Megafun Forces Worksheet
Problem Solving Suggestions:
ü Draw a Free Body Diagram showing all of the forces exerted on the object by its surroundings (but none of the forces exerted by the object on its surroundings). This is important in determining what forces contribute to the net force on an object.
ü If any of the forces are at an angle, draw in components.
ü Use Fnet = ma and Fnet = F1 + F2 + F3 + . . .
ü Treat perpendicular directions separately; take a two dimensional problem and make it into two one dimensional problems – i.e. apply the 2nd Law to the components of force and acceleration: SFx = max; SFy = may.
1. On a car traveling on level ground there is a forward force of 550 N and a friction force of 550 N in the opposite direction. Is the car at rest, moving with constant velocity, or moving with constant acceleration? How do you know? Which Law or Laws support your answer?
2. Speaking of cars, that forward force has to come from the road pushing forward on the tires. (What else is there to push the car?!). This forward force is one force in a pair as described by the 3rd Law. Use Newton's Laws to describe the other force in this pair. State what object is exerting a force on what object and in what direction and how much.
3. Two sumo wrestlers with unequal masses are stationary on a level mat. As the less massive wrestler pushes on the other with greater and greater force one of them eventually slides across the mat. Assuming they are both wearing identical Nike Air-Sumo wrestling shoes, which one slips first, the more or less massive wrestler? Explain using Newton's Laws and the properties of friction.
4. Consider a skydiver. (a) What is the net force acting on a 895 N man falling through the air with acceleration 5.00 m/s2, 270.0° (b) What is the air resistance (friction) force on the man?
5. For Thanksgiving Mom lifts a 20.0 lb turkey (1 kg weighs 2.20 lb) from the lower part of the fridge to the countertop. Find the force Mom exerts for each part of the turkey's trip: (a) Initially accelerating the turkey from rest at 1.00 m/s2, 90.0°. (b) Moving the turkey upward at a constant 0.50 m/s. (c) Slowing the turkey to a stop 1.00 m/s2, 270.0°.
6. For Thanksgiving Dad has to carve the same turkey Mom was working on. He tries to hold it in place by exerting a force of 66.0 N, 225.0° with a fork, but the bird starts sliding across the level countertop. If μ = 0.300, what is the bird's acceleration across the coutertop?
7. A cable is used to lift an old treasure chest from the depths of the sea. The chest has a mass of 61.0 kg. Due to the volume of water it displaces, the force of buoyancy acting on the chest is 343 N, 90.0° (this same type of force makes it possible for things to float). In order to accelerate it 2.50 m/s2, 90.0°, the cable must be able to exert what force?
8. A 4.00 g coin rests on a closed book on a table. The coefficient of starting friction between the coin and the book's cover is 0.20; the coefficient of sliding friction is 0.15. (a) If the cover of the book is slowly opened at what angle of inclination will the coin begin to slide? (b) What will be the coin's acceleration just after it starts moving?
9. Two hockey players struggle on a surface of ice where μ = 0.075. Mario, mass 75.0 kg, shoves Wayne, mass 95.0 kg, with a force of 355 N, 0.0°. Find the acceleration of each player.
10. A tight rope walker with mass 55.0 kg stands at rest on a cable which is deflected 3.0° from the horizontal on either side due to her weight. Find the tension in the cable.
11. A physics poem/problem:
An 80.0 kg man pushes (at 0°),
on a 125 kg chest while his woman mushes (her hero to tease). Man and chest,
are initially at rest. The force increases until something releases. Find the
acceleration of the thing that slid, knowing as you did:
starting friction coefficients: man: μ = 0.80; chest: μ = 0.50.
sliding coefficients: man: μ = 0.60; chest: μ = 0.40.
12. An airplane in flight has only four forces on it: a forward force called thrust, a backward force called drag (friction), an upward force called lift, and a downward force (gravity). Suppose the "black box" in a 1750 kg plane indicates the thrust was 2550 N, 0.0° and the plane's acceleration was 5.00 m/s2, 225.0° just before an accident. Investigators use this information to find: (a) the lift force; (b) the drag force. Hint: find the x and y components of acceleration and treat this 2D problem like two 1D problems.
Answers
1.
2.
3.
4. a. 457 N, 270.0°
b. 438 N, 90.0°
5. a. 98.1 N, 90.0°
b. 89.1 N, 90.0°
c. 80.0 N, 90.0°
6. 0.65 m/s2, 180.0°
7. 407 N, 90.0°
8. a. 11.3°
b. 0.480 m/s2 down the incline
9. Wayne: 3.00 m/s2, 0.0°
Mario: 4.00 m/s2, 180.0°
10. 5150 N
11. 0.98 m/s2, 0°
12. a. 10.9 kN, 90.0°
b. 8740 N, 180.0°