Rate of Gravitational Force (Power)

The gravitational force is doing work on the projectile at a rate of -39 W when the projectile has a velocity of 4.0 m/s at an angle of 30 degrees above the horizontal. The negative sign indicates that the work being done is in the opposite direction of the force. The angle of 120 degrees is used because it is the angle between the force of gravity and the velocity of the projectile, and this is the angle that is used in the calculation of work. It is important to pay attention to the signs and angles when solving problems involving work and energy.
  • #1
Lori

Homework Statement


At what rate is the gravitational force doing work on a 2.0 kg projectile at an instant when the velocity of the projectile is 4.0 m/s directed 30 degrees above the horizontal.

the answer is -39 W but i need help on why!

Homework Equations


P = W/T
Power = Work*V
=Force*cos(theta)*V

The Attempt at a Solution


I know that the force of the gravity is just 9.8*2, but why is the angle that i plug in cos(120) ? I'm not sure why/how we use angle of 120 as oppose to 90-30 = 60 degrees...

If you can provide a sketch for me visually, it would help a lot...

Here's what i thought:
upload_2017-11-7_19-40-49.png
 

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  • #2
How does cos(60) compare to cos(120)? This might help answer your question.
 
  • #3
phyzguy said:
How does cos(60) compare to cos(120)? This might help answer your question.
Ohh, it's just the negative of it... but i get why work would be negative in this case, but how can i draw it so that i don't mess up the angles next time? Since i have to do it right on the exam, i just draw the diagram and don't think of the signs! Is there a actual sketch of it that it should look like, so that i can learn how to draw it?
 
  • #4
Look at the definition of work done. Eg.. W=f.s

The angle between the velocity and force of gravity vectors is 120 degrees when they are drawn from the same point.
 
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  • #5
CWatters said:
Look at the definition of work done. Eg.. W=f.s

The angle between the velocity and force of gravity vectors is 120 degrees when they are drawn from the same point.
Ohhhh. I see it now. Thanks
20171107_201215.jpg
 

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  • #6
This is the way I would draw it...

Vectors.jpg
 

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What is the rate of gravitational force?

The rate of gravitational force, also known as the power of gravitational force, is the amount of work done per unit time by the force of gravity on an object. It is measured in watts (W).

How is the rate of gravitational force calculated?

The rate of gravitational force is calculated by multiplying the mass of the object by the acceleration due to gravity (9.8 m/s^2) and the height at which the object is located. The equation is P = mgh, where P is the power, m is the mass, g is the acceleration due to gravity, and h is the height.

Does the rate of gravitational force change with distance?

Yes, the rate of gravitational force decreases with distance. This is because as the distance between two objects increases, the force of gravity between them decreases. This relationship is described by the inverse square law, which states that the force of gravity is inversely proportional to the square of the distance between two objects.

How does the rate of gravitational force affect objects on Earth?

The rate of gravitational force affects objects on Earth by causing them to accelerate towards the center of the Earth at a rate of 9.8 m/s^2. This is why objects fall towards the ground when dropped. The rate of gravitational force also determines the weight of an object, as weight is the force of gravity acting on an object's mass.

Can the rate of gravitational force be manipulated?

No, the rate of gravitational force cannot be manipulated. It is a fundamental force of nature and cannot be changed or altered. However, the effects of gravity can be counteracted by other forces, such as the force of air resistance or the force of thrust from a rocket.

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