A sled of mass 50kg is pulled along snow-covered, flat ground. The static friction coefficient is 0.3, and the sliding friction coefficient is 0.1.

a) What force is needed to start the sled moving?

b) What force is needed to keep the sled moving at a constant velocity?

c) Once moving, what total force must be applied to the sled to accelerate it at 3m/s^2?

Can you please show all your working.... and explain what you are doing so.... i can easily understand... when similar problems come up on my test tomorrow!

THANKS

*Posted: June 14, 2013*

To start, you should draw a free body diagram of the sled and label all the forces. There will be four forces. The weight of the sled due to gravity, the normal force of the snow pushing up on the sled, the force to pull the sled, and the frictional force. The weight of the sled and the normal force will be the same in magnitude, but opposite in direction.

The weight is 50 kg * 9.81 N/kg, which results in a normal force close to 500N.

To determine the force to get the sled moving, you need to take the normal force and multiple it by the coefficient of static friction. This results in about 150N. That means you would have to pull with a minimum of 150 N to get the sled going.

Once moving, the friction decreases, and you will not need to use as much force. To determine the friction force when moving, multiply the normal force by the sliding coefficient. That should be about 50N.

Now to accelerate the sled, you will have to increase the force. F = m*a. Therefore, to accelerate it at 3m/s2, it requires an extra 150 N (50kg*3). Therefore you need roughly 200N.

Note: for simplicity I used 10 m/s instead of 9.81 for the gravitational constant. Just makes the rough math easier.