# Physics

posted by .

A force of 40 N is applied at an angle of 30oabove the horizon to a 5 kg block that is at rest on a rough horizontal surface. After the block starts to move with an acceleration of 3 m/s2, what is the coefficient of kinetic friction between the block and the horizontal surface?

How do I solve a friction problem without knowing the coefficient and vice versa?

• Physics -

You know friction force is equal to the horizontal pulling force (adjusted for acceleration).

40cos40=friction force-mass*acceleration
= (mg-40sin30)mu-ma
solve for mu

• Physics -

I understand that friction WOULD equal to the horizontal pulling force IF the object is moving at constant velocity, but the question asked about friction force when there is acceleration.

Are you saying that whether or not there is acceleration, the friction force would always equal to that of the pulling force?

• Physics -

The friction force is opposite to the direction of motion and acceleration in this case. As Bob has noted, the friction force is reduced because the vertical component of the applied force reduces the force on the ground.

40 cos30 - friction force = mass*acceleration
40 cos30 - (mg -40 sin30)*mu = m a

mu = (40 cos30 -ma)/(mg -40 sin30)

• Physics -

I have a sign error in my equality.

40cos40=friction force+mass*acceleration

• Physics -

<<Are you saying that whether or not there is acceleration, the friction force would always equal to that of the pulling force?>>

No. That is not what we are saying. The imbalance between the pulling force and the friction force provides the scceleration, in a kinetic friction situation, as here. That is reflected in the equations that Bob and I have written.

In a static-friction situation, the pulling and friction forces are equal, until enough force is applied to cause motion.

## Similar Questions

1. ### Physics

A force of 50 N directed 30 degrees above the hoizontal is applied to a 10-kg block at rest on a rough horizontal surface. What is the coefficient of kinetic friction between the block and the horizontal surface if the acceleration …
2. ### physics

A force of 8.7 N is applied to a steel block initally at rest on a horizontal frictionless surface. The force, which is directed at an angle of 30.0o below the horizontal, gives the block a horizontal acceleration of 5.3 m/s2. a) Draw …
3. ### physics

A 450 g block is at rest on a horizontal surface. The coefficient of static friction between the block and the surface is 0.670, and the coefficient of kinetic friction is 0.380. A force of magnitude P pushes the block forward and …
4. ### physics

A 450 g block is at rest on a horizontal surface. The coefficient of static friction between the block and the surface is 0.670, and the coefficient of kinetic friction is 0.380. A force of magnitude P pushes the block forward and …
5. ### physics

A 3.24 kg block located on a horizontal frictionless floor is pulled by a cord that exerts a force F=11.8N at an angle theta=23.0degrees above the horizontal, as shown. What is the magnitude of the acceleration of the block when the …
6. ### Physics.

A block of mass 3m is placed on a frictionless horizontal surface, and a second block of mass m is placed on top of the first block. The surfaces of the blocks are rough. A constant force of magnitude F is applied to the first block. …
7. ### Physics

A block of mass 3m is placed on a frictionless horizontal surface, and a second block of mass m is placed on top of the first block. The surfaces of the blocks are rough. A constant force of magnitude F is applied to the first block …
8. ### Physics

A block whose weight is 34.0 N rests on a horizontal table. A horizontal force of 27.2 N is applied to the block. The coefficients of static and kinetic friction are 0.600 and 0.472, respectively. Will the block move under the influence …
9. ### physics

Two blocks connected by a string are pulled across a rough horizontal surface by a force applied to one of the blocks, as shown. The acceleration of gravity is 9.8 m/s Block 1 has a mass of 5 kg while block two has a mass of 8 kg. …
10. ### PHYSICS

A 6.53-kg block initially at rest is pulled to the right along a horizontal surface by a constant force of 125 N applied at an angle θ above the horizontal. The coefficient of kinetic friction between the block and the horizontal …

More Similar Questions