?
0 Newtons.
• What happens when the net force acting on an object is zero?
The object is at rest or will remain in motion at constant speed.
• Units used to measure acceleration are meters/second2.
• The newton is defined as force that can give a 1-kilogram mass an acceleration of 1
•
m/sec2.
Determine how the acceleration of an object will change if:
a. The mass is doubled. The acceleration will be less (specifically the
acceleration will be cut in half.)
b. The force is doubled. The acceleration will be greater (specifically the
acceleration will double.)
c. The mass and force are doubled. The acceleration will remain the
same.
d. The acceleration of an object is directly proportional to force (doubling
the force doubles the acceleration) and inversely proportional to mass
•
(increasing the mass decreases the acceleration). Think about this
logically. Is it easier to accelerate something heavier or lighter?
What is the mass of a car that accelerates at 20 m/s2 with a force of 10000 N?
Looking For
mass
•
Relationship
m = F/a
Solution
10000 N / 20 m/s2
=500 kg
How much force is needed to accelerate a 2-kg soccer ball at a rate of 5 m/s2?
Looking For
force
•
Given
a=20 m/s2
F=10000 N
Given
m=2 kg
a=5 m/s2
Relationship
F=ma
Solution
2 kg x 5 m/s2
=10 N
What is the acceleration of a 5-kg ball that has a 10-N force applied to it?
Looking For
acceleration
Given
m=5 kg
F=10 N
Relationship
a=F/m
Solution
10 N / 5 kg
=2 m/s2
•
When an object has no acceleration what can you conclude about:
a. The net force acting on it is zero.
b. Yes the object could be at rest. An object at rest has a net force of zero
and will not move unless acted upon by an unbalanced force.
c. Yes the object could move with constant velocity/speed. When an
object moves with constant velocity/speed its acceleration is zero.
•
Therefore, if two objects of different mass have the same acceleration, which will
require more force to stop?
o A truck moving at 20 m/s2or a bicycle moving at 20 m/s2
o A truck moving at 20 m/s2
Newton’s 3rd Law
• Action Reaction
• For every action, there is an equal and opposite reaction
• A bullet fired from a rifle causes the rifle to kick back. This is because the bullet
pushes against the rife with an equal and opposite force. Therefore their forces
are equal in size and opposite in direction.
• Newton stated that if you push gently on something it will push gently on you.
• The force the bat exerts on the ball is the same as the force the ball exerts on the
bat.
• 100 N
• The ball hitting the bat.
Word Problems
A force of 500-N propels a train down a track at a rate of 5.0 m/s2. If a frictional force of
200 N acts on the train determine the net force acting on the train?
Looking For
Given
Relationship
Solution
Net force
F2 =500 N
Fnet = F2 – F1
F1 =200 N
300 N
What is the acceleration of a 18 kilogram bowling ball if a force of 36 N is applied to it?
Looking For
Given
Relationship
Solution
2
acceleration
F=36 N
a = F/m
2 m/s
m=18-kg