Physics ch 4 newtons laws

newtons first law

An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an net force.

What is force measured in?

Newtons

Newton's 2nd law indicates that when a net force acts on an object, it must accelerate. Does this mean that when 2+ forces are applied to an object it must accelerate?

No because 2 or more forces can cancel each other, leading to a net force of zero

If a car weighs 1850kg and 1 person is pushing the car with 275N of force and another is exerting 395N, while the friction force is exerting 560N of force, what is the acceleration

+.059 m/s^2

A father and his 7 year old daughter are facing each other on ice skates. With their hands, they push off against one another. Which one or more of the following statements is true 1. each experiences an acceleration that has a different magnitude 2. each experiences an acceleration of the same magnitude 3. Each experiences a pushing force that has a different magnitude 4. Each experiences a pushing force that has the same magnitude

1+4 1. each experiences an acceleration that has a different magnitude 4. Each experiences a pushing force that has the same magnitude

Which of Newton's Laws explains why you lunge forward when a roller coaster suddenly comes to a stop? 1. Law of Inertia (1st Law) 2. Law of Acceleration (2nd Law) 3. Law of Interaction/Reciprocity (3rd Law) 4. Law of Universal Friction (4th Law) 5. No law... it's just because you have to puke.

1. Law of Inertia (1st Law) - Landing at a lower height increases the overall "hang time" of the flight. Therefore the projectile may be fired more horizontally in order to move as far sideways as possible during this increased hang time.

What type of friction typically makes it possible for your car to accelerate forwards from rest? 1. static friction 2. kinetic friction 3. rolling friction 4. drag

1. static friction

When you first throw a ball up in the air, what force(s) are acting on it immediately after it leaves your hand? 1. Gravity 2. Gravity and air resistance 3. Gravity, air resistance and some force from your hand 4. The force of your hand only 5. The answer depends on your Jedi abilities

2. Gravity and air resistance - The object retains no "memory" of the force from your hand after it leaves contact with it, it is only the forces of gravity and air resistance which are acting on it.

Which of Newton's Laws explains why it is easier to push an empty grocery cart than a full one. 1. Law of Inertia (1st Law) 2. Law of Acceleration (2nd Law) 3. Law of Interaction/Reciprocity (3rd Law) 4. Law of Universal Friction (4th Law) 5. No law... it isn't any easier because the cart has a sticky wheel

2. Law of Acceleration (2nd Law)

If you are standing on a scale in an elevator, what happens when the elevator accelerates downward? 1. Your apparent weight increases 2. Your apparent weight decreases 3. Your apparent weight remains unchanged 4. more info is needed

2. Your apparent weight decreases

An object is moving at a constant velocity. All but 1 of the statements could be true. Which on cannot be true? 1. no forces act on the object 2. a single force acts on the object 3. two forces are simultaneously acting on the object 4. 3 forces are simultaneously acting on the object

2. a single force acts on the object

Two ice skaters, Paul and Tom, are each holding on to opposite ends of the same rope. Each pulls the other toward him. The magnitude of paul's acceleration is 1.25 times greater than the magnitude of toms acceleration. What is the ratio of Paul's mass to Tom's mass? 1. 0.67 2. 0.8 3. 0.25 4. 1.25 5. 0.5

2.) 0.8 1.25 = 5/4 * acceleration is inversely proportional to mass - inverse of 5/4 is 4/5 which equals .8

Two forces act on a moving object that has a mass of 27kg. One force has a magnitude of 12 N and points due south, while the other force has a magnitude of 17N and points due west. What is the acceleration of the object? 1. 0.63 m/s^2 @ 55 deg S of W 2. 0.44 m/s^2 @ 24 deg S of W 3. 0.77 m/s^2 @ 35 deg S of W 4. 0.77 m/s^2 @ 55 deg S of W 5. 1.1 m/s^2 @ 35 deg S of W

3. 0.77 m/s^2 @ 35 deg S of W

All of the following cause acceleration to double, Which one is the exception? 1. all forces acting on the object double 2. the net force acting on the object doubles 3. Both net force on the object and the mass of the object double 4. the net force acting on the object remains the same, while the mass of the object is reduced by a factor of 2

3. Both net force on the object and the mass of the object double

An object is rolling East when acted upon abruptly and for a short duration by a force directed North, what is the most likely direction of the object after the kick? 1. North 2. East 3. Northeast 4. South

3. Northeast - Since the kick is abrupt and of short duration, the ball will be deflected a bit from it's original path, but will continue rolling in a straight line afterwards.

The net external force acting on an object is zero. Which one of the following statements is true? 1. The object can only be stationary 2. The object can only be traveling with a constant velocity 3. The object can either be stationary or traveling with a constant velocity 4. the object can only be traveling with a velocity that is stationary

3. The object can either be stationary or traveling with a constant velocity

A 10 kg object sits on an inclined plane tilted at 30 degrees to the horizontal, what is the approximate value of the normal force? 1. 50N 2. 10N 3. 100N 4. 87N

4) 87N - Mass* Gravity* cos(30) The normal force is equal to the vertical component of the weight, given by mgcos30

In Case A an object is moving straight downward at with a constant speed of 9.8 m/s while in case B an object is moving straight downward with a constant acceleration magnitude 9.8 m/s^2. Which of the following is true? 1. A nonzero net external force acts on the object in both cases 2. A nonzero net external force acts on the object in neither case 3. A nonzero net external force acts on the object in Case A only 4. A nonzero net external force acts on the object in Case B only

4. A nonzero net external force acts on the object in Case B only

If no net force is acting on an object, it will... 1. Remain stationary 2. Move with a constant acceleration 3. Move with a constant velocity 4. Remain stationary OR move with a constant velocity 5. Freak out

4. Remain stationary OR move with a constant velocity

Which of the following is true, according to Newton's laws of motion, ignoring friction? 1. an SUV hits a stationary motorcycle. Since its stationary the the motorcycle sustains a greater force than the SUV 2. A semitrailer truck crashes all the way through the wall. Since the wall collapses, the wall sustains a greater force than the truck 3. Sam and his sister go ice skating. They push off against one another and fly apart. Sam flies off with the greater acceleration 4. Two astronauts on a space walk are throwing a ball back and forth between each other. In this game of catch the distance between them remains constant 5. None of the above are true according to the third law.

5. None of the above are true according to the third law.

what is force

A push or a pull

Object Mass Speed ----------------------------- A. 2. 6 B. 4. 5 C 6. 4 D. 8. 2 The table above lists the speeds and masses of four different objects. Which one has the greatest inertia? A B C D

D

Force equation

F=ma (force = mass x acceleration)

T/F: A mosquito collides with a car windshield, the car exerts a much larger force on the mosquito than the mosquito exerts on the car.

False - By Newton's third law, the forces the bug and car windshield exert on each other will be equal in magnitude but opposite in direction.

T/F: Kinetic friction is generally greater than static friction.

False - Static friction, the friction which must be overcome to set a stationary object into motion, is generally greater than kinetic friction, the friction which must be overcome to keep an object in motion.

T/F: If you kick a bowling ball in gravity free space, you won't stub your toe because it has no weight

False - The bowling ball still has mass, and therefore inertia, you will stub your toe.

T/F: The normal force will always be equal and opposite of the weight (or gravity) vector.

False - While it is possible the normal force is equal and opposite to the weight, there is no guarantee it is. There are many instances, such as when someone is pushing down or pulling up on an object, or the object is on an inclined plane (ramp), that the normal force will differ from the weight.

T/F: If a force is acting upon you, you will accelerate.

False - While possible, it depends on whether that force is a net force. If the force is unbalanced by another force(s), you will accelerate. Simply, a force acting is no guarantee of acceleration. I know, this was tricky and now you're mad, but at least you understand the concept.

A bucket is accelerated upward by a rope. Which of the following expressions best describes the net force acting on the bucket? 1. T 2. T-mg 3. mg-T 4. mg

T-mg The net force is the difference between the tension and weight.

Newton's 2nd Law of Motion

The acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

net force

The combination of all forces acting on an object

T/F: Friction is caused by intermolecular forces.

True - At the atomic level, friction is caused by electrostatic repulsion and attraction.

T/F: If you double the mass of an object that is being acted upon by a constant net force, its acceleration will be halved.

True - By Newton's 2nd Law, the acceleration is inversely proportional to the mass of the object.

T/F: An object of mass 5 kg at rest has more inertia than an object of mass 1 kg moving at 10 m/s

True - Inertia is dependant only on mass and is independent of velocity

T/F: The coefficient of friction is equal to the ratio of the frictional force to the normal force.

True - Yes, this is the definition of coefficient of friction.

T/F: You are riding on a CATA bus when it suddenly brakes to a stop, you fall forward because your body is trying to stay in motion.

True - Your body has mass and therefore inertia and desires to maintain the state of motion it was in.

Object A weighs twice as much as Object B at the same spot on earth. Would the same be true at a given spot on Mars?

Yes, because the ratio of the two weights depends only on the masses of the objects, which are the same on earth and mars.

What is the mass on Mercury of an object that weighs 784N on the earth's surface? a) 80kg b) 48kg c) 118kg d) 26kg e) not enough info

a- 80kg F=ma m=F/a. -----> m=784/9.8 =80

acceleration equation

a=f/m

which of the following statements can be explained by newtons first law? 1. when your car suddenly comes to a halt, you lunge forward 2. when your car rapidly accelerates, you are pressed backwards into the seat a) neither a or b b) both a and b c) a but not b d) b but not a

b) both a and b

A cup of coffee is sitting table in a RV. The cup slides towards the rear of the RV. According to newton's first law, which of the following statements could describe the motion of the RV 1. the RV is at rest and the driver suddenly accelerates 2. the RV is moving forward and the diver suddenly accelerated 3. the RV is moving backward and the diver suddenly hits the breaks a) 1 b) 2 c) 3 d) 1+2 e) 1+2+3

e) 1+2+3

correlation between mass and inertia

greater mass=greater inertia

correlation between force and acceleration

greater the force, greater the acceleration * 2X the force=2X acceleration

mass is a quantitative measure of

inertia

Inertia

natural tendency of an object to remain at rest or in motion at a constant velocity. - The tendency of an object to resist a change in motion

twice the mass = ____ the acceleration

twice the mass = 1/2 the acceleration

which part of the component is normal force

vertical component