- F= ma (Force = mass * acceleration)
- A force is defined as a pushed or pull
- In the metric system, force is measured in Newtons (N).
- A newton is approximately equal to 1/4 lb (pound)
- In physics, mass is usually measured in kilograms
- Acceleration is measured in m/s^2
a) At what rate will the rocket accelerate?
F = ma => 4500 N = 1750 kg * a, solve for a,
a = 4500/1750 = 2.57 N/g = 2.57 m/s^2
b) How fast will the rocket be moving at the end of 25 s?
Sf = si + at => Sf= 0 + (2.57 m/s^2) (25s) = 64.25 m/s
Example 2: Two forces act on a box. One force is 500 N and acts to the right. The second force is 840 N, and act to the left.
a) What is the net force acting on the box?
Fnet = Fa - Ff = 840 -500 = 340 N left
b) If the box mass is 120 kg, at what rate does it accelerate?
F = ma => 340 N = (120 kg) a, a = 2.83 N/kg or m/s^2
Weight:
- Weight refers to the force of gravity acting on an object
- The formula for calculating the weight of an object is a variation of Newton's 2nd Law.
- W= mg (Weight = mass * gravitational acceleration (9.8 m/s^2))
- Remember, mass is the amount of matter contained within an object. If you shoot a heavy object into space. Its weight will disappear, but its mass will be unchanged.
Example 1: An object has a mass of 320 kg. How much does this object weight?
W = mg = 320 kg (9.8 m/s^2) = 3136 N
Example 2: A man weight 850 N. What is his mass?
W = mg => 850 N = m (9.8 m/s^2), m = 86.7 kg
Example 3:
a) A man stands on a scale in an elevator. The elevator is not moving. The man's mass is 75 kg. What is the reading on the scale?
b) If the elevator beginning to move upward 2 m/s^2. What is the reading on the scale now?
F = ma = 75 kg ( 2 m/s^2) = 150 N
Fnet = 735 N + 150 N = 885 N
c) The elevator come to stop and then start accelerating downward at 3 m/s^2. What is the reading on the scale now?
F = ma = 75 (3) = 225 N
Fnet = 735 - 225 = 510N
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