Transcript Newton`s 2nd Law

Newton’s Second Law (The
Mathematical Calculation of Force)
Newton’s Second Law of Motion
Newton’s Second Law States:
The acceleration of an object is related
to the net force acting on it and the mass
of the object
Newton’s Second Law of Motion
Newton’s Second Law States:
The acceleration of an object is related
to the net force acting on it and the mass
of the object
The mathematical relationship looks like
this:
F
=
m
Force equals mass
X
a
times
acceleration
Forces
• A force is a push or a pull that
one object exerts upon another
Units
• Remember – ALL of our numbers are
measurements and thus MUST HAVE
UNITS
– Force is measured in a units called:
• Newtons (N)
• Named after . . . . . . .
– 1 N = 1 kg m/s2 (kg is the unit for mass and
m/s2 is the unit for acceleration)
• 1 N is the amount of force required to accelerate
one kg of mass 1 meter per second every second
Solving Force Problems
• Solving force problems can be especially difficult
because:
• Sometimes you will have to calculate acceleration first
before you can find force.
• You may also have to go the other way and find
velocity, acceleration, distance, time, or mass using
force problems.
• Because there are so many variables involved in
the mathematic equations for force, acceleration,
and velocity (speed), it is best to follow proper
steps when problem solving.
Problem-Solving Steps
1. Read the problem and figure out what you are looking for
in the final answer
2. Write the equation(s) you need to get the final answer
Ex. If trying to find acceleration, you could write either
F
Vf – Vi
a=
OR a =
m
t
3. Write down all of the variables that you are given and their
quantities and units.
4. If you have all of the variables needed to solve for the
final answer, convert your units and solve!
5. If you don’t have all of the variables needed to solve for
the final answer. . .
Problem Solving Steps If You Don’t
Have All of the Necessary Variables
6. Write down the equation(s) you need to solve for the
missing variables in your final equation.
7. Convert your units and solve for the missing variables.
8. Plug in those missing variables in your final equation and
solve (remember to check your units!!!)
Things to Remember When Solving
Force Problems
-Your mass MUST be in kg, if it’s in grams, you must
use your metric stairs to convert.
-Your acceleration must be in m/s2, if it’s not, you must
use your metric stairs to convert.
- Your final answer for force must be in Newtons (N)
- You might have to decide, based on your variables
given, which of two different acceleration equations you
have to use (you may end up using both).
- Just go step-by-step and you’ll be okay.
Practice Problems (they get more
difficult as they go on. . .)
1. A 52-kg water skier is being pulled by a speedboat. The
force causes her to accelerate at 2 m/s2. Calculate the net
force that causes this acceleration.
Click For the Answer
1. What net force is needed to accelerate a 55-kg cart at 15
m/s2?
Click For the Answer
3. A remote control car has a mass of 24 kg and an
acceleration of 120 cm/s2. What is the force of this car
when it crashes into a brick wall?
Click For the Answer
4. A 5-gram pencil is dropped out of an airplane where it
accelerates toward Earth at 9.8 m/s2. With how much
force does that pencil hit the ground?
Click For the Answer
5. A 1,500 kg car is stopped at a red light. The light turns
green and the car accelerates for 7 seconds until it
reaches a velocity of 42 m/s, at which points it smashes
into a car waiting to turn. With how much force did the
1,500 kg car hit the car waiting to turn?
Click For the Answer
6. A 3,000-kg car is stopped at a red light. The car accelerates
for 9 seconds when it smashes into a car waiting to turn.
How fast was the car going at the time of the accident if it
smashed into the car waiting to turn with a force of 36,000
N?
Click For the Answer
Final Questions
• What causes objects to accelerate?
Answer
• What is a ‘Force’?
Answer
• If the same force acts on two objects with
different masses, what will you notice about the
more massive object?
Answer