07 newton's law of motion

Newton’s Laws of Motion Topic 4 (cont.)

Lecture Outline ,[object Object],[object Object],[object Object],[object Object]

Weight and Normal Force ,[object Object],[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object]

[object Object],[object Object],[object Object],[object Object],[object Object]

Example 4-7: Accelerating the box. What happens when a person pulls upward on the box in the previous example with a force greater than the box’s weight, say 100.0 N?

Free-Body Diagrams ,[object Object],[object Object]

A hockey puck is sliding at constant velocity across a flat horizontal ice surface that is assumed to be frictionless. Which of these sketches is the correct free-body diagram for this puck? What would your answer be if the puck slowed down?

[object Object],[object Object],[object Object]

[object Object],ConcepTest 4.7a Gravity and Weight I 1) F g is greater on the feather 2) F g is greater on the stone 3) F g is zero on both due to vacuum 4) F g is equal on both always 5) F g is zero on both always

[object Object],The force of gravity (weight) depends on the mass of the object!! The stone has more mass, therefore more weight. ConcepTest 4.7a Gravity and Weight I 1) F g is greater on the feather 2) F g is greater on the stone 3) F g is zero on both due to vacuum 4) F g is equal on both always 5) F g is zero on both always

[object Object],1) it is greater on the feather 2) it is greater on the stone 3) it is zero on both due to vacuum 4) it is equal on both always 5) it is zero on both always ConcepTest 4.7b Gravity and Weight II

[object Object],The acceleration is given by F/m so here the mass divides out. Since we know that the force of gravity (weight) is mg , then we end up with acceleration g for both objects. 1) it is greater on the feather 2) it is greater on the stone 3) it is zero on both due to vacuum 4) it is equal on both always 5) it is zero on both always ConcepTest 4.7b Gravity and Weight II Follow-up: Which one hits the bottom first?

ConcepTest 4.8 On the Moon An astronaut on Earth kicks a bowling ball and hurts his foot. A year later, the same astronaut kicks a bowling ball on the Moon with the same force. His foot hurts... 1) more 2) less 3) the same Ouch!

The masses of both the bowling ball and the astronaut remain the same, so his foot feels the same resistance and hurts the same as before. ConcepTest 4.8 On the Moon An astronaut on Earth kicks a bowling ball and hurts his foot. A year later, the same astronaut kicks a bowling ball on the Moon with the same force. His foot hurts... 1) more 2) less 3) the same Follow-up: What is different about the bowling ball on the Moon? Ouch!

[object Object],ConcepTest 4.9a Going Up I 1) N > mg 2) N = mg 3) N < mg (but not zero) 4) N = 0 5) depends on the size of the elevator m v

[object Object],The block is moving at constant speed, so it must have no net force on it. The forces on it are N (up) and mg (down), so N = mg , just like the block at rest on a table. ConcepTest 4.9a Going Up I 1) N > mg 2) N = mg 3) N < mg (but not zero) 4) N = 0 5) depends on the size of the elevator m v

[object Object],1) N > mg 2) N = mg 3) N < mg (but not zero) 4) N = 0 5) depends on the size of the elevator ConcepTest 4.9b Going Up II m a

[object Object],The block is accelerating upward, so it must have a net upward force . The forces on it are N (up) and mg (down), so N must be greater than mg in order to give the net upward force ! 1) N > mg 2) N = mg 3) N < mg (but not zero) 4) N = 0 5) depends on the size of the elevator  F = N – mg = ma > 0  N > mg ConcepTest 4.9b Going Up II Follow-up: What is the normal force if the elevator is in free fall downward? m a > 0 mg N

[object Object],ConcepTest 4.10 Normal Force 1) case 1 2) case 2 3) it’s the same for both 4) depends on the magnitude of the force F 5) depends on the ice surface Case 1 Case 2

[object Object],In Case 1, the force F is pushing down ( in addition to mg ), so the normal force needs to be larger . In Case 2, the force F is pulling up , against gravity, so the normal force is lessened . ConcepTest 4.10 Normal Force 1) case 1 2) case 2 3) it’s the same for both 4) depends on the magnitude of the force F 5) depends on the ice surface Case 1 Case 2

[object Object],ConcepTest 4.11 On an Incline 1) case A 2) case B 3) both the same ( N = mg ) 4) both the same (0 < N < mg ) 5) both the same (N = 0) A B

[object Object],1) case A 2) case B 3) both the same ( N = mg ) 4) both the same (0 < N < mg ) 5) both the same ( N = 0) N W W y x y f   ConcepTest 4.11 On an Incline In Case A , we know that N = W . In Case B , due to the angle of the incline, N < W . In fact, we can see that N = W cos(  ). A B

[object Object],ConcepTest 4.12 Climbing the Rope 1) this slows your initial velocity, which is already upward 2) you don’t go up, you’re too heavy 3) you’re not really pulling down – it just seems that way 4) the rope actually pulls you up 5) you are pulling the ceiling down

[object Object],When you pull down on the rope, the rope pulls up on you!! It is actually this upward force by the rope that makes you move up! This is the “ reaction ” force (by the rope on you ) to the force that you exerted on the rope . And voilá, this is Newton’s Third Law. ConcepTest 4.12 Climbing the Rope 1) this slows your initial velocity, which is already upward 2) you don’t go up, you’re too heavy 3) you’re not really pulling down – it just seems that way 4) the rope actually pulls you up 5) you are pulling the ceiling down

[object Object],1) the bowling ball exerts a greater force on the ping-pong ball 2) the ping-pong ball exerts a greater force on the bowling ball 3) t he forces are equal 4) t he forces are zero because they cancel out 5) there are actually no forces at all ConcepTest 4.13a Bowling vs. Ping-Pong I F 12 F 21

[object Object],The forces are equal and opposite by Newton’s Third Law! ConcepTest 4.13a Bowling vs. Ping-Pong I 1) the bowling ball exerts a greater force on the ping-pong ball 2) the ping-pong ball exerts a greater force on the bowling ball 3) t he forces are equal 4) t he forces are zero because they cancel out 5) there are actually no forces at all F 12 F 21

[object Object],1) they do not accelerate because they are weightless 2) accels. are equal, but not opposite 3) accelerations are opposite, but bigger for the bowling ball 4) accelerations are opposite, but bigger for the ping-pong ball 5) accelerations are equal and opposite ConcepTest 4.13b Bowling vs. Ping-Pong II F 12 F 21

[object Object],The forces are equal and opposite -- this is Newton’s Third Law!! But the acceleration is F/m and so the smaller mass has the bigger acceleration . 1) they do not accelerate because they are weightless 2) accels. are equal, but not opposite 3) accelerations are opposite, but bigger for the bowling ball 4) accelerations are opposite, but bigger for the ping-pong ball 5) accelerations are equal and opposite ConcepTest 4.13b Bowling vs. Ping-Pong II Follow-up: Where will the balls meet if they are released from this position? F 12 F 21

[object Object],ConcepTest 4.14a Collision Course I 1) the car 2) the truck 3) both the same 4) it depends on the velocity of each 5) it depends on the mass of each

[object Object],ConcepTest 4.14a Collision Course I 1) the car 2) the truck 3) both the same 4) it depends on the velocity of each 5) it depends on the mass of each According to Newton’s Third Law, both vehicles experience the same magnitude of force.

[object Object],ConcepTest 4.16a Tension I 1) 0 N 2) 50 N 3) 100 N 4) 150 N 5) 200 N

[object Object],The tension in the rope is the force that the rope “feels” across any section of it (or that you would feel if you replaced a piece of the rope). Since you are pulling with a force of 100 N , that is the tension in the rope. ConcepTest 4.16a Tension I 1) 0 N 2) 50 N 3) 100 N 4) 150 N 5) 200 N

[object Object],1) 0 N 2) 50 N 3) 100 N 4) 150 N 5) 200 N ConcepTest 4.16b Tension II

[object Object],This is literally the identical situation to the previous question. The tension is not 200 N !! Whether the other end of the rope is pulled by a person, or pulled by a tree, the tension in the rope is still 100 N !! 1) 0 N 2) 50 N 3) 100 N 4) 150 N 5) 200 N ConcepTest 4.16b Tension II

[object Object],1) you and your friend each pull on opposite ends of the rope 2) tie the rope to a tree, and you both pull from the same end 3) it doesn’t matter -- both of the above are equivalent 4) get a large dog to bite the rope ConcepTest 4.16c Tension III

[object Object],Take advantage of the fact that the tree can pull with almost any force (until it falls down, that is!). You and your friend should team up on one end, and let the tree make the effort on the other end. 1) you and your friend each pull on opposite ends of the rope 2) tie the rope to a tree, and you both pull from the same end 3) it doesn’t matter -- both of the above are equivalent 4) get a large dog to bite the rope ConcepTest 4.16c Tension III

07 newton's law of motion

Recomendados

Recomendados

Más contenido relacionado

La actualidad más candente

La actualidad más candente (20)

Destacado

Destacado (10)

Similar a 07 newton's law of motion

Similar a 07 newton's law of motion (20)

Más de IZZUDIN IBRAHIM

Más de IZZUDIN IBRAHIM (16)

Último

Último (20)

07 newton's law of motion

Notas del editor