Newton's Second Law
The acceleration of an
object is directly proportional to the net force acting
upon it. The constant of proportionality is the mass.
Fnet=MA A=Fnet/M
Force = A push or pull that one object exerts on another object. M
= Mass. A= Acceleration.
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A car is moving at 6 m/s. Let’s assume that the wheels of a 5-kg
car apply 10 N of net force. What is the acceleration if friction and drag
are negligible? |
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Net Force |
= MA |
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10 |
= 5A |
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Acceleration = |
2 m/s2 |
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A car is moving at 6 m/s. What is the net force if the wheels of
the 5-kg car apply 10 Newtons but a 1-kg
parachute applies 3 Newtons in the other
direction? The net force would equal 3 Newtons.
The total mass = 6 kg. |
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What is the acceleration
of the car? |
Acceleration |
= Fnet/M |
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Acceleration |
= 3/6 |
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Acceleration = |
0.5 m/s2 |
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A car is moving at 6 m/s. A rocket is added to the car and
applies an additional force of 10 Newtons. The
wheels still apply 10 N. What is the net force if the parachute continues to apply
7 Newtons in the other direction? The
total mass of the car, rocket and parachute is 10 kg. The net force would equal 13 Newtons.
The total mass = 10 kg. |
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What is the acceleration
of the car? |
Acceleration |
= Fnet/M |
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Acceleration |
=13/10 |
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Acceleration |
=1.3 m/s2 |
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Big masses are hard to accelerate. It is hard to speed up
or slow down big masses. Big masses require big forces to change speed. It is
hard to change the direction of motion of a big mass. |
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Small masses are easy to accelerate. Small masses require
small forces to change speed. It is relatively easy to speed them up and/or
slow them down. It is easy to change their direction of motion. |
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Objects move in the direction they are pushed or pulled. Objects accelerate more quickly when a greater force is used. |
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Assume that both steam engines below apply the
same amount of force.
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A heavy train has a difficult time accelerating. Big masses
require big forces to change speed. |
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Acceleration = |
Force /
Mass |
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Acceleration = |
100% /
100% |
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Acceleration = |
1 |
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When the same
force is applied to a less massive train its acceleration is greater. Small masses
require small forces to change speed. |
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Acceleration = |
Force /
Mass |
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Acceleration = |
100% /
10% of the big train |
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Acceleration = |
10 times
greater than the big train |