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HW 1 FORCES
1. Driving down the road you hit the brakes suddenly. As a result, your body moves toward the front of the car. Explain, using
Newton’s laws.
2. When a plane accelerates down a runway during takeoff, the passengers feel as if they are being pushed back into their seats.
Why?
3. A young girl slides down a rope. As she slides faster and faster, she tightens he grip, increasing the force exerted on her by the
rope.
What happens when this force is equal in magnitude to her weight?
Explain.
4. An astronaut on a space walk discovers that his jetpack no longer works; leaving him stranded 50m from the spacecraft. If the
jetpack is removable, explain how the astronaut can still use it to return to the ship.
5. A Cadillac bumps into a Volkswagen. Is the force exerted by the Cadillac on the Volkswagen greater than, less than, or equal
to the force exerted on the by the Volkswagen on the Cadillac?
Explain.
6. Suppose you jump from the cliffs of Acapulco and perform a perfect swan dive. As you fall you exert an upward force on the
earth equal in magnitude to the downward force the earth exerts on you. Why, then, does it seem that you are the one doing
all the accelerating? Since the forces are the same why aren’t the accelerations?
7. Give some everyday examples of situations where friction is beneficial.
8. At the local farm you buy a flat of strawberries and place them on the backseat of your car. On the way home you begin to
brake as you approach a stop sign.
seat. Explain.
At first the strawberries stay put, but as you brake a bit harder, they begin to slide off the
9.
Felicia, the ballet dancer, has a mass of 45.0 kg. a) What is Felicia’s weight on Earth? b) What is Felicia’s mass on Jupiter, where
the acceleration due to gravity is 25.0 m/s2? c) What is Felicia’s weight on Jupiter?
10.
Butch, the 72.0-kg star quarterback of Belmont High School’s football team, collides with Trask, a stationary left tackle, and is
brought to a stop with an acceleration of —20.0 m/s2. a) What force does Trask exert on Butch? b) What force does Butch exert
on Trask?
11.
A 20-g sparrow flying toward a bird feeder mistakes the pane of glass in a window for an opening and slams into it with a force
of 2.0 N. What is the bird’s acceleration?
12.
Gunter the weightlifter can lift a 230.0-kg barbell overhead on Earth. The acceleration due to gravity on the sun is 274 m/s 2. a)
Would the barbells be heavier on the sun or on Earth? b) How much (in newtons) would the barbells weigh on the sun (if it
were possible to stand on the sun without melting)?
13.
Sammy Sosa swings at a 0.15 kg baseball and accelerates it at a rate of 3.0 x104 m/s2. How much force does Sosa exert on the
ball?
14. If an elephant were chasing you, its enormous mass would be very threatening. But if you zigzagged, the elephant’s mass would
be to your advantage. Why?
15. What is the weight in newtons of Lily Lightweight, who has a mass of 40 kg?
16. What is the net force on a 1000-N barrel falling in air with an air drag of 400 N?
17. When a 1000-N barrel is sinking through the water at a constant speed, what is the upward force of the water on the barrel?
18. If a woman weighs 500 N on Earth, what would she weigh on Jupiter, where the acceleration of gravity is 26 m/s2?
19. Suppose a cart is being moved by a certain net force. If the net force is doubled, by how much does the cart’s acceleration
change?
20. What is the net force acting on a 25-N freely falling object? What is the net force when the object encounters
15 N of air resistance? When it falls fast enough to encounter 25 N of air resistance?
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HW 1 FORCES
Answer Section
1. The force exerted on the car by the brakes causes it to slow down, but your body continues to move forward with the same
velocity (due to inertia) until the seat belt exerts a force on it to decrease its speed.
2. The thrust form the engines accelerates the plane. For your body to have the same acceleration a force must act on it; this is
the force you feel as the seat pushes on your back.
3. When the magnitude of the force exerted on the girl by the rope equals the magnitude of her weight, the net force acting on
her is zero.
As a result she moves with constant velocity.
4. Turn their back to spacecraft and throw their jetpack away.
5. the same
6. The acceleration of an object is inversely proportional to its mass. The diver and the earth experience the same force, but the
earth – with its much larger mass – has a much smaller acceleration.
7. walking, swimming, driving,....
8. As you brake harder you car has a greater acceleration. The greater the acceleration of the car the greater the force required
to give the flat of strawberries the same acceleration.
the strawberries begin to slide.
When the required force exceeds the minimum force of static friction
9. a.
10.
11.
12.
w = mg = (45.0 kg)(10.0 m/s2) = 450. N
b. w = mg = (45.0 kg)(25.0 m/s2) = 1130 N
Since a newton is equivalent to 0.22 pounds, little Felicia would weigh about 260 lb on Jupiter. It should be noted, however, that it
would be impossible to stand on Jupiter due to its entirely gaseous surface.
a. The force depends upon the rate at which Butch’s mass is brought to rest.
F = ma = (72.0 kg)( —20.0 m/s2) = —1440 N
The negative sign in the answer implies that the direction of the force is opposite that of Butch’s original direction of motion.
b. Newton’s third law states that for every action there is an equal and opposite reaction. Therefore, if Trask exerts a force of
—1440 N on Butch, Butch will exert the same 1440 N force back on Trask, but in the opposite direction.
Since the sparrow exerts 2.0 N of force on the window, the window must provide —2.0 N back in the opposite direction. Don’t forget
to convert grams into kilograms before beginning.
a = F/m = (-2.0 N)/(0.02 kg) = - 100 m/s2
(about 10 g’s!)
Therefore, the bird experiences a very rapid negative acceleration, as the window brings the bird to a sudden stop. Ouch!
a) they would be heavier on the Sun
b) W = mg = 6.30x104 N
F = ma = 45000 N
13.
14. Because of the large mass, the elephant would have difficulty in zigzagging - in changing its motion.
15.
16.
17.
18.
19.
20.
W = mg = 400 N
Fnet = mg – Fdrag = 600 N
Fupward = 1000 N
m = 50 kg
W = mg = 1300 N
It doubles.
25 N downward;
10 N downward;
zero