How does load friction affect acceleration

Describe a model for friction on a molecular level. Describe matter in terms of molecular motion. The description should include diagrams to support the description; how the temperature affects the image; what are the differences and similarities between solid, liquid, and gas particle motion; and how the size and speed of gas molecules relate to everyday objects.

The two blocks of Figure are attached to each other by a massless string that is wrapped around a frictionless pulley. When the bottom 4. Find the magnitude of the force necessary to move the blocks at constant speed.

Assume that the coefficient of kinetic friction between all surfaces is 0. We analyze the motions of the two blocks separately. The top block is subjected to a contact force exerted by the bottom block. The components of this force are the normal force. The bottom block is subjected to contact forces due to the top block and due to the floor. The first contact force has components. The components of the contact force of the floor are. Since the top block is moving horizontally to the right at constant velocity, its acceleration is zero in both the horizontal and the vertical directions.

When these values are substituted into the second set of equations, we can determine. Understanding what direction in which to draw the friction force is often troublesome. Notice that each friction force labeled in Figure acts in the direction opposite the motion of its corresponding block. The coefficients of friction between the surfaces are.

Find the frictional force on the crate when the truck is accelerating forward relative to the ground at a 2. The forces on the crate are its weight and the normal and frictional forces due to contact with the truck bed. We start by assuming that the crate is not slipping. In this case, the static frictional force.

We can now check the validity of our no-slip assumption. The maximum value of the force of static friction is. Thus, the assumption of no slipping is valid. If the crate is to move with the truck when it accelerates at.

Since this exceeds the maximum of N, the crate must slip. The frictional force is therefore kinetic and is. Earlier, we analyzed the situation of a downhill skier moving at constant velocity to determine the coefficient of kinetic friction. The snowboarder of Figure glides down a slope that is inclined at. The forces acting on the snowboarder are her weight and the contact force of the slope, which has a component normal to the incline and a component along the incline force of kinetic friction.

Because she moves along the slope, the most convenient reference frame for analyzing her motion is one with the x -axis along and the y -axis perpendicular to the incline. In this frame, both the normal and the frictional forces lie along coordinate axes, the components of the weight are. The glue on a piece of tape can exert forces. Can these forces be a type of simple friction? Explain, considering especially that tape can stick to vertical walls and even to ceilings.

When you learn to drive, you discover that you need to let up slightly on the brake pedal as you come to a stop or the car will stop with a jerk. Explain this in terms of the relationship between static and kinetic friction.

When you push a piece of chalk across a chalkboard, it sometimes screeches because it rapidly alternates between slipping and sticking to the board. Describe this process in more detail, in particular, explaining how it is related to the fact that kinetic friction is less than static friction. The same slip-grab process occurs when tires screech on pavement.

A physics major is cooking breakfast when she notices that the frictional force between her steel spatula and Teflon frying pan is only 0. Knowing the coefficient of kinetic friction between the two materials, she quickly calculates the normal force. What is it? N of force to insert a dry steel piston into a steel cylinder. What is the normal force between the piston and cylinder? What is the maximum force of friction under such conditions? The frictional forces in joints are relatively small in all circumstances except when the joints deteriorate, such as from injury or arthritis.

Increased frictional forces can cause further damage and pain. Suppose you have a kg wooden crate resting on a wood floor, with coefficient of static friction 0. The coefficient of sliding friction is known to be 0.

A team of eight dogs pulls a sled with waxed wood runners on wet snow mush! The dogs have average masses of Consider the Remember that friction always acts in the direction opposite that of motion or attempted motion between surfaces in contact. Show that the acceleration of any object down a frictionless incline that makes an angle.

Note that this acceleration is independent of mass. Show that the acceleration of any object down an incline where friction behaves simply that is, where. Note that the acceleration is independent of mass and reduces to the expression found in the previous problem when friction becomes negligibly small. The result of the preceding problem may be useful, but be careful to consider the fact that the snow boarder is going uphill. A machine at a post office sends packages out a chute and down a ramp to be loaded into delivery vehicles.

You can neglect air resistance in both parts. If an object is to rest on an incline without slipping, then friction must equal the component of the weight of the object parallel to the incline. This requires greater and greater friction for steeper slopes. Show that the maximum angle of an incline above the horizontal for which an object will not slide down is. You may assume that the weight of the car is evenly distributed on all four tires and that the coefficient of static friction is involved—that is, the tires are not allowed to slip during the deceleration.

Ignore rolling. Calculate for a car: a On dry concrete. Assume that only half the weight of the car is supported by the two drive wheels and that the coefficient of static friction is involved—that is, the tires are not allowed to slip during the acceleration. This is not a large frictional force for such a massive system. Rolling friction for trains is small, and consequently, trains are very energy-efficient transportation systems.

Assume that the force is exerted parallel to her legs. Also, assume negligible force exerted by her arms. A contestant in a winter sporting event pushes a The contestant now pulls the block of ice with a rope over his shoulder at the same angle above the horizontal as shown below.

Calculate the minimum force F he must exert to get the block moving. At a post office, a parcel that is a The coefficient of kinetic friction between the box and plane is 0. Static and Kinetic Friction The basic definition of friction is relatively simple to state. Friction Friction is a force that opposes relative motion between systems in contact. Static and Kinetic Friction If two systems are in contact and stationary relative to one another, then the friction between them is called static friction.

Figure 6. Note that constant velocity means constant speed plus constant direction that means along a straight line. The acceleration has the same direction as the applied net force. The above examples were on Newton's 1st and 2nd laws. For Newton's 3rd law , look at the following example :. Example 4: A Example 5: An 8 0 kg man is standing in an elevator. Determine the force of the elevator onto the person if the elevator is a accelerating upward at 2.

Solution: The force of the elevator onto the person is nothing but the normal reaction, N , of the elevator floor onto his feet. For each case, a force diagram must be drawn. Make sure that you carefully draw each force diagram with minimal looking at the following force diagrams.

This is the case that the elevator has just started going upward. Since its speed has to change from zero to some value, it has to accelerate upward and the person feels heavier because the floor of the elevator exerts a normal reaction, N , onto the man that is greater than his weight.

This creates a nonzero net force and therefore accelerates the person. Zero acceleration means zero net force acting on the man. This requires N to be equal to w in magnitude. In other words, it decelerates as it goes upward. We all have this experience that during such slowing down, we feel lighter. We will notice that the magnitude of the normal reaction, N , becomes less than that of w.

This time we use Example 6: In the figure shown, determine the acceleration of each block if a the string is cut, and b if only the stopper is removed.

B can't fall freely and must pull A as well ; therefore, its acceleration will not be 9. According to Newton's 3rd law , the vertical forces on A cancel each other. See the right side figure. The force of gravity on B is the only cause of motion of the two blocks. In fact, we are only interested to find the magnitude of acceleration of the system of blocks. Since the system is connected, both blocks move at the same acceleration magnitude. Although both blocks move at the same acceleration magnitudes ; however , A accelerates horizontally while B accelerates vertically.

We may write:. Example 7: Vehicles A and B are shown in nine different cases. In each case, a statement is made on the left. Refer to the figure in the middle and determine if the statement is true T or false F. After you have answered all, then click here to check your answers. Chapter 4 Test Yourself Is the normal force of that surface on the box 35N?

Is the normal force on the box 75N? An astronaut looking through a window notices a bolt becoming detached from the ship.

How many miles behind the ship will the bolt be in an hour? How long will it take? How many miles does it travel for the change to happen? Friction is the result of engagement of surface irregularities between two surfaces in contact. Note that whoever tries to pull the block to the right will face a resistant frictional force F k as shown below.

For constant velocity motion, we may write:. It is easy to see that F k force of kinetic friction is the force that the horizontal surface exerts on the object and F appl. If F appl. This means that, if the coefficient of friction between two surfaces is known, and we also know the normal force, we can easily calculate the force of friction.

Physics Tutorial. My Cart Subscription Selection. Student Extras. Since there is no vertical acceleration, the normal force equals the gravity force. We Would Like to Suggest Sometimes it isn't enough to just read about it. You have to interact with it! And that's exactly what you do when you use one of The Physics Classroom's Interactives. We would like to suggest that you combine the reading of this page with the use of our Force Interactive.

You can find it in the Physics Interactives section of our website. The Force Interactive allows a learner to explore the effect of variations in applied force, net force, mass, and friction upon the acceleration of an object. Visit: Force. There are two vertical forces - gravity and normal force. There are two horizontal forces - friction and the applied force. N The F net is the vector sum of all the forces: 4.