Question

forces and friction test

1. when a rifle is fired, the bullet leaves the end of the barrel faster than the rifle is thrown backward because

a. there is a greater force on the bullet than the rifle
b. there is a greater force on the rifle than the bullet
c. the force is equal, but the bullet has more inertia
d. the force is equal, but the bullet has less inertia

1. a spaceship is towing 4 pods behind it, which are attached with a very light fiber. the ship and all the pods are accelerating to the right. which fiber will have the greatest tension on it?
2. what are the units of the coefficient of friction?

a. n
b. 1/n
c. n²
d. the coefficient is unitless

1. every action force is accompanied by an equal and opposite reaction force. the reason that these forces do not cancel each other is...

a. action forces last longer
b. the two forces are not always in the same direction
c. one of the two forces is always greater
d. the two forces act on different bodies
e. they do always cancel; all objects are in equilibrium

1. the acceleration of gravity is less on the moon. if you went to the moon:

a. your weight and mass would be less
b. your weight would be less but your mass would remain the same.
c. your weight would remain the same but your mass would be less.
d. both your mass and weight would remain the same.

1. a box with a weight of 50 n is sitting on a rough surface and is pulled at an angle diagonally to the horizontal. which of the following statements is true regarding the normal force between the box and the surface?

a. the normal force is 50 n.
b. the normal force must be more than 50 n due to the applied force.
c. the normal force must be less than 50 n due to the applied force.
d. the normal force cannot be calculated unless either the coefficient of friction or the acceleration of the box is known.

Explanation:

• Question 9: According to Newton's third law, the force on the bullet and the rifle is equal. But the bullet has less inertia (mass - related to inertia, \(F = ma\), for the same force \(F\), smaller \(m\) leads to larger \(a\)).
• Question 10: Using \(F = ma\), for fiber \(D\), the mass it needs to pull is \(m=(200 + 300+100 + 200)\text{kg}=800\text{kg}\), \(F_D=800\times3 = 2400\text{N}\); for fiber \(C\), \(m=(300 + 100+200)\text{kg}=600\text{kg}\), \(F_C=600\times3=1800\text{N}\); for fiber \(B\), \(m=(100 + 200)\text{kg}=300\text{kg}\), \(F_B=300\times3 = 900\text{N}\); for fiber \(A\), \(m = 200\text{kg}\), \(F_A=200\times3=600\text{N}\). So fiber \(D\) has the greatest tension.
• Question 11: The formula for frictional force is \(F_f=\mu F_N\), where \(F_f\) (frictional force) and \(F_N\) (normal force) have units of Newtons (\(N\)). So \(\mu=\frac{F_f}{F_N}\), and the units cancel out, making the coefficient of friction unitless.
• Question 12: Action - reaction forces act on different bodies. For example, when you push a wall (\(F_{you - wall}\)), the wall pushes you back (\(F_{wall - you}\)). One acts on the wall, and the other acts on you.
• Question 13: Mass is an intrinsic property of an object (\(m\), amount of matter). Weight \(W=mg\). On the moon, \(g\) (acceleration due to gravity) is less, so \(W\) is less, but \(m\) remains the same.
• Question 14: The normal force \(N\) in the vertical direction: \(N+F\sin\theta=mg\) (where \(F = 40\text{N}\), \(\theta = 30^{\circ}\), \(mg = 50\text{N}\)). \(N=mg - F\sin\theta=50-40\times\frac{1}{2}=50 - 20=30\text{N}<50\text{N}\)

Answer:

• Question 9: d
• Question 10: Fiber \(D\)
• Question 11: d
• Question 12: d
• Question 13: b
• Question 14: c