Physics 20 - STA
Work Booklet
Unit I – Dynamics
Chapter 3 – Forces
Name: ______________________________________________________________________________
3.1 – The Nature of Force
1.
2. Shana, Kelsey, and Amanda pull on a toy exerting forces described in the diagram below. Let North be positive and south Negative. Determine the net force acting on the toy.
3. Three forces act on an object. Two are pulling to the south, one with a magnitude of 35N and the other with a magnitude of 23N. The third force is pulling towards the north with a magnitude of 29N. Determine the net force acting on the object.
4. Two forces pull on an object. One force pulls with 30N to the west. The second force is unknown. Determine the unknown force if the net force is 40 N to the East?
5. A car is speeding up. The weight of the car is 12 000 N [down]. The force of air resistance is 3600 N [backward]. With the engine engaged, the engine applies a 20 000 N [forward] force to the axels. Draw a free-body diagram for this situation.
6. Two people, A and B, are dragging a canoe out of a lake onto a beach using light ropes. Each person applies a force of 60.0 N [forward] on the rope. The force of friction exerted by the beach on the canoe is 85.0 N [backward]. Starting with a free-body diagram, calculate the net force on the canoe.
7. Two dogs, A and B, are pulling a sled across a horizontal, snowy surface. Dog A exerts a force of 200 N [forward] and dog B a force of 150 N [forward]. The force of friction exerted by the snow on the sled is 60 N [backward]. The driver attempts to slow down the sled by pulling on it with a force of 100 N [backward]. Starting with a free-body diagram, calculate the net force on the sled.
8. In a tractor pull, four tractors are connected by strong chains to a heavy load. The load is initially at rest. Tractors A and B pull with forces of 5000 N [E] and 4000 N [E] respectively. Tractors C and D pull with forces of 4500 N [W] and 3500 N [W] respectively. The magnitude of the force of friction exerted by the ground on the load is 1000 N.
(a) Starting with a free-body diagram, calculate the net force on the load.
(b) If the load is initially at rest, will it start moving? Explain.
9. The total weight of a biker and her motorbike is 1800 N [down]. The engine applies a force of 500 N [forward]. The air resistance and friction acting on the biker and bike is 200 N [backward]. The normal force exerted by the road on the biker and bike is 1800 N [up].
(a) Consider the biker and bike as a single object. Draw a free-body diagram for this object.
(b) Calculate the net force.
Answers
1) FBD 2) 140N [North] 3) 29N [South] 4) 70N [East] 5) FBD
6) 35.0N 7) 190N [forward] 8) a) 0N b) No, not moving.
9) a) FBD b) 300N [forward]
3.2 – Newton’s First and Second Law
1. Use Newton’s first law to explain why
(a) steel barriers usually separate the cab of a truck from the load
(b) trucks carrying tall loads navigate corners slowly, and
(c) customers who order take-out drinks are provided with lids.
2. Give two examples that illustrate the property of inertia for both a stationary and a moving object.
3. Use Newton’s first law to describe the motion of
(a) a car that attempts to go around an icy curve too quickly
(b) a lacrosse ball after leaving the lacrosse stick.
4. Apply Newton’s first law and the concept of inertia to each of these situations.
a) How could you remove the newspaper without toppling the plastic beaker?
b) While moving at constant speed on a level, snowy surface, a snowmobiler throws a ball vertically upward. If the snowmobile continues moving at constant velocity, the ball returns to the driver. Why does the ball land ahead of the driver if the snowmobile stops? Assume that the air resistance acting on the ball is negligible.
5. Determine the accelerations which result when a 12-N net force is applied to a 3-kg object and then to a 6-kg object.
6. A net force of 15 N is exerted on an encyclopedia to cause it to accelerate at a rate of 5 m/s2. Determine the mass of the encyclopedia.
7. Suppose that a sled is accelerating at a rate of 2 m/s2. If the net force is tripled and the mass is doubled, then what is the new acceleration of the sled?
8. A lacrosse player exerts an average net horizontal force of 2.8 N [forward] on a 0.14-kg lacrosse ball while running with it in the net of his stick. Calculate the average horizontal acceleration of the ball while in contact with the lacrosse net.
9. The net force acting on a 6.0-kg grocery cart is 12 N [left]. Calculate the acceleration of the cart.
10. A net force of 34 N [forward] acts on a curling stone causing it to accelerate at 1.8 m/s2 [forward] on a frictionless icy surface. Calculate the mass of the curling stone.
Answers
1) Answers Vary 2) Answers Vary 3) Answers Vary 4) Answers Vary
5) a=4 m/s2 and a=2 m/s2 6) 3kg 7) 3m/s2 8) 20 m/s2
9) 2.0 m/s2 10) 19 kg
3.3 – Multi Force Analysis
1. Two forces act on an object. The first force pulls 400N to the East while the second pulls 600 N to the West. The object accelerates 5.00 m/s2 to the West. Determine the mass of the object.
2. A ball has a mass of 0.240 kg and is thrown straight up into the air, the initial acceleration of the ball is 2.69 m/s2. What is the applied force on the ball?
3. Bryce applies a horizontal force of 120 N to the East on her 10.0kg toboggan causing it to accelerate 2.00 m/s2 to the East. Determine the force of friction opposing the toboggan’s motion.
4. Three forces act simultaneously on an object: F1 is 65 N [30.0°], F2 is 80 N [115°], and F3 is 105 N [235°]. Calculate the net force and direction acting on the object.
5. While in the water, a force of 320 N [up] exerted by the water on a dolphin causes the dolphin to jump up. The acceleration of the dolphin is 2.6 m/s2 [up].
a) Calculate the mass of the dolphin.
b) What would be the acceleration of the dolphin if it had the same strength but half the mass?
6. Two people, A and B, are dragging a sled on a horizontal, icy surface with two light ropes. Person A applies a force of 65.0 N [30.0°] on one rope. Person B applies a force of 70.0 N [300°] on the other rope. The force of friction on the sled is negligible and the ropes are parallel to the icy surface. Calculate the net force on the sled.
7. An ice hut used for winter fishing is resting on a level patch of snow. The combined mass of the hut and angler inside is 80 kg. A wind exerts a horizontal force of 205 N on the hut, and causes it to accelerate. While in motion, the magnitude of the force of friction acting on the hut is 196 N. What is the acceleration of the hut?
8. A 25.0-kg store sign is suspended as shown in Figure 3.18. Calculate the forces FT1 and FT2 exerted at the point at which the sign is suspended.
Answers
1) 40.0kg 2) 3.00 N 3) 100N 4) 42N [27° E of S] or [63° S of E]
5) a=5.2m/s2 6) 95.5N [17.1° S of E] or [72.9° E of S] 7) 0.12 m/s2 (forward)
8) FT1=299N [125°] and FT2=172N [0°]
3.4 Force of Friction
1. A 2.6 kg sled is pulled along a horizontal surface at a constant velocity to the south. If the coefficient of kinetic friction between the surfaces is 0.17, determine the force of friction acting on the sled?
2. Due to a frictional force of 3.30 N, a 20 kg curling rock slides to rest along a horizontal sheet of ice. Calculate the coefficient of kinetic friction between the curling rock and the ice.
3. The coefficient of static friction can be calculated knowing the foce applied at the threshold of motion. (i.e.) just as the object beging to move. A 220 g block is placed on a horizontal surface. If a force of 1.2 N is applied and just starts the object in motion, determine the coefficient of static friction between the two objects. [0.56]
4. A larger force is required to start an object moving than is needed to keep the object moving. A 250 g glass block is set on a glass plate and a horizontal force is applied. The force is gradually increased in magnitude. When the applied force reaches a value of 2.3 N the block starts to move. The force required to keep the block moving at constant velocity is 0.98 N. Deteremine the coefficient of :
a) Static friction between the block and the surface [0.94]
b) Kinetic Friction between the block and the surface [0.40]
5. The driver of a 2600 kg truck travelling at 13.1 m/s along a level, but icy road steps on the brakes. The wheels lock and the tires skid along the icy surface for a distance of 26.2 m before coming to a stop. Calculate the coefficient of kinetic friction between the tires’ surface and the ice [0.33]
6. A 100N force is applied to accelerate a 20 kg box along a horizontal sidewalk where the kinetic coefficient of friction between the box and sidewalk is 0.40. Determine the magnitude of the box’s acceleration.
7. A force of 31 N [forward] is needed to start an 8.0-kg steel slider moving along a horizontal steel rail. What is the coefficient of static friction?
8. A biker and his motorcycle have a weight of 2350 N [down]. Calculate the force of kinetic friction for the rubber tires and dry concrete (u=0.70) if the motorcycle skids.
Answers
1) 4.3N 2) 0.017N 3) 0.56 4) a) 0.94 b) 0.40
5) 0.33 6) 1.1 m/s2 7) 0.40 8) 1.6 X 103 N
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Physics 20 Unit 2: Dynamics