Concept #1: Work Done by Gravity

Practice: You push on 3 kg box against a wall for a distance of 2 m with a 100-N force that makes 53° with the horizontal, as shown. The box-wall coefficient of friction is 0.3. Calculate the work done by:

(a) you,

(b) friction,

(c) gravity.

Practice: A 70 kg person hikes from the bottom to the top of a 1,000 m hill with varying speeds. The path you take is very irregular, with varying inclinations. How much total work does gravity do on you during the entire hike?

Concept #2: Work on Inclined Planes

Practice: You push a 2-kg box from the bottom of an inclined plane with 50 N for 10 m. The incline makes 37° with the horizontal, and the box-incline coefficient of friction is 0.6. Find the work done by:

(a) you,

(b) friction, and

(c) gravity.

Example #1: Work on Inclined Planes

A 50 g mass is lifted upwards 20 cm. How much work is done by the gravitational force?

An object released from rest at the top of a 30.0° incline slides down the incline to the bottom of the incline. During this motion the work done on the block by the friction force on the block isA) positiveB) negativeC) zero

A box is pulled by a force F up a ramp that is inclined at 37° above the horizontal. The direction of this force is 60.0° above the horizontal. The force has magnitude 60.0 N. The box travels a distance of 5.00 m along the surface of the ramp. How much work does the force F do during this displacement of the box?A) 300 JB) 150 JC) 240 JD) 276 JE) None of the above answers

A block of mass 10.0 kg slides 16.0 m down a 36.9° incline, from point A at the top of the incline to point B at the bottom. As the block moves from point A to point B, the surface of the incline exerts a constant friction force that has magnitude 42.0 N.As the block moves from A to B, how much work is done on it by the friction force? (Be sure to indicate whether the work is positive or negative).

A block of mass 10.0 kg slides 16.0 m down a 36.9° incline, from point A at the top of the incline to point B at the bottom. As the block moves from point A to point B, the surface of the incline exerts a constant friction force that has magnitude 42.0 N.As the block moves from A to B, how much work is done on it by the gravity force? (Be sure to indicate whether the work is positive or negative).

A box with mass 5.00 kg is pulled up a 36.9° incline by a constant force F that has magnitude 75.0 N and that is parallel to the incline. The distance along the incline from the bottom to the top is 6.00 m. During the motion of the box, the surface of the incline exerts a constant friction force fk = 18.0 N on the box, in a direction opposite to the motion.
For the motion from the bottom of the incline to the top, how much work is done by each of the following forces? In addition to giving the magnitude of the work, be sure to indicate whether the work done is positive or negative.
(i) the force F that pulls the box
(ii) the friction force fk
(iii) the gravity force
(iv) the normal force

A ramp is inclined at 36.9° above the horizontal. A block with mass 0.500 kg is pulled up the ramp by a force F. The block starts at point A at the bottom of the ramp and ends up at point B at the top of the ramp. The distance from A to B, measured along the ramp is 5.00 m. The work done on the block by gravity as the block moves from point A to point B is
A) +24.5 J
B) -24.5 J
C) +19.6 J
D) -19.6 J
E) +14.7 J
F) -14.7 J
G) None of the above answers

A 10 kg object slides down an incline of 30 o. If the object starts at a height of 30 cm, and feels a coefficient of kinetic friction of 0.2 down the slope, answer the following questions:a. How much work is done on the object as it moves down the slope? b. What is the change in the object’s potential energy down the slope? c. What is the change in the object’s kinetic energy down the slope

A block with mass 5.0 kg slides a distance of 6.0 m from point A at the top of a ramp to point B at the bottom of the ramp. The ramp is inclined at 53° above the horizontal. For the displacement of the block from A to B the work done on the block by gravity is
A) 177 J
B) -177 J
C) 235 J
D) -235 J
E) 294 J
F) -294 J
G) zero
H) None of the above answers

When an object is moved from rest at point A to rest at point B in a gravitational field, the net work done by the field depends on the mass of the object and
1. the nature of the external force moving the object from A to B.
2. the velocity of the object as it moves between A and B.
3. both the positions of A and B and the path taken between them.
4. the path taken between A and B only.
5. the positions of A and B only.