I don't know what to do. I asked this yesterday but got no response and I still don't know what to do :(
A child is sitting on a stationary skateboard with a pile of rocks which she throws in one direction in order to make the skateboard travel in the opposite direction. If the rocks each have a mass of 0.70 kg and she can throw them with a speed of 13 m/s relative to the ground, determine the number of rocks she must throw per minute in order to maintain a constant average speed against a 2.0 N force of friction. (Note: because this is a rate, the answer may not be a whole number.)
To determine the number of rocks the child must throw per minute to maintain a constant average speed against the force of friction, we can use the principle of conservation of momentum.
Let's break down the problem:
1. First, determine the initial momentum of the child and the skateboard system.
- The initial momentum is the total momentum before any rocks are thrown.
- Since the child is sitting on a stationary skateboard, the initial momentum is zero.
2. Secondly, we need to determine the momentum change required to maintain a constant average speed against the force of friction.
- The force of friction is given as 2.0 N.
- To counteract this force and maintain a constant average speed, the child needs to throw rocks in the opposite direction.
3. Calculate the momentum of a single rock:
- The momentum of an object is given by the formula P = mv, where P is momentum, m is mass, and v is velocity.
- Each rock has a mass of 0.70 kg and is thrown with a speed of 13 m/s relative to the ground.
- Calculate the momentum of a single rock using P = (0.70 kg) * (13 m/s).
4. Determine the number of rocks needed per minute:
- Divide the momentum change required (opposing the force of friction) by the momentum of a single rock.
- Since the question asks for the answer in terms of rocks thrown per minute, we need to ensure the momentum change required and the momentum of a single rock have the same units.
- Convert the force of friction from Newtons to kg·m/s² by dividing by the acceleration due to gravity (9.8 m/s²).
- Divide the converted force by the momentum of a single rock to get the number of rocks per second.
- Multiply the number of rocks per second by 60 to get the number of rocks per minute.
By following these steps, you should be able to determine the number of rocks the child must throw per minute to maintain a constant average speed against the force of friction.