Download Forces Review Answers

Physics
Unit 3 Review
Know these things:
1. Newton’s three laws of motion
2. How to draw force (free body) diagrams for objects with forces acting on them.
3. How to solve problems using force diagrams and Newton’s second law.
4. How to solve problems that have two parts. Part one is a force problem. The force is used to find
acceleration and acceleration is used to solve a 1 or 2-D motion problem.
1. 1. A force of 20 N acts upon a 5 kg block. Calculate the acceleration of the object.
4m/s2
2. An object of mass 300 kg is observed to accelerate at the rate of 4 m/s2. Calculate the
force required to produce this acceleration. 1200N
3. A 5 kg block is pulled across a table by a horizontal force of 40 N with a frictional
force of 8 N opposing the motion. Calculate the acceleration of the object. 6.4m/s2
4. An object of mass 30 kg is in free fall in a vacuum where there is no air resistance.
Determine the acceleration of the object. 9.8m/s2 down
5. An object of mass 30 kg is falling in air and experiences a force due to air resistance of
50 Newtons.
a. Determine the net force acting on the object and 244N
b. calculate the acceleration of the object. 8.13m/s2
6. A student pushes on a crate with a force of 100 N directed to the right. What force
does the crate exert on the student? 100N to the left
7. A force of 200 N is exerted on an object of mass 40 kg that is located on a sheet of
perfectly smooth ice.
a. Calculate the acceleration of the object. 5m/s2
b. If a second object identical to the first object is placed on top of the first object,
what acceleration would the 200 N force produce? 2.5m/s2
8. Just before opening her parachute a skydiver of mass 50 kg reaches terminal velocity.
Calculate the force of air resistance. 490N
9. For a person who has a mass 60 kg, calculate the weight in Newtons and in pounds.
588N, 132 lbs you can ignore pounds for the test
10. An object of mass 10 kg is accelerated upward against gravity at 2 m/s2. What force
is required? 118N up
11. An aircraft with a mass of 10,000kg has an engine providing 100,000N of thrust.
The aircraft’s wing is providing 90,000N of lift. The drag on the aircraft is 95,000N. At
time zero the aircraft is travelling at 250m/s horizontally. Find the velocity in the x and y
directions at 5s. x: 252.5m/s Y: -4m/s
12. We have a block with a mass of 120kg resting next to a 250kg block. If we push on
the 120kg block with 1000N force what is the acceleration of the system? 2.7m/s2
13. What is the acceleration of the system in problem 12 if the system is on a friction
surface with coefficient of kinetic friction = .2? .74m/s2
14. A 2kg block is resting on an inclined plane that is at an angle of 60 degrees. The
coefficient of friction between the block and surface is .1. Find:
A. The normal force of the plane on the block 9.8N
B. The force of gravity acting to pull the block down the plane 16.97N
C. The frictional force on the block .98N
D. The acceleration of the block down the plane with the friction. 7.995m/s2
E. The acceleration of the block down the plane if the plane were frictionless. 8.48m/s2
15. Two movers want to push a 150kgdresser across the floor. µs = 0.45, µk =
0.30. A. What force is needed to set the dresser in motion? 661.5N
B. What force is needed to keep it in motion? 441N
16. A crane is lifting a 2500kg truck with a wire. Find:
A. The tension when the truck is not accelerating 24,500N
B. The tension when the truck is accelerating upwards at 1m/s2 27000N
C. The tension when the truck is accelerating downwards at 1m/s2 22000N
D. The tension of the truck is accelerating downward at 9.8m/s2 0N
17. A boy pushes on a box of mass 11 kg and it moves at an acceleration of 2.2m/s2.
With what force does the boy push on the box? 24.2N
18. A 17,250 kg rocket is pushed with a thrust of 1730564 N. What is the acceleration of
the rocket? 100m/s2
19. If a force of 3000 N can move a crate at an acceleration of 2.2 m/s2, what is the mass
of the object? 1364kg
20. If a force of 20N is applied to a 10kg object on a smooth surface in the horizontal
direction, find: A. The acceleration 2m/s2
B. The velocity after 4s. 8m/s
C. The displacement after 4s. 16m
D. If the system is changed to a rough surface with a 5N frictional force redo the
problem. 1.5m/s2, 6m/s 12m
21. A 10kg block rests next to a 20kg block on a smooth surface. A 150N force is
applied to the 10kg block in the horizontal direction. Find:
A. The force of the 10kg block on the 20kg block. 100N
B. The acceleration of the system. 5m/s2
C. The acceleration if the system were moved to a surface with a coefficient of kinetic
friction of .1 4.02m/s2