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Newton and Friction Quick Fact Study Sheet
Newton’s Laws
1) An object in motion will remain in motion unless an unbalanced force acts on it.
If an object is moving at constant velocity, there is no acceleration or net force.
Mass and inertia are proportional, the higher the mass the higher the inertia regardless of velocity.
2) Net Force = (mass)(acceleration) Forces are in Newtons (N), Mass in kg
Weight (force) = (mass)(acceleration due to gravity)
{you weigh between 500 N to 600 N} {1 stick of butter = 1 Newton}
3) For every action (force) there is an equal and opposite reaction (force). If a bat hits a ball with a
force of 100 N then the ball hits the bat with a force of 100 N. Same force, different mass, different
acceleration.
Friction
Friction forces are opposite the direction in which an object is moving.
It takes more force to start an object in motion then to keep it moving (s > k)
The coefficient of friction is a result of the roughness of the two surfaces in contact; it only changes
when the surfaces in contact change (see reference table for values)
A 10 N force is applied to a desk and it does not move. A 10 N friction force acts in the opposite
direction (Net force = 0, no acceleration)
Since gravity acts toward the center of the Earth, only a component of gravity is responsible for
moving the box down the incline.(mg or weight in Newtons)(sin )
mg sin 
mg cos 

Looking at the picture, the normal force acts perpendicular to the surface and the force that opposes
this is the component of gravity in the y direction (mg cos)
When drawing the forces acting on the box you would not show the components of gravity, just the
weight vector.
Elevator problems – when moving at constant velocity, your weight in an elevator is normal. When
accelerating upwards, your weight measures higher, when accelerating downward, your weight
measures less. In problems use the Net Force = (mass)(acceleration)
Practice Problems
1) The diagram below shows a 10.0-kilogram mass held at rest on a frictionless 30.0° incline by
force F. What is the approximate magnitude of force F?
(1) 9.81 N (3) 85.0 N
(2) 49.1 N (4) 98.1 N
2) Calculate the Normal force. 1) 10 N
2) 49.1N
3) 85 N
4) 8.6 N
3) A person is standing on a bathroom scale in an elevator car. If the scale reads a value greater
than the weight of the person at rest, the elevator car could be moving
(1) downward at constant speed
(2) upward at constant speed
(3) downward at increasing speed
(4) upward at increasing speed
4) An apple weighing 1 newton on the surface of Earth has a mass of approximately
(1) 1 x 10–1 kg
(3) 1 x 101 kg
0
2
kg
kg
5) In which situation is the net force on the object equal to zero?
(1) a satellite moving at constant speed around Earth in a circular orbit
(2) an automobile braking to a stop
(3) a bicycle moving at constant speed on a straight, level road
(4) a pitched baseball being hit by a bat
Do in workbook Regents practice 6 and Regents practice 7