Question

true/false: select a for true, and b for false. mark your scantron.

1. for any two surfaces in contact, the coefficient of kinetic friction is usually greater than that of static friction
2. any object with mass also has inertia.
3. air resistance depends on the size and velocity of the moving object.
4. the sun pulls harder on the earth than the earth does on the sun.
5. if your tires are already spinning in the snow, spinning them faster will give a greater force of friction.
6. the shape of an object has no effect on its air resistance.
7. rockets work by pushing off the ground behind them.

Explanation:

• 21: The coefficient of static friction is usually greater than that of kinetic friction. So, this statement is false.
• 22: Inertia is the property of an object to resist changes in its state of motion, and it is directly related to mass. Any object with mass has inertia. So, this statement is true.
• 23: Air resistance \(F_d=\frac{1}{2}

ho v^{2}C_dA\) (where \(
ho\) is air density, \(v\) is velocity, \(C_d\) is the drag coefficient related to shape, and \(A\) is the cross - sectional area). So, air resistance depends on the size (related to \(A\)) and velocity (\(v\)) of the moving object. This statement is true.

• 24: According to Newton's third law, the force that the sun exerts on the earth and the force that the earth exerts on the sun are an action - reaction pair. They have the same magnitude. So, this statement is false.
• 25: When tires are spinning in the snow, it is kinetic friction. Spinning them faster does not increase the normal force (which is related to the weight of the vehicle on the snow, assuming no vertical acceleration). Since \(F_f = \mu_kN\) (where \(\mu_k\) is the coefficient of kinetic friction and \(N\) is the normal force), the frictional force does not increase. This statement is false.
• 26: As mentioned in the formula for air resistance \(F_d=\frac{1}{2}

ho v^{2}C_dA\), the shape (related to \(C_d\)) affects air resistance. So, this statement is false.

• 27: Rockets work based on the principle of conservation of momentum. They expel gas at high speed in one direction, and the reaction force propels them forward. They do not push off the ground (in space, there is no ground to push off). So, this statement is false.

Answer:

1. B. False
2. A. True
3. A. True
4. B. False
5. B. False
6. B. False
7. B. False