Laws of Motion Review

Vocabulary Acceleration Equilibrium Force Inertia Kinetic Friction Mass Motion Net Force Newton Static Friction Terminal Velocity Vector

Types of Forces Pushing a crate across the floor – Applied Air Friction – Drag Opposes motion – Friction Support force – Normal Created in a rope, etc when connected to another mass Tension Force due to gravity – Weight

Free Body Diagrams What is the net force if the applied is 15N and friction is 5N? Applied is 22N and friction is 6N? What is the acceleration for each if the weight is 10N?

1st Law of Motion Law of Inertia An object at rest will stay at rest, an object in motion will stay in motion unless acted upon by a net force greater than zero Ex. A car rider continues forward when the driver suddenly applies the brakes

2nd Law of Motion ∑F = ma W = mg Ff = µFn What is the net force? The sum of the forces Remember: Mass must be in kilograms

2nd Law Practice A 20 N block is being pushed across a horizontal table by an 18 N force. If the coefficient of kinetic friction between the block and the table is 0.4, find the acceleration of the block. A. 0.5 m/s2 B. 1 m/s2 C. 5 m/s2 D. 7.5 m/s2 E. 9 m/s2

2nd Law Practice If all of the forces acting on an object balance so that the net force is zero, then A. the object must be at rest B. the object’s speed will decrease C. the object will follow a parabolic path D. the object’s direction of motion can change, but not its speed E. none of the above

2nd Law Practice A block of mass m is at rest on a frictionless horizontal table placed on a laboratory on the surface of the Earth. An identical block is at rest on a frictionless horizontal table placed on the surface of the Moon. Let F be the net force necessary to give the Earth-bound block an acceleration of a across the table. Given the gmoon is one-sixth of the gEarth the force necessary to give the Moon-bound block the same acceleration a across the table is A. F/12 B. F/6 C. F/3 D. F E. 6F

2nd Law Practice A cart of mass 10 kg moves on a frictionless table surface. A 10 N force pulls on the cart horizontally from the right. At the same time, a 30 N force acts at an angle of 30 degrees to the horizontal to the left. What is the magnitude of the horizontal acceleration of the cart? A. 0.5 m/s2 B. 1.6 m/s2 C. 2.0 m/s2 D. 2.5 m/s2 E. 2.6 m/s2

2nd Law Practice What is the vertical component of a resultant F acting at an angle? A. 1/2F B. F C. F cosӨ D. F sinӨ E. F tanӨ

2nd Law Practice Two astronauts tug on opposite sides of a satellite. The first astronaut tugs to the left with a force of 30 N. With what force does the second astronaut tug in order to keep the satellite moving toward him at a constant velocity of 20 m/s? A. 30 N B. – 30 N C. 0 N D. 50 N E. 1.5 N

2nd Law Practice Two people, one of mass 100 kg and the other of mass 50 kg, stand facing each other on an ice-covered (essentially frictionless) pond. If the heavier person pushes on the lighter one with a force of F, then A. the force felt by the more massive person is – 1/2F B. the force felt by the more massive person is -2F C. the magnitude of the acceleration of the less massive person will be ½ of the magnitude of the acceleration of the more massive person D. the magnitude of the acceleration of the less massive person will be twice the magnitude of the acceleration of the heavier person E. None of the above

2nd Law Practice The maximum (or terminal) velocity of a skydiver (m=70kg) with arms and legs extended is about 120 mi/h (= 53.6 m/s). What force does air resistance exert on the skydiver? A. 0 N B. 13 N C. 70 N D. 700 N E. 1000 N

2nd Law Practice An airplane has a mass of 3.1 x 104 kg and takes off under the influence of a constant net force of 3.7 x 104 N. What is the net force that acts on the plane’s 92 kg pilot?

2nd Law Practice Two forces act on a 6.00 kg object. The first force has a magnitude of 25 N and is directed west. The second force has a magnitude of 72 N at an angle of 47 degrees south of west. What is the horizontal acceleration (magnitude and direction) of the block?

2nd Law Practice Weight and mass are A. always the same B. inversely proportional C. based on location D. none of the above

2nd Law Practice A helicopter is lifting a 50 kg flood victim out from a rushing stream of water. How much tension is there in the rescue line pulling the victim up if the victim is accelerating upwards at a rate of 12 m/s2? a.1300 N b.1250 N c.1600 N d.1100 N

2nd Law Practice A large (150 kg) refrigerator needs to be moved across the kitchen. Fred pulls with a force of 100 N and Wilma pulls with 75 N in the same direction. If the force of friction was 40 N, what will be the acceleration of the refrigerator? a.1.17 m/s2 b..90 m/s2 c..43 m/s2 d.135 m/s2

2nd Law Practice A box of mass 10kg slides down a frictionless inclined plane that forms a 40º to the horizontal. A. Find the magnitude of the parallel force B. Find the magnitude of the perpendicular force C. What is the acceleration of the block? D. How do these values change if a 7N force of friction is present?

3rd Law of Motion For every action, there is an equal and opposite reaction

3rd Law Practice A mass is suspended on a spring. The reaction force to the force of the gravity from the Earth acting on the mass is the force exerted by the A. mass on the Earth B. mass on the spring C. spring on the mass D. spring on the Earth E. Earth on the mass

3rd law Practice A bucket is being supported by 2 ropes, each at an angle of 35o with the vertical. What is the tension force on each rope?