Mechanics | Relativity Answers

1. Two crates, A and B, sit at rest side by side on a frictionless horizontal surface. The crates have masses mA and mB. A horizontal force is applied to crate A and the two crates move off to the right. (a) Draw clearly labelled free-body diagrams for crate A and for crate B. Indicate which pairs of forces, if any, are third-law action-reaction pairs. (b) If the magnitude of force is less than the total weight of the two crates, will it cause the crates to move? Explain.

1. An athlete throws a ball of mass m directly upward, and it feels no appreciable air resistance. Draw a free-body diagram of this ball while it is free of the athlete's hand and (a) moving upward; (b) at its highest point; moving downward. (d) Repeat parts a, b, and c if the athlete throws the ball at a 60-degree angle above the horizontal instead of directly upwards.

1. We know that Earth pulls on the moon. Does the moon pull on Earth? If so, which pull is stronger?

1. If the action is a bowstring acting on an arrow, what is the reaction force?
2. A person stands still. The upward support of the floor is a reaction force. What is the action force?

1. When you walk along a floor, what pushes you along?
2. When swimming, you push the water backward – call this action. What is the reaction?

1. You are driving a car at a red stoplight. A small stuff toys hangs from your rear-view mirror. As you wait inside the car, the stuff toy is at rest relative to you. Gravity pulls the stuff toy down while the string pulls it up. These two forces are equal, resulting to a zero total force. The stoplight signals "Go" and you step on the gas. The stuff toy swings towards the back of the car. Is Newton's First Law applicable on the stuff toy?
2. Let say, an observer is standing on the sidewalk watching your car in number 1. As it sits at the red light the stuff toy is stationary relative to you. When the light turns green and the car begins to accelerate to the right, the stuff toy remains stationary relative to you until the force exerted on them by the string forces them to follow the motion of the car. Is Newton's law of inertia tru to the observer? What about to you who is sitting in the car?

1. Suppose you are in a bus that is travelling on a straight road and speeding up. If you could stand in the aisle on roller skates, you would start moving backward relative to the bus as the bus gains speed. If instead the bus was slowing to a stop, you would start moving forward down the aisle. Is Newton's law valid? Explain.
2. You are driving a car at a red stoplight. A small stuff toys hangs from your rear-view mirror. As you wait inside the car, the stuff toy is at rest relative to you. Gravity pulls the stuff toy down while the string pulls it up. These two forces are equal, resulting to a zero total force. The stoplight signals "Go" and you step on the gas. The stuff toy swings towards the back of the car. Is Newton's First Law applicable on the stuff toy?

1. A 3.0 meters long solid shaft makes an angle of 35 degrees with the horizontal. A vertical force of 50 N
is applied 0.60 m from the upper end. What is the magnitude of the torque due to the vertical force
about each end?

2. A Ladder of 6.0 meters in length weighs 150 N rests on horizontal ground and leans at an angle of 65
degrees with the horizontal against a smooth vertical wall. How far up the ladder may a 700 N person
go before the ladder slips? The coefficient of friction between ladder and ground is 0.40.

3. A Flat bar is loaded with boxes as shown in figure at
right. Box M = 50 kg, what must be the magnitude
of mass m so that the bar becomes level? Neglect
the weight of the bar. What torque about each end
of the bar?

How pressure depends on fermi energy write only expression for 1d

Draw a free body diagram of a 16,000kg bus accelerating at 2 m/s2 with a Ff of 20,000N from a stop sign. Remember Fg=mg and Fnet=ma

Find Fa to the bus from the road