Physics Workbook 5 d: Focus on Friction

Quest for the bag of goodies: The saga continues.....Part II.

When we left our battling twosome , the bear was finally making his triumphant get-away dragging the bag behind him. But alas, it was not to be, the bag snags on a root, ripping open with goodies scattering everywhere. Both the bear and the orangutan quickly begin to eat as fast as they can. After stuffing themselves, they suddenly discover how ridiculous they are and begin to laugh. The bear (Hank) invites the orangutan (Sally) over to his place for a cup of tea. He picks up the rope as they leave saying it may come in handy getting his sled full of firewood home.

1. As they are finishing up the tea, Sally asks Hank about the sled of firewood he mentioned, saying she might be of help. He says "come on , I'll show you. They go down into the valley where the 200 kg sled sits. The bear attaches the rope and tugs with a force of 1000 N parallel to the ground, but just as the sled begins to move, the rope breaks. Hank says "humm, the breaking strength of this rope must be 1000N", as he ties it back together.

Draw a FBD of the sled

Draw a x and y axis.

what is the weight of the sled ?

What is the apparent weight of the sled ?

Is the sled in equilibrium in either the x or y directions?

Sum the forces acting on the sled in both x and y directions.

What are these sums equal to?

What is the friction force equal to?

Is it static or kinetic friction ?

What is the acceleration of the sled ?

2. Sally leaps behind to push, trying to help. Being taller than the sled, she must lean over to push, applying a force of 600N at an angle of 53° below the horizontal, while Hank is careful to pull on the rope with exactly 600 N so as not to break it again. Considering that the "Big Bear Book of Wisdom" reveals that the static coefficient of friction between a sled and dirt is 0.5, while the kinetic coefficient of friction is 0.3, do the two together manage to budge the sled? If so, what is it's acceleration?

.Draw a FBD of the sled

Draw a x and y axis.

what is the weight of the sled ?

What is the apparent weight of the sled ?

Is the sled in equilibrium in either the x or y directions?

Sum the forces acting on the sled in both x and y directions.

What are these sums equal to?

What is the friction force equal to?

Is it static or kinetic friction ?

What is the acceleration a_x of the sled?

What is the acceleration a_y of the sled?

3. Next Sally hunkers down so that she is pushing horizontally with 600 N, in the same direction as Hank.

What is the acceleration of the sled ?

Draw a FBD of the sled

Draw a x and y axis.

what is the weight of the sled ?

What is the apparent weight of the sled ?

Is the sled in equilibrium in either the x or y directions?

Sum the forces acting on the sled in both x and y directions.

What are these sums equal to?

What is the friction force equal to?

Is it static or kinetic friction ?

What is the acceleration a_x of the sled?

What is the acceleration a_y of the sled?

4. Ahah! For those of you who haven't figured it out, the sled is finally moving! Sally, decides to play a joke on Hank. She jumps onto the sled for a ride. The question is, with Sally no longer pushing and her added mass of 80 kg , is Hank still able to keep the sled moving?

Draw a FBD of the sled

Draw a x and y axis.

what is the weight of the sled ?

What is the apparent weight of the sled ?

Is the sled in equilibrium in either the x or y directions?

Sum the forces acting on the sled in both x and y directions.

What are these sums equal to?

What is the friction force equal to?

Is it static or kinetic friction ?

What is the acceleration a_x of the sled?

What is the acceleration a_y of the sled?

5. Bored with the free ride, Sally leaps off to help again and pushes upward at an angle of 37° above the horizontal hoping to lighten the load a bit, so they move along faster.

Draw a FBD of the sled

Draw a x and y axis.

what is the weight of the sled ?

What is the apparent weight of the sled ?

Is the sled in equilibrium in either the x or y directions?

Sum the forces acting on the sled in both x and y directions.

What are these sums equal to?

What is the friction force equal to?

Is it static or kinetic friction ?

What is the acceleration a_x of the sled?

What is the acceleration a_y of the sled?

6. Presently, when they begin to go down a hill that slopes down at an angle of 26° below the horizontal, they stop for a rest. The question is, does the sled stay at rest on the hill, or does it begin to slide down the hill by itself? (recall that the static coefficient of friction is .5 and the kinetic coefficient of friction is .3.)

Draw a FBD of the sled

Draw a x and y axis.

what is the weight of the sled ?

What is the apparent weight of the sled ?

Is the sled in equilibrium in either the x or y directions?

Sum the forces acting on the sled in both x and y directions.

What are these sums equal to?

What is the friction force equal to?

Is it static or kinetic friction ?

What is the acceleration a_x of the sled?

What is the acceleration a_y of the sled?

7. However, when Sally accidently bumps the sled it begins to move down the hill!

Draw a FBD of the sled

Draw a x and y axis.

what is the weight of the sled ?

What is the apparent weight of the sled ?

Is the sled in equilibrium in either the x or y directions?

Sum the forces acting on the sled in both x and y directions.

What are these sums equal to?

What is the friction force equal to?

Is it static or kinetic friction ?

What is the acceleration a_x of the sled?

What is the acceleration a_y of the sled?

8. Sally leaps in front of the sled and begins to push as hard as she can ( 600 N ) with a force that is horizontal. Does she manage to stop the sled?

Draw a FBD of the sled

Draw a x and y axis.

what is the weight of the sled ?

What is the apparent weight of the sled ?

Is the sled in equilibrium in either the x or y directions?

Sum the forces acting on the sled in both x and y directions.

What are these sums equal to?

What is the friction force equal to?

Is it static or kinetic friction ?

What is the acceleration a_x of the sled?

What is the acceleration a_y of the sled?

9. The Grand Slam!

A car traveling at 20 m/s stops in a distance of 50 m. Assume that the deceleration is constant. The coefficients of friction between the passenger and the seat are µ_s = 0.5 and µ_k = 0.3 . Will a 70 kg passenger slide off the seat if not wearing a seat belt?