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Dynamics: the study of the effect of forces on matter.
Haplosciences.com |
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Mass and Weight - Friction |
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MASS AND WEIGHT
It is important to understand the difference between mass and weight. Mass depends only of the amount of matter in an object. That is the number of atoms. You can be on Earth, Jupiter or inside a black hole, your mass stays the same. The mass is also a measure of the inertia of an object. A larger mass means a smaller acceleration if the same force is applied. Mass is measure in kg (slugs for the British system).
The weight of an object is the amount of gravitational pull exerted by the earth (or by the moon if you are standing on the moon). The weight depends on your mass but also on the mass of the planet or moon or.. you are on.
The gravitational pull is a force that will cause an object to accelerate. On Earth, if we neglect air resistance, the acceleration of any free-falling object =g = 9.8 m/s2. (32ft/s2)
W = m g
1) Find the weight of 5.00 kg
2) Find the weight of an 1150 kg automobile
3) Find the mass of an 11,500N automobile.
4) Find the 75kg astronaut’s weight on the moon, where g=1.63 m/s2.
5) John’s is 65kg. (typical adult’s mass) A) What is his weight on Mars ? (g= 3.72 m/s2) B) Found his weight in pounds on Mars (convert Newton to lbs) C) What is his weight on Jupiter ( g= 29.9 m/s2) D) Found his weight in pounds on Jupiter E) Same questions for the moon. (acceleration is 6th of the Earth’s)
6) Use the words mass, weight, and volume, to complete the table
7) can you think of 2 ways to measure mass ? You will get the inertial mass and the gravitational mass. The are both numerically equal.
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FRICTION
When we attempt to slide or roll two objects across each other, a force that resists the motion is produced. The force is called friction. Friction can be caused by the irregularities of the two surfaces sliding or sliding across each other. However, if you polish the surfaces, friction will decrease only to a certain point. If the surfaces are too smooth, friction will increase again. (electric force will hold the molecules of the surface together).
· Friction is a force that always acts parallel to the surface in contact and opposite to the direction of motion.
· Starting friction, also called static friction, is greater than moving friction also called kinetic friction. Static friction Fs acts only when the objects is acted upon by a force (push or pull) but is not moving because friction overcomes the push of the pull. Kinetic friction Fk acts only when the objects is moving because acted by a force greater than friction.
· Friction increases as the force between the surfaces increases. It is much easier to slide a light crate across the floor than a heavy one. The area of contact is not relevant. The force of friction is proportional to the force of contact between the 2 objects.
Fs = µs FN Fk = µK FN
Fs or Fk is the frictional force, µs or µk, is the coefficient of friction and FN is the normal force that is the force perpendicular to the contact surfaces. Usually, FN is balanced by the force so its magnitude = weight of the object.
Try first without hints. If you are confused you may read the hints
7) look at the following picture. f (magnitude = 10N) is the sliding or kinetic friction and P (magnitude = 100N) is the push. A) What is the net force acting Fnet on the chair ? Which way does it point ? Hint: to the right = positive and to the left = negative. Fnet = P + f A) Find N the normal force if m = 50kg. Hints : N balanced W and W = mg A) Find the coefficient of friction µK Hints f = µK N solve for µK
8) Look at the following picture. A) what is the net force Fnet? Hints: forces are vectors, they have direction. Left = negative and right = positive. Fnet = F + f A) What is the acceleration of the block ? Find the magnitude and direction. Hints: Fnet = m ax. Solve for ax A) What is the coefficient of kinetic friction µK ? Hints: write f = µK N with N = mg solve for µK
9) A force of 170N is needed to keep a 530N wooden block sliding on a wooden floor. What is the coefficient of kinetic friction ? (hints: use Fk = µK N = µK W)
10) A cart on wheels weighs W = 2400N. The coefficient of kinetic friction between the wheels and floor is 0.16. What force F is needed to keep the cart rolling uniformly ?
Hints: there is no acceleration so forces balance each other. That means F = frictional force f = µK W so F = µK W solve for F.
11) A piano weighs 4700N. What force is needed to start the piano rolling across the floor when the coefficient of static friction is 0.23 ?
Hints: there is no acceleration so forces balance each other. That means F = frictional force f = µs W so F = µs W solve for F.
12) A snowmobile is pulling a large sled across a snow-covered field. The weight W of the sled is 3560 N. If the coefficient of friction of the sled is 0.12, what is the pulling force supplied by the snowmobile ?
Hints: pulling P = frictional force so P = µk W .
13) 2 workers push in the same direction on a large pallet. The force exerted by one of the worker is F1 = + 645N. The force exerted by the other worker is F2 = 755N. The motion is opposed by a frictional force of f = -1175 N. Find the net force. Hint: Fnet = F1 + F2 + f
14) Find the acceleration of an automobile of mass 1500kg acted upon by a driving force of 2200N (F = + 2200N) that is opposed by a frictional force of 450N ( f = - 450N). Hint Fnet = m a and Fnet = f + F
15) A truck of mass 13,100kg is acted upon by a driving force of 8900N. The motion is opposed by a frictional force of 2230N. Find the acceleration. Hint: see 14)
16) fill the gap in the picture:
LABS: Frictions levers |
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The force of the engine has to balance the friction forces between the tires and the road. If they each balance each other, the car moves at a constant speed. If the force of the engine is greater, you accelerates according to Newton’s second law. Actually, there is also air resistance that slows down the car and has to be counter balanced by the engine too/ |
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push |
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Fs = µs mg |
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2 . you push but it is not moving. Your push is balanced by the frictional force. AS you push harder, the frictional force increases and it is still not moving. Push = FS = µS FN = µS mg Fs = push As you increase the push, Fs increases too. |
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1. A block is sitting on the ground. The weight is balanced by the normal force. So mg = FN |
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3. You push even harder and it is finally moving. Your push overcomes the static frictional force. The frictional force decreases and is now called the kinetic frictional force. If you are moving at a constant speed, your push is exactly balanced by the friction. If you are accelerating, the push is greater than then frictional force. If the block is accelerating : push > Fk push—Fk is the net force acing on the block |
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The force due to gravity on an object |
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The quantity of matter in an object |
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The amount of space an object occupies |
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