A truck is traveling horizontally to the right. When the truck starts to slow down, the crate on the friction-less truck bed starts to slide. In what direction could the net force be on the crate?
no direction; the net force is zero
You are trying to push your stalled car. Although you apply a horizontal force of 400 N to the car, it doesn't budge, and neither do you. Which forces must also have a magnitude of 400 N?
the force exerted by the car on you, the friction force exerted by the car on the road, and the friction force exerted by the road on you
A strongman is able to move a large truck because...
the ground exerts a greater friction force on the strongman than it does on the truck
A bear sling is used for placing backpackers' food out of reach of bears. As the backpacker raises the pack by pulling down on the rope, the force needed...
increases but the rope always sags where the pack hangs
What causes a row boat to move forward?
the force the water exerts on the paddle
A person stands on a scale in an elevator. His apparent weight will be the greatest when the elevator...
is accelerating upward
When a skier skis down a hill, the normal force exerted on the skier by the hill is...
less than the weight of the skier
A golf ball is hit with a golf club. While the ball flies through the air, which forces act on the ball is air resistance is negligible?
the force of gravity on the ball
An object is accelerated by a force of 100 N. Suddenly a second force of 100 N in the opposite direction is exerted on the object, so that the forces cancel. The object...
continues at the velocity it had before the second force was applied
You are pushing a heavy box across a rough floor. When you are initially pushing the box and it is accelerating...
the force you exert on the box is equal to the force of the box pushing back on you
A 50 N crate sits on a floor where the coefficient of static friction between the crate and the floor is .5; a 20 N force is applied t the crate acting to the right. What is the resulting static friction force acting on the crate?
20 N to the left
The normal force on an extreme skier descending a very steep slope can be zero if...
he leaves the slope and no longer touches the snow or if the slope is vertical
To pull an old stump out of the ground, you and a friend tie two ropes to the stump. You pull on it with a force of 500 N to the north while your friend pulls with a force of 450 N to the northwest. The total force from the two ropes is...
less than 950 N
the basic classical laws describing motion
Newton's Laws of Motion
a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.
inertia
if the net force on an object is zero, an object originally at rest remains at rest, and an object in motion remains in motion in a straight line with constant velocity
Newton's 1st Law (Law of Inertia)
the acceleration of an object is directly proportional to the net force acting on it, and inversely proportional to its mass
Newton's 2nd Law
whenever one object exerts a force on a second object, the second object always exerts a force on the first object, which is equal in magnitude but in opposite direction
Newton's 3rd Law
the measure of the inertia of an object
mass
the gravitational force on an object, and is equal to the product of the object's mass and the acceleration of gravity
weight
a vector that can be considered a push or pull, or an action capable of giving rise to acceleration
force
the vector sum of all forces acting on an object
net force
the resistance that one surface or object encounters when moving over another
friction
the force each object exerts in the other perpendicular to their contact surfaces
normal force
formula for friction with a kinetic coefficient
formula for friction with a static coefficient
a force that acts between moving surfaces
kinetic friction
a force that keeps an object at rest; It must be overcome to start moving the object
static friction
a diagram that shows all the forces acting on an object
free body diagram
True or false, acceleration is always the same.
true
True or false, tension is always the same.
true
Why does a child in a wagon seem to fall backward when you give the wagon a sharp pull forward?
The friction between the child and the wagon acts on the child to move them forward, but the friction acts at the contact point so the lower part of the child moves forward with the wagon but the upper part of the child remains stationary, making it look
A box rests on a friction-less bed of a truck. The truck driver starts the truck and accelerates forward. Discuss the motion of the box as seen by someone watching the truck and someone riding on the truck.
The person outside of the truck would see the box remain motionless while the truck moves, and would say there is no force on the box so it does not accelerate according to Newton's first law. The person in the truck would see the box accelerate backwards
If an object is moving, is it possible for the net force acting on it to be zero?
Yes, this would happen if there is no acceleration, but there can be a constant velocity.
If the acceleration of an object is zero, are no forces acting on it?
The vector sum of the forces can be zero, so there could be forces acting on an object without causing it to accelerate.
Only one force acts on an object. Can the object have zero acceleration? Zero velocity?
If only one force acts on an object, the net force cannot be zero, so the object cannot have zero acceleration. It impossible for the object to have zero velocity, but only for an instant.
When a golf ball is dropped to the pavement, it bounces back up. Is a force needed to make it bounce back up? If so, what exerts the force?
Yes a force is needed to bounce the ball back up, because the ball changes direction, or accelerates, and for their to be acceleration, there needs to be a force. The pavement exerts the force on the golf ball.
If you walk along a log floating on a lake, why does the log move in the opposite direction?
As you take a step on the log, your foot exerts a force on the log in the direction opposite to the direction in which you are going and since water has little friction, the log is pushed backwards.
Why do you push down harder on the pedals of a bicycle when first starting out than when moving at constant speed? Why do you need to pedal at all when cycling at constant speed?
When you first start riding a bike, you need to exert a strong force to accelerate the bike and yourself forward and to overcome static friction. Once you are moving at a constant speed you need to exert a force that will equal the opposing forces of fric
A stone hangs by a fine thread from the ceiling, and a section of the same thread dangles from the bottom of the stone. If a person gives a sharp pull on the dangling thread. where is the thread likely to break? What if the person gives a slow and steady
When a large, sudden force is applied to the bottom string, the bottom string will have a large tension in it. Because of the stone's inertia, the upper string does not immediately experience the large force and the bottom string breaks. If you pull slow
The force of gravity on a 2 kg rock is twice as great as that on a 1 kg rock. Why doesn't the heavier rock fall faster?
The acceleration of both rocks is found by dividing their weight (the force of gravity on them) by their mass. The 2 kg rock has a force of gravity on it that is twice as great as the force of gravity on the 1 kg rock, but also has twice as great a mass a
You pull a box with a constant force across a friction-less table using an attached rope held horizontally. If you pull the rope with the same force at an angle, does the acceleration of the box increase, decrease, or stay the same? What happens if there
When you pull the rope at an angle, only the horizontal component of the pulling force will be accelerating the box across the table. This is a smaller horizontal force than originally used, so the acceleration will decrease. When there is friction, there
When an object falls freely under the influence of gravity there is a net force mg exerted on it by Earth. Yet by Newton's third law the object exerts an equal and opposite force on the Earth. Does the Earth move?
The Earth moves but the movement is so small and the Earth so large that the movement is negligible.
Compare the force needed to lift a 10 kg object on the moon compared to lifting the same thing on the Earth. Compare the force needed to throw a 2 kg object horizontally.
The acceleration due to gravity is less on the moon than it is on Earth. A 10 kg object will weigh less on the moon, which means it will be easier to life on the moon. When throwing an object horizontally, the force needed to accelerate it to the desired
According to Newton's 3rd law, each team in a tug of war pulls with equal force on the other team. What determines which team will win?
In a tug of war, the team that pushes hardest against the ground wins. Both teams have the same force on them due to the tension in the rope, but the winning team pushes harder against the ground and thus the ground pushes harder on the winning team, maki
When you stand still on the ground, how large of a force does the ground exert on you? Why don't you lift up into the air?
When you at rest, the net force on you is zero. Hence the ground exerts a force on you exactly equal to your weight. The two forces acting on you add up to zero, so you do not accelerate.
Explain why the head of someone in a car crash seems to be thrown backward when the car is struck from the rear?
The impact on the car suddenly pushes the car, seat, and body forward. The head is attached by a flexible neck, however, and can momentarily remain where it was and lag behind the body.
You exert an upward force of 40 N to hold a bag of groceries. Describe the reaction force (Newton's 3rd Law) by stating its magnitude, its direction, on what object it is exerted, and by what object it is exerted.
The reaction force has a magnitude of 40 N that points downward and is exerted on your hands and arms by the bag of groceries.
A father and young daughter are ice skating. They face each other at rest and push each other, moving in opposite directions. Which one has the greater final speed?
Both the father and daughter will have the same magnitude of force acting on them as they push each other away (Newton's 3rd Law). Since the daughter has less mass than her father, her acceleration should be greater (a=f/m). Both forces and accelerations
A heavy crate rests on the bed of a flatbed truck. When the truck accelerates, the crate stays fixed on the truck - so it accelerates. What force causes the crate to accelerate?
Static friction between the crate and the truck bed causes the crate to accelerate.
A block is given a brief push so that it slides up a ramp. After the block reaches its highest point, it slides down, but the magnitude of its acceleration is less on the descent than the ascent. Why?
On the way up, there are two forces on the block that are parallel to each other causing the deceleration - the component of weight parallel to the plane and the force of friction on the block. Since the forces are parallel to each other, both pointing do
Why is the stopping distance of a truck much shorter than for a train going the same speed?
The net force slowing the vehicles is friction. Assuming they are on a level surface, the normal force is equal to the weight. The coefficient of friction between the train and train tracks is smaller than the coefficient of friction for the truck and roa
What would a scale read if you weighed yourself on an inclined plane? assume the mechanism functions properly even at an angle.
Your weight is W. The scale is on an inclined plane that is on an inclined angle of Θ. The scale would read the magnitude of normal force between you and the plane, which would be W cosΘ.