11Blockhawk -Stephen Hawking is moving a 100 kg block from one side of his laboratory to the other. To accomplish this he ties a rope to the block and then ties it to his wheelchair such that the rope is pulled horizontally to the floor. If the coefficient of sliding friction is 0.35, what force must Mr. Hawking apply to the rope to move at constant speed?Ans: 343 N
12Warm Up1. A 30 N block has a coefficient of friction of What force is required to slide it along a level surface at constant velocity?2. A 35 kg crate requires a force of 110 N to slide at an acceleration of 2 m/s2. What is the coefficient of friction?
13F= MA PracticeIn the figure below the coefficient of friction is Find the tension in the rope then find the acceleration of the 18 N block.
14A crate having a mass of 60 kg falls horizontally off the back of a truck which is traveling at 30 m/s. Compute the coefficient of kinetic friction between the road and the crate if the crate slides 45 m on the ground with no tumbling along the road before coming to rest. Assume that the initial speed of the crate along the road is 30m/s.30 m/sAns. 1.02
15Radio Control CarA 35 kg radio control car runs out of gas and coasts to a stop in 3 seconds. Knowing the coefficient of friction between the ground and the car is 0.4, find the initial velocity and the distance the car rolled.
16Atwood Again…A cord running over a pulley connects two objects. The coefficient of friction between the object and the table is Find the acceleration of the system andthe tension ofthe rope.7 kg5 kg
17Angle Block Problem:A 40 kg lawnmower is being pushed at a constant speed along the ground. The angle at which the force is being applied is 35º below the horizontal. What force will keep the lawnmower moving at a constant velocity if the coefficient of kinetic friction is .3?
18How does pulling a block at an upward angle change the normal force? …friction force?FFNyxFg
19How does pulling a block at a downward angle change the normal force How does pulling a block at a downward angle change the normal force? …friction force?FNxyFgF
20If all the boxes are the same, which requires a greater force to pull If all the boxes are the same, which requires a greater force to pull? Why?FFF
21Warm-upA 925 N crate is being pulled across a level floor by a force of 325 N at an angle 25° above the horizon. The coefficient of kinetic friction between the crate and the floor is Find the acceleration of the crate.325 N25°925 NAnswer: m/s2
22What if we’re on an incline? If you were sledding down a hill would mass affect your acceleration?
23There are always at least two forces acting upon any object that is positioned on an inclined plane - the force of gravity and the normal force. The force of gravity (also known as weight) acts in a downward direction; yet the normal force acts in a direction perpendicular to the surface.
24Important! FN F 30º Ff Fg - Rotate the x & y axis Fg-yFgFg-x- Rotate the x & y axis- Break Fg into x and y components- Angle between Fg and y is equal to the angle of the incline
25Inclined to be Friction Old Grandma Jones is trying to load a wooden crate full of apples into her truck. To accomplish this, she plans to slide the crate up along a board. If the crate has a mass of 50 kg and the board is at a 30º incline, what is the maximum force she must apply parallel to the board to move the crate at constant velocity into her truck if the coefficient of friction is .25?
26That DEAD Cat! That Darn Cat! David is speeding down a hill (incline) when he notices a cat in the road. He immediately switches from the gas pedal to the brakes in order to stop the car. The slope of the incline is 20º and the car has a mass of 1500 kg.a. Draw a force diagram showing all the forces actingon the car.b. Determine the acceleration of the car if the coefficient of friction between the road and thetires is 0.55.c. If the cat is 25 m away, and David is going 10 m/swhen he slams on the brakes, does he hit the cat?
27Quiz part 2 10 kg a. Draw a free body diagram of the block. 1. The block given an initial velocity of 10 m/s is sliding down the hill and will come to a stop due to friction (µk=.7). Hint (The only force acting on the block down the hill is Fgx)a. Draw a free body diagram of the block.b. Write the Newton’s 2nd law equations.10 kgc. Find the acceleration of the blockd. How far will it travel before it stops?
28Quiz: Part 3 Atwood Again… A cord running over a pulley connects two objects. The coefficient of friction between the object and the table is Find the acceleration of the system andthe tension ofthe rope.7 kg5 kg
29The brakes are slammed on a speeding truck and it skids to a stop The brakes are slammed on a speeding truck and it skids to a stop. If the truck were heavily loaded so it had twice the total mass the skidding distance would be:a) the sameb) 1 ½ times as farc) twice as fard) four times as far
30Answer: a) the sameTwice the mass means the skidding tires will bear against the road with twice the force, which results in twice the friction. Twice as much friction acting on twice as much mass produces the same deceleration and hence the same stopping distance.