gravitational fields physics 30s. outcomes the student will be able to: s3p-4-01: define the...
TRANSCRIPT
Outcomes• The student will be able to:• S3P-4-01: Define the gravitational field
qualitatively as the region of space around a mass where another point mass experiences a force.
• S3P-4-02: Diagram the Earth’s gravitational field, using lines of force.
• S3P-4-03: Define the gravitational field quantitatively as a force per unit mass.
• S3P-4-04: Compare and contrast the terms “mass” and “weight.”
Outcomes Continued
• S3P-4-05: Describe, qualitatively and quantitatively, apparent weight changes in vertically accelerating systems. Examples: elevators, spacecraft…
• S3P-4-06: Derive the acceleration due to gravity from free fall and Newton’s laws.
• S3P-4-07: Perform an experiment to calculate g near the surface of the Earth.
• S3P-4-08: Solve free-fall problems.
Gravitational Fields
• What is a field?• A field is a region in space where an object
would experience a force.• The field type is determined by the type of
force– A gravitational field is an area in space where an
object would experience a gravitational force– An electric field is an area in space where an
object would experience an electric force
What is Gravity?
• Gravity is an inherent attraction between two objects with mass.
• Gravity is rare in that it is only an attractive force – it cannot repel
• All matter has mass, and therefore is affected by gravity
Drawing Fields
• Michael Faraday (experimental physicist known for his contributions to electricity) determined a method to represent fields symbolically
• Fields around an object are indicated by arrows• A greater number of arrows per unit area
indicates a stronger field• Arrow head indicates direction of field
(direction of the force experienced by a test mass.
What about Size?
• The field diagrams for a 1 kg mass and Earth show no difference, but the fields obviously have different strengths.
• There are no moons orbiting the 1 kg mass• Qualitative definition:• The strength of a gravitational field is the force
experienced by an object in that field per unit mass.
• Units: N/kg
Example 3
• A 50.0 kg mass experiences a 490.0N gravitational force down. Find the strength of the gravitational field.
• G = 9.8 N/kg
Example 4
Robert lifts a 10.0 kg medicine ball. Could he be on Earth if he lifts the ball with a:
a) 50.0 N force?b) 500.0 N force?a) No b) Yes
Homework
1. Suppose you needed a formula to calculate the gravitational force on a 1 kg mass on the surface of another planet. What factors would need to be taken into account?
2. Suppose you needed a formula to calculate the gravitational force on a 1 kg mass floating in space due to another planet. What factors would need to be taken into account?
Apparent Weight
• Mass: a measure of the amount of matter in an object
• Weight: the amount of gravitational force acting on an object.
• Is your weigh always the same? When could it change?
Apparent Weight
• Apparent weight is when the normal force acting on a body is different from the gravitational force acting on a body.
• From the FBD, Fnet ≠0• Occurs when the reference frame is
accelerating• Examples: rollercoaster, swings, elevator,
spacecraft, etc.
Example 1
• Suppose you are riding in the elevator when the cable is cut. If the elevator is freefalling, draw your free body diagram and come up with an Fnet statement.
• Fnet = Fg + Fn
• Fnet = Fg, so Fn = 0• This is what the physics is behind
weightlessness.
Example 2
• Suppose Sam (m = 60.0 kg) is riding in the elevator and the elevator accelerates upwards at 0.150 m/s2. If Sam were standing on a scale, what would it read?
• The scale would read 600. N.
Example 3
• Suppose Sam (m = 60.0 kg) is riding in the elevator when the cable is cut. Suppose the elevator accelerates downwards at 1.50 m/s2. If Sam were standing on a scale, what would it read?
• The scale would read 498N.
Derivation of g
Imagine a mass free falling towards earth. The only forces acting on the object are gravity and air resistance. Assuming no air resistance,
Fnet = Fg
a = g
g
• Ever heard of pulling 2g through a bend?• What it means is that the apparent weight on
you is twice the force of gravity.• You are accelerating at 19.6 m/s2