Physics Unit 3 (Newtons Laws)

Is it possible for an object to be moving if no net force is acting on it? Explain.

Yes, it can be moving at a constant velocity

If an object remains at rest, can you conclude that no forces are acting on it?

No, only that the net force is zero.

What determines how much inertia an object has?

Mass – more mass = more inertia

Is inertia a type of force? Why or why not?

Inertia is not a type of force because forces cause acceleration, and inertia is the tendency to resist acceleration

Why does your body tend to lunge forward when you are riding in a car and the driver suddenly slams on the brakes? Which of Newton’s laws applies most directly to this situation?

Your body has inertia, so it will tend to continue moving at the same velocity it was moving at before the car slowed down. This is an example of Newton’s 1st Law

What will happen to an object when all the forces acting on it do not balance each other out?

The object will accelerate in the direction of the net force

What is terminal velocity? Why will a falling object eventually reach a certain terminal velocity as it falls through the air?

Terminal velocity is the maximum velocity an object can reach as it falls through the air. As an object falls through the air, it accelerates towards the ground, and the force of air resistance increases (the faster it moves, the larger the air resistance will be). Eventually the air resistance force reaches the same value as the downward force of gravity (the weight). At this point, the forces will be balanced (equilibrium), so the object will no longer accelerate. Since it is no longer accelerating, the air resistance will not become any larger, and the object will continue to fall with a constant velocity (the terminal velocity).

When a baseball player hits a baseball, the bat exerts a force on the ball, and the ball exerts an equal force back on the bat in the opposite direction. Even though these forces are equal in magnitude but opposite in direction, the ball still goes flying. Why do these equal but opposite forces not cancel each other out?

The forces are acting on different objects. Since the ball experiences only one of these forces, it will accelerate away from the bat.

When a gun fires a bullet, the gun exerts a force on the bullet, and the bullet exerts an equal and opposite force back on the gun. If these two forces are equal, why is it that the bullet moves so much faster than the gun?

Since the bullet has much less mass than the gun, it experiences a greater acceleration

What are two ways you can increase the friction between two surfaces?

Change the characteristics of the surfaces (e.g. make them rougher, coat them with materials that have higher coefficients of friction); increase the amount of normal force between the surfaces

What force does gravity exert on a 100 kg object?

Fg = 980 N

The forces acting on a motorboat are 390 N north, 180 N east, and 230 N at 35 south of west. (There’s also a downward force of gravity downward and an upward force from the water keeping the boat afloat, but they cancel out.) The motorboat has a mass of 950 kg. a. What is the net force acting on the motorboat? (magnitude and direction) b. What is the acceleration of the motorboat? (magnitude and direction) c. How much does the motorboat weigh?

a. Fnet = 260 N at 88 north of west b. a = 0.27 m/s2 at 88 north of west c. Fg = 9310 N

You're trying to lift a heavy box and exert a force of 400 N upward on it (but it's not enough!). If the box has a mass of 50 kg, what normal force is the floor exerting on the box?

FN = 90 N

A 5.0 kg bucket of water is raised from a well by a rope. a. If the upward acceleration of the bucket is 3.0 m/s2 , find the tension force exerted by the rope on the bucket of water. b. If the tension force is decreased to 15 N, what will be the acceleration of the bucket (magnitude and directio

a. FT = 64 N b. a = 6.8 m/s2 downward

Starting from rest, a 65 kg skateboarder rolls down a 3.0 m long ramp in 2.0 s. a. What is his acceleration? (magnitude only) b. What amount of net force is acting on him to accelerate him down the ramp?

a. a = 1.5 m/s2 b. Fnet = 97.5 N

The coefficient of friction between rubber and concrete is 0.8. If it takes 300 N of force to keep a tire sliding at a constant velocity along the street, how massive is the tire?

m = 38.3 kg

An object has a mass of 50 kg and a coefficient of static friction equal to 0.7. If I push on the object with a force of 200 N, what is the force of static friction?

Ff = 200 N (I haven’t exceeded the maximum static friction force yet, so the static friction force will only go large enough to match my applied force)

How hard do you have to push on a 40 kg object to move it at a constant velocity if μ = 0.3? How hard do you have to push to accelerate it at 3.5 m/s2?

Fpush = 118 N (at constant velocity); Fpush = 258 N (at 3.5 m/s2 of acceleration)

A 22 kg pet rock is attached to a leash and pulled on with a force of 45 N at an angle of 25° above the horizontal. As a result, the rock slides sideways at a constant velocity. How big is the normal force that the ground exerts on the rock? What is the coefficient of friction between the rock and the ground as it slides?

FN = 197 N;  = 0.207

A 750-kg elevator experiences 9,000 N of tension force from the cables as it passes the 4th floor. a. What is the net force acting on this elevator? b. Is it possible that this elevator is currently moving up? If so, is it maintaining a constant velocity, speeding up, or slowing down? c. Is it possible that this elevator is currently moving down? If so, is it maintaining a constant velocity, speeding up, or slowing down?

a. Fnet = 1650 N upward b. yes, it must be speeding up c. yes, it must be slowing down

A 45 kg crate is pushed up a 10 incline with a force of 300 N. If the coefficient of friction between the crate is 0.4, what is the acceleration of the crate?

a = 1.1 m/s2

A 15 kg object is on a 35º frictionless incline that is 20 m long. a. Calculate the object’s weight. b. Calculate the normal force exerted on the object by the incline. c. Calculate the amount of force pushing the object down the incline. d. What is the acceleration of the object down the incline? e. What would the acceleration be if there IS friction, and μ = 0.3? f. Assuming it started from rest, how long would it take the object to slide to the bottom (with friction)?

a. Fg = 147 N b. FN = 120 N c. Fg,parallel = 84.3 N d. a = 5.6 m/s2 e. 3.2 m/s2 e. 3.5 s