How can this sled move if the forces are all balanced? Example 4-5, showing only horizontal forces. Seventy-year-old Michelangelo has selected a fine block of marble for his next sculpture. Shown here is his assistant pulling it on a sled away from the quarry. Forces on the assistant are shown as red (magenta) arrows. Forces on the sled are purple arrows. Forces acting on the ground are orange arrows. Action-reaction forces that are equal and opposite are labeled by the same subscripts but reversed (such as FGA and FAG) and are of different colors because they act on different objects.

To determine if the assistant moves or not, we must consider only the forces on the assistant.

NORMAL FORCE AND DIFFERENT SCENARIOS (a) A box of mass 10.0 kg is resting on the frictionless surface of a table. Determine the weight of the box and the normal force on it. (b) Now you push down on the box with a force of 40.0 N. What is the normal force on the box? Example 4-6. (a) A 10-kg gift box is at rest on a table. (b) A person pushes down on the box with a force of 40.0N. (c) A person pulls upward on the box with a force of 40.0N. The forces are all assumed to act along a line; they are shown slightly displaced in order to be distinguishable. Only forces acting on the box are shown. (c) Then you pull up on the box with a force of 40.0 N. What is the normal force on the box?

What is the acceleration of the box and the normal force of the box? Free Body Diagram (a) Pulling the box, Example 4-11; (b) is the free-body diagram for the box, and (c) is the free-body diagram considering all the forces to act at a point (translational motion only, which is what we have here).

What happens when you pull up on the box with a force of 100.0 N?

Forces in angle and Net force

Find Net force and Draw FBD

Which requires less force, pushing or pulling? (assume the angles are equal)

Why it is easier to push than pull? Notice that F*sinθ acts upwards along with the weight m*g and therefore decreases the normal reaction N. Therefore the frictional force is reduced. Therefore it is easier to Pull than to Push.

TENSION FORCE The tension force is the force that is transmitted through a string, rope, cable or wire when it is pulled tight by forces acting from opposite ends. The tension force is directed along the length of the wire and pulls equally on the objects on the opposite ends of the wire.

TENSION AND Newton’s Laws Find the acceleration of each box Find the Tension in the cord connecting the boxes.

The car is stuck in the mud The car is stuck in the mud. The driver ties a strong rope to the car bumper and the other end to a boulder as shown. The driver pushes at the midpoint of the rope with a force of 300 N. The car just begins to budge with the rope at an angle of 5 degrees. With what force is the rope pulling on the car? Example 4-15. (a) Getting a car out of the mud, showing the forces on the boulder, on the car, and exerted by the person. (b) The free-body diagram: forces on a small segment of rope.

Calculate the acceleration. 40.0 N 10.0 Kg μk = 0.30 When an object is pulled by an applied force (FA) along a surface, the force of friction Ffr opposes the motion. The magnitude of Ffr is proportional to the magnitude of the normal force (FN). If Ff were greater than FPX, what would you conclude?

Suppose in the previous example that the snow is slushy and the skier moves down the 30˚ slope at constant speed. What can you say about the coefficient of kinetic friction?

What is the tension in the two ropes in Picture 1 and Picture 2 Question 1: A rope used to the lift a 50 kg box with and upward acceleration of 2.3m/s2 . What is the tension in the rope? What would be the tension in the rope, if the box descended slowly with a downward accelerated of 0.75 m/s2 Question 2: What is the tension in the two ropes in Picture 1 and Picture 2